9th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy

31 Aug 2026, 00:30 → 4 Sept 2026, 23:00 Europe/Berlin

Neue Aula (Lecture Halls of the "Neue Universität")

Jim Hinton (MPIK), Felix Aharonian, Brian Reville (MPIK), Ralf Klessen (Universität Heidelberg), Ruth Crespo (MHKP)

 

The 9^th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy (γ-2026) will take place from August 31 to September 4 2026, in Heidelberg, Germany.

The Gamma-Ray Symposium has been held since 1994 in Heidelberg (see previous editions in 2000, 2004, 2008, 2012, 2016), with the last two symposia taking place in Barcelona (2022) and Milan (2024). We are glad to announce that the 2026 edition will again be held in Heidelberg at the Neue Universität in the heart of the historical old town, hosted by Heidelberg University and MPIK. The format will closely follow that of previous symposia, covering all major observational and theoretical aspects of the field with a focus on the high-energy (GeV), very-high-energy (TeV), and ultra-high-energy (>100 TeV) intervals of the electromagnetic spectrum.

Emphasis will be placed on recent advances in the field, in areas including microquasars, extragalactic jets and transients, the acceleration of cosmic rays, PeV sources, and progress in understanding diffuse emission. Major progress on new facilities, upgrades, instrumentation, and techniques will be reported. Synergies with other wavebands and messengers, including X-rays, radio, neutrinos, and gravitational waves are expected to be a strong theme of the symposium. The symposium will also address cosmological issues related to dark matter, intergalactic radiation and magnetic fields, as well as topics in fundamental physics.

SOC

 

 

 

                 

Gamma26-poster.pdf

Posters.xlsx

Monday 31 August

Organisation: Register

10:30 Coffee break

Plenary: I

Convener: Brian Reville (MPIK)

1 Introduction

Gamma2026Intro.pdf

Gamma2026Intro.pptx

2 Unveiling the Extreme Universe with Recent IACT Discoveries

Speaker: Rubén López-Coto (IAA-CSIC)

2026_08_31_IACTs_summary.pptx

3 Measurement of Galactic diffuse gamma-ray emission in the very-to-ultra-high energy band

The diffuse Galactic gamma-ray emission is a very important tool used to study the propagation and interaction of cosmic rays in the Milky Way. The measurements of diffuse emission above TeV energy are limited in the sky coverage and energy coverage for a long time, hindering effective constraints on cosmic ray models. Using the data from the Large High Altitude Air Shower Observatory (LHAASO), precise measurements of the spectra and spatial distributions of the diffuse emission across the Galactic plane with $15<l<235$ in a wide energy range from TeV to PeV have been achieved. In this talk, the results and possible interpretation of the LHAASO measurements will be described.

Speakers: Qiang Yuan, Qiang Yuan, Qiang Yuan

2026_Heidelberg_YuanQ.pdf

12:30 Lunch

Plenary: II

Convener: Alicia López-Oramas (Instituto de Astrofísica de Canarias (IAC))

4 Gamma-ray view of Supernova Remnants

Speaker: Marianne Lemoine-Goumard

SNR_gamma2026.pdf

5 Non-thermal emission from microquasars

Speakers: Laura Olivera Nieto (MPIK), Laura Oliviera-Nieto

2026-09-GAMMA26.pdf

6 Gamma-ray emission from young stellar ecosystems

Speaker: Lars Mohrmann

2026-08-31_StellarEcosystems_Gamma26_LarsMohrmann.pdf

15:30 Coffee break

Plenary: III

Convener: Gustavo E. Romero (Instituto Argentino de Radioastronomía (IAR))

7 Multimessenger diagnostics of hadronic acceleration from PeV to EeV

Speaker: Prof. Foteini Oikonomou (NTNU)

GAMMA26_Oikonomou.pdf

8 Detection of variable PeV gamma-rays from Cygnus X-3

Recent observations have unveiled that microquasars may represent a new class of powerful particle accelerators and may contribute to the high-energy cosmic rays in our Galaxy. However, very-high-energy (VHE) and ultra-high-energy (UHE) gamma-rays from the central system have not been detected to date. Cygnus X-3, a well-known microquasar situated at the center of the Cygnus Bubble, has historically attracted significant interest while also posing considerable confusion in the field. Utilizing approximately four years of data from the Large High-Altitude Air Shower Observatory (LHAASO), we have confirmed the detection of petaelectronvolt (PeV) gamma-rays from Cygnus X-3 and provided strong evidence for their hadronic origin. In this presentation, I will provide a brief introduction about the recent findings on Cygnus X-3 from LHAASO observations.

Speaker: Cong Li (The Institute of High Energy Physics of the Chinese Academy of Sciences)

Cygnus X-3 _CongLi_gamma2026.pdf

9 The Heidelberg Gamma-Ray Symposia and Three Decades of Milestones in Ground-Based Gamma-Ray Astronomy

Founded in 1994, the Heidelberg Gamma-Ray Symposia have accompanied and reflected the emergence of ground-based gamma-ray astronomy as a truly astronomical discipline and one of the great success stories of modern astrophysics. Over the past three decades, the meetings have mirrored many of the field's defining scientific and instrumental breakthroughs. In this talk, I will revisit these milestones through the evolution of the Heidelberg Symposia and conclude with some thoughts on the prospects and challenges for the coming years

Speaker: Frank Rieger (IPP)

Gamma_History.pdf

Poster Session I

10 A Deep Learning Approach to Improving the Arrival-Direction Accuracy of Gamma-Ray Air-Shower Measurements with a Ground-Based Detector

The Tibet AS experiment employs an array of ground-based air-shower detectors (Tibet-III), consisting of plastic scintillators, to observe cosmic gamma rays in the energy range from several TeV to a few PeV.
To determine the arrival direction, a conventional method is used in which the shower-front surface is fitted to the detector hit-time data; the resulting angular resolution is typically between 0.5 degrees and 0.2 degrees in the energy range of 10–100 TeV.
To further improve the directional accuracy, this study develops a new arrival-direction reconstruction method that integrates a convolutional neural network (CNN) with the conventional approach.
An evaluation using gamma-ray events generated by Monte Carlo (MC) simulations shows that the directional resolution is improved by approximately 10–15% compared to the conventional method.

Speaker: Masato Kobayashi (for the Tibet ASγ Collaboration) (Yokohama National University)

11 Molecular Gas towards stellar cluster C1 1806-20 and HESS J1808-204

This work investigates the ISM gas towards TeV gamma-ray source HESS J1808-204 and stellar cluster C1 1806-20. We will also show results from a single-zone modelling of the leptonic and hadronic processes to determine the particle origin of the TeV emission, to understand the roles of cluster itself and the other potentially extreme particle accelerators such as the soft gamma repeater and magnetar SGR 1806-20 and luminous blue variable star LBV J1808-20. Out initial results suggest that molecular cloud MC 73, located at ~ 6kpc, exhibits a spatial match to the TeV gamma-ray morphology.

Speaker: Tiffany Collins (GNOI)

12 Discovery of a Low-Energy Cutoff in the Injected Spectrum of a Pulsar Wind Nebula

Bow-shock pulsar wind nebulae (PWNe) are synchrotron sources formed when the outflow of supersonic pulsars is confined by the surrounding interstellar medium. These sources are dominated by freshly injected particles, thus providing a unique laboratory for studying particle acceleration in relativistic outflows. The Mouse is a prototypical bow-shock PWN and is bright in the radio and X-ray bands, enabling detailed multi-wavelength spectral modeling. Using data from 17 telescopes, we identify a clear spectral break at $\sim3.7$\,GHz.The radio spectrum exhibits a rising trend at low frequency, peaks at the break and then declines. We found a change in spectral index $\Delta\alpha=0.65\pm0.05$ across the break. We attribute this to an intrinsic cutoff or steepening in the injected particle distribution. This could indicate the characteristic energy for leptons leaving the pulsar magnetosphere and from the energy dissipation after the particles crossed the termination shock.

Speaker: Zhihong Shi (University of Hong Kong)

13 Reassessing GRB high-energy emission correlations using a large, homogeneous sample of X-ray afterglows

Gamma-ray bursts (GRBs) exhibit diverse X-ray afterglow lightcurves, including breaks and plateau phases, whose physical origins remain debated. Previous studies have often relied on small and heterogeneous samples, limiting their statistical power and leading to apparently conflicting claims. Most notably, correlations have been suggested between high-energy ($E\ge100$MeV) detection and X-ray afterglow complexity or plateau incidence.
We present a comprehensive and unbiased statistical analysis of GRB X-ray afterglows based on the full Swift-XRT catalog, comprising over 1400 events. Our approach relies on a fully automated and model-independent pipeline, including uniform flare removal and segmented power-law fitting, ensuring consistent measurements across the entire dataset.
We find that both lightcurve complexity and plateau occurrence are strongly driven by the XRT observation start time ($t_{XRT}$). Apparent correlations between high-energy detection and X-ray morphology arise when $t_{XRT}$ is not accounted for; however, these correlations disappear once the sample is properly controlled or stratified by $t_{XRT}$. This indicates that X-ray afterglow features are not intrinsically linked to high-energy emission.
These results resolve longstanding claims in the literature and demonstrate that controlling for observational biases, particularly $t_{XRT}$, is essential in large-sample GRB studies. The automated analysis framework introduced here provides a robust basis for future population studies and will be critical for interpreting data from upcoming missions such as SVOM, Einstein Probe, and THESEUS.

Speaker: Alessandro Armando Vigliano (University of Trieste - INFN Trieste)

14 4FHL: The Deepest Fermi-LAT Catalog of Hard Gamma-Ray Sources

The Fermi Large Area Telescope (LAT), operating since 2008, continuously surveys the gamma-ray sky. A series of hard-source catalogs has been produced to focus on the highest-energy gamma-ray sources detected by the LAT. The 2FHL catalog mapped the sky above 50 GeV using 6.5 years of data in the 50 GeV-2 TeV energy band, while the 3FHL extended the energy coverage down to 10 GeV over a 7-year baseline. Building on these efforts, the Fourth Catalog of Hard Fermi-LAT Sources (4FHL) presents the most sensitive and extensive survey above 50 GeV to date, based on the first 16 years of observations, significantly improving the detection of faint and hard sources. The catalog includes 673 sources, nearly doubling the number of sources detected in the 2FHL catalog at the same energies, providing evidence for new extreme particle accelerators. By focusing on this high-energy regime, the 4FHL catalog provides a fundamental link between space-based gamma-ray observations and the TeV range probed by current and future ground-based observatories, such as CTAO. We present the main results from the 4FHL catalog and discuss its scientific potential across Galactic and extragalactic source populations, as well as its relevance to multiwavelength and multimessenger studies.

Speaker: Alba Rico (Clemson University)

15 A Large Number of New Fermi-LAT Blazar Identifications in the 1FLAT and 2FLAT Catalogs: A Citizen Science Effort with Firmamento and Prospects for CTAO

We present 1FLAT (1st Firmamento LAT AGN Table) and the preliminary results of its successor, 2FLAT — two independently derived catalogs of blazar counterparts for high-Galactic latitude γ-ray sources from the Fermi-LAT 4FGL-DR4 and FL16Y source catalogs.
1FLAT and 2FLAT have been built using Firmamento — a web-based multi-frequency platform — which combines data from over 90 multi-frequency catalogs covering the full electromagnetic spectrum, and includes SED construction, variability analysis, and machine-learning-based synchrotron peak estimation.
A defining feature of this work is its inclusive model of scientific participation: high-school and undergraduate students working within the Firmamento project at NYUAD and elsewhere participated in the analysis, enabling human supervision of thousands of SEDs at a scale difficult to achieve otherwise.
The reassessment involves more than five thousand high-Galactic latitude γ-ray sources and confirms 4FGL-DR4, 4LAC-DR3 and FL16Y associations in about 90% of the cases. Most notably, numerous new blazar identifications are found among previously unassociated sources, reducing the fraction of unidentified extragalactic Fermi-LAT sources from ~25% to ~17% in 1FLAT and to less than 15% in 2FLAT.
Ongoing related work includes the study of redshift distributions of the newly identified blazars and an assessment of their detectability with CTAO.

Speaker: Michele Doro (University of Padova)

doro_2026_gamma_poster.pdf

16 A Multiwavelength Approach to Multipolar Magnetic Field Modeling in Millisecond Pulsars

Recent X-ray observations of millisecond pulsars (MSPs) with the Neutron Star (NS) Interior Composition Explorer (NICER) have enabled precise constraints on NS masses and radii. These observations also indicate that simple dipolar magnetic field geometries are insufficient to reproduce the observed surface hotspot structures, motivating the exploration of more complex magnetic configurations.

We model the thermal X-ray light curve (LC) of PSR J0030+0451 (J0030) using a swept-back multipolar magnetic field configuration including stellar rotational effects. The magnetic field is described through a complete expansion in vector spherical harmonics, allowing for a flexible and self-consistent representation of arbitrary field geometries. We introduce quantitative measures to characterize magnetic field complexity and illustrate their application with representative examples.

Assuming a vacuum magnetosphere and including multipole components up to the octupole order, we determine the magnetic configuration that best reproduces the bolometric thermal X-ray LC of J0030. Parameter estimation is performed using Markov chain Monte Carlo methods accelerated by a neural network (NN) surrogate model to efficiently explore the high-dimensional parameter space. We find that a centered multipolar field including components up to the octupole order provides an adequate fit to the NICER data.

We further outline an extension of this framework to joint multiwavelength modeling by incorporating Fermi-LAT gamma-ray LCs, and discuss the associated challenges in NN training for combined datasets. This methodology is readily applicable to other NICER-observed MSPs and offers a systematic approach toward more realistic magnetic field modeling in NSs.

Speaker: Anu Kundu (NASA Goddard Space Flight Center, North West University South Africa)

17 A Ring of Fire Orphan γ-ray Flare in the Neutrino Candidate Radio Galaxy 3C 120

We present a multi-wavelength and 43 GHz VLBI study of the brightest γ-ray outburst observed from the radio galaxy 3C 120, which occurred in March 2018 and has recently been discussed in connection with the IceCube neutrino alert IC-180213A. Despite reaching a peak γ-ray luminosity of about 3.7 × $10^{44}$ erg s$^{-1}$, contemporaneous X-ray, optical, and radio observations show no corresponding variability, establishing the event as a clear orphan γ-ray flare. High-cadence VLBI imaging reveals the emergence of a new disturbance propagating through the inner parsec-scale jet, with the γ-ray peak occurring when this disturbance crosses a quasi-stationary feature downstream of the VLBI core. The accompanying polarization changes indicate localized magnetic-field compression at the interaction site. We interpret the event within a Ring of Fire scenario, where the moving disturbance inverse-Compton scatters synchrotron photons from the stationary jet feature. This provides a direct observational link between VLBI-resolved jet dynamics and orphan γ-ray emission in a radio galaxy, with possible implications for structured jets and multi-messenger activity in mildly misaligned AGN.

Speaker: Dr Efthalia Traianou (Universität Heidelberg)

18 A time-resolved, systematic approach to gamma-ray burst physics

The Fermi mission observes Gamma-Ray Bursts (GRBs) over a broad energy range through its two instruments, the Gamma-ray Burst Monitor (GBM) and the Large Area Telescope (LAT), which operate in distinct yet complementary energy bands spanning from ~10 keV to >300 GeV. A joint spectral analysis of GRBs simultaneously detected by both instruments enables a comprehensive characterization of their prompt emission across this wide energy range, allowing for detailed studies of spectral evolution. Since the launch of Fermi, several catalogs based on single-instrument analyses have been published. However, only a few joint time-integrated studies combining GBM and LAT data are available, and these are based on relatively small samples.

In this contribution, we present the first comprehensive systematic spectral analysis of the complete sample of GRBs jointly observed by GBM and LAT over the first 17 years of the Fermi mission. The analysis is performed within the Multi-Mission Maximum Likelihood (3ML) framework, and includes both time-integrated and time-resolved studies. The sample comprises ~200 GRBs spanning a wide range of durations, fluences, and spectral properties, providing a representative dataset for statistical studies of prompt emission. We report preliminary results from the joint fits and quantitatively evaluate the impact of high-energy emission on the spectral parameters, including systematic shifts in the inferred values and a reduction in their uncertainties compared to previous results reported in the literature, as well as their evolution across time bins. We also investigate the presence of additional spectral components required to account for emission at the highest energies, such as extra power-law components or possible high-energy exponential cutoffs. By exploiting Fermi’s broadband coverage, this study provides new constraints on the physical mechanisms driving GRB prompt emission.

Speaker: Aldana Holzmann Airasca (University of Trento and INFN Bari)

19 Air shower simulations with CORSIKA 8

This contribution will present the status and features of CORSIKA 8, a complete C++ redesign of the widely used Fortran-based air shower simulation framework CORSIKA 7. The progress of the ongoing implementation of Cherenkov light emission, a critical feature for gamma-ray observations, will also be discussed.

Speaker: Radek Privara (ESO)

20 Calibration and Characterization of PANOSETI Telescopes for Ultra-High-Energy Gamma-Ray Detection

Detecting ultra-high-energy gamma-rays requires instrumenting large effective areas, which motivates the deployment of arrays of compact, cost-effective Imaging Air Cherenkov Telescopes (IACTs).

The Panoramic Search for Extraterrestrial Intelligence (PANOSETI) implements this approach using small IACTs equipped with 0.5 m Fresnel lenses and 1024-pixel SiPM cameras at Palomar Observatory. Each telescope provides a field of view of approximately 10°, enabling observations of large sky regions and supporting a broad range of applications both within and beyond gamma-ray astronomy, including searches for ultra-heavy dark matter for the Dark100 project. The concept is designed to scale to large arrays, enabling coverage of very large effective areas.

A detailed understanding of the detector response, including optics, mechanical alignment, and focal plane electronics, is essential for reliable event reconstruction and gamma-hadron separation. We present results from calibration and characterization campaigns performed in laboratory and field environments, including test bench measurements, photoelectron calibration, and characterization of the optical point spread function, as well as studies of detector stability and uniformity.

These efforts establish the basis for consistent performance across multiple units and demonstrate the viability of a cost-effective, wide-field IACT array for gamma-ray astronomy at the highest energies.

Speaker: Yuriy Popovych (Ruhr University Bochum)

Gamma2026_Poster_final.pdf

21 Characterization and Evaluation of Legacy PMTs for SWGO

SWGO will be a next-generation wide-field-of-view gamma-ray observatory. It will be located close to ALMA in Chile, feature thousands of Water Cherenkov Detector Units (WCDUs), and cover an energy range from 100s of GeV up to the PeV scale.
The current baseline layout for the innermost WCDUs features a two-layer design (two volumes of water stacked on top of one another), with two large-area PMTs. The two PMTs are arranged back-to-back so that each one instruments one water volume.
This poster presents a study of multiple large-area PMTs reclaimed from other experiments (ANTARES, Dooble-Chooz, Borexino). They are characterized and compared with one another and a new reference PMT. Their performance is then evaluated with respect to their use in SWGO WCDUs, as their use could significantly reduce the costs of such an array.

Speaker: Frederik Wohlleben (MPIK)

22 Collective gamma-ray emission from protostellar jets embedded in star forming regions

Protostellar jets, which can reach speeds of up to 1000 km/s, are present throughout the stellar formation process. In some cases, these jets are synchrotron radio emitters, indicating the presence of relativistic electrons. Models of particle acceleration and non-thermal multi-wavelength emission from single protostellar jets suggest that only the most powerful sources, such as HH80-81 (recently associated with the Fermi source 4FGL J1818.5-2036), will produce detectable levels of gamma-ray emission. However, we note that massive stars are formed in groups and that the collective effects of all the protostellar jets within the star-forming region should be considered in detectability studies in the gamma-ray domain. We compute the injection of cosmic rays into molecular clouds due to the presence of multiple protostellar jets and the collective non-thermal emission.
Gamma-ray fluxes depend on the protostellar mass function and ambient densities. Our results are relevant for those star forming regions detected in gamma rays and without a supernova remnant, as well as to explain recent associations of Fermi sources with massive protostars. Although protostellar jets were only considered candidate gamma-ray emitters until recently, growing observational evidence now suggests that they are beginning to emerge as a new class of high-energy sources.

Speaker: Anabella Araudo (GNOI)

23 Constraining TeV Flare Origins in M 87 Using Multi-Zone and Time-Dependent SSC Modeling

The radio galaxy M 87 is known for the fast day-scale flares it has undergone at very high energies (VHE) since it was first detected in gamma-rays. The most recent flare, lasting approximately 8 days, was observed by LHAASO in 2022 in the TeV band. The origin of these flares remains elusive despite the numerous observations and multi-wavelength (MWL) monitorings of this source. We construct a physically constrained MWL baseline model of the quiescent state of M 87 using a multi-zone leptonic scenario. Building on this baseline, we simulate rapid VHE flares using time-dependent synchrotron self-Compton (SSC) scenarios with particle injection descriptions motivated by different acceleration mechanisms, such as Fermi-type acceleration and magnetic reconnection, in the downstream of the jet or in the vicinity of the black hole. We evolve the emitting particle population and compute the resulting spectral energy distribution and light curves, comparing our predictions with the available observations. With this approach, we aim to assess the viability of different emission sites and acceleration processes for the latest TeV flare of M 87.

Speaker: Paloma Thevenet (Observatoire de Paris (PSL))

24 Constraints on Hadronic Emission from Microquasars Detected by LHAASO

Recently, the Large High-Altitude Air Shower Observatory (LHAASO) collaboration reported ultra-high-energy gamma-rays from six microquasar systems.
For five of these sources, the emission exceeds $100$ TeV, making microquasars promising Galactic PeVatrons.
In this work, we investigate whether gamma-rays around $100$ TeV can originate from hadronic interactions of accelerated cosmic rays with the ambient medium and estimate the contribution of these sources to the CR proton knee.
Two transport scenarios are considered: one in which particle transport around the source is dominated by advection with the velocity of the jet, and another in which it is dominated by diffusion, with the diffusion coefficient reduced with respect to the Galactic value. In both scenarios, the diffusion coefficient matches the Galactic value at large distances from the source.
We find that in both advection and diffusion scenarios, hadrons alone cannot fully explain the observed emission in most sources, allowing us to place upper limits on the hadronic contribution to the observed gamma-ray flux.
Finally, we estimate the contribution of these sources to the Galactic cosmic-ray spectrum; this quantity strongly depends on source age and injection history.

Speaker: Vittoria Vecchiotti (INAF-OAA, TDLI)

25 Decoding Blazar Gamma-Ray Quasi-Periodic Oscillations with a Morphology-Based Search for Supermassive Binary Black Holes

Gamma-ray quasi-periodic oscillations (QPOs) in blazars offer a unique probe of relativistic jet dynamics and supermassive black hole environments. However, stochastic red noise often mimics true periodicity, complicating their physical interpretation. We present a new morphology-based framework to evaluate year-scale QPO candidates using the 17-year Fermi-LAT baseline. Our pipeline uses Singular Spectrum Analysis to isolate oscillatory components, followed by Weighted Wavelet Z-transform ridge tracking to map their temporal evolution. By deriving physically motivated parameters, we definitively distinguish rigid, long-lived geometric modulation from unstable plasma-driven variability. Applying this methodology to a large Fermi-LAT sample, we will present a "gold sample" of highly stable QPO candidates. Finally, I will discuss how these systems are used to constrain supermassive binary black hole parameters and highlight their role as optimal targets for continuous gravitational wave searches.

Speaker: Adithiya Dinesh (Universidad Complutense de Madrid)

26 Detection of TeV emission during early afterglow from poorly localized GRBs with ground based IACTs

Gamma-ray bursts (GRBs) are among the most luminous and rapidly evolving transients in the Universe, yet their very-high-energy (VHE; $E>100$ GeV) emission remains difficult to capture from the ground, particularly during the prompt and early afterglow phases. This is primarily due to rapid temporal fading, attenuation by the extragalactic background light, observational delays, and the typically large localization uncertainties associated with MeV instruments such as Fermi/GBM.

In this work, we explore the detectability of TeV ($\sim$100 GeV–1 TeV) emission from poorly localized GRBs by implementing optimized follow-up strategies based on rapid tiling of wide sky regions. We simulate a realistic GRB population informed by over fifteen years of observations from Fermi/GBM and Swift/XRT, combined with recent developments in afterglow emission modeling. Using these simulations, we assess the performance of next-generation Imaging Atmospheric Cherenkov Telescopes with large fields of view, exploring the impact of latency, exposure time, and observing strategy on detection prospects.

We find that optimized tiling strategies can significantly enhance detection rates. For instruments such as ASTRI and LACT, the detection rate can increase by up to a factor of two compared to strategies focusing only on well-localized events. For CTAO, our approach yields up to four VHE detections per year. I will discuss the implications of these results for future GRB follow-up programs and how optimized observational strategies can maximize the scientific return of upcoming VHE facilities.

Speaker: Samanta Macera (INAF)

27 FlashCam - development, verification and performance

FlashCam is a high-performance camera system designed for imaging atmospheric Cherenkov telescopes. Its fully-digital trigger and readout architecture enables high event rates without dead-time and provides precise control over the trigger setup. Its state-of the-art front-end electronics design based on photomultiplier tubes achieves a wide dynamic range up to several thousand photoelectrons per pixel using a single gain channel. This contribution summarizes the development and performance of FlashCam, with emphasis on the verification and calibration measurements obtained with a fully-equipped camera unit in the lab and the long-term performance of the advanced prototype operating in H.E.S.S. for more than 6 years. Furthermore, the preparatory steps for installation of the first unit into the first Medium-Sized Telescope of CTAO-South as well as for the mass-production of in total 14 units are discussed.

Speaker: Simon Steinmaßl (Max-Planck-Institut für Kernphysik)

gamma-26-Poster-fc-print.pdf

28 Follow-up observations of TDE2025aarm with LST-1 and MAGIC

Tidal disruption events (TDEs) are energetic astrophysical transients that occur when a star is torn apart by a supermassive black hole. Shocks within the infalling debris are believed to provide efficient sites for particle acceleration. In these environments, cosmic rays may be accelerated up to PeV energies, potentially producing a detectable very-high-energy (VHE; E>100 GeV) gamma-ray component. Although over 100 events have been identified through high-cadence optical and X-ray surveys, follow-up observations of TDEs in the VHE regime are scarce. To date, no detection of TDEs in the GeV or TeV ranges has been confirmed.

We present the first target of opportunity observations for the follow-up of TDEs with the joint Cherenkov Telescope Array Observatory’s (CTAO) first Large-Sized Telescope (LST-1) and Major Atmospheric Gamma-ray Imaging Cherenkov Telescopes (MAGIC) configuration. This setup provides the best detection sensitivity among the current generation of imaging atmospheric Cherenkov telescopes (IACTs) in the tens of GeV energy range. The planned exposure of 50 hours, combined with observations during both opaque and transparent emission phases, allows to obtain strong constraints on the non-thermal emission components in TDEs. We report results from the follow-up campaign of TDE2025aarm (z = 0.0136), the second closest TDE reported to date. Approximately 11 hours of observations with LST-1 and 30 hours with MAGIC were obtained within -15 and +86 days from the optical peak, constituting the deepest and earliest TDE observation campaigns conducted with IACTs to date. Additionally, we discuss additional constraints for the non-thermal emission in light of a broad multiwavelength dataset collected for TDE2025aarm.

Speaker: Maria Kherlakian (Ruhr University Bochum)

final_poster_TDE.pdf

29 Forecasting the MeV Gamma-ray Sky: A Predicted All-Sky Map and a Promising MeV Source Catalog

The MeV gamma-ray sky remains the least explored window in high-energy astrophysics, the so-called "MeV gap", with only 32 steady sources and 31 GRBs detected by COMPTEL. With COSI launching soon and other future MeV missions, quantitative predictions of what these telescopes will see are urgently needed to sharpen MeV gamma-ray science cases in the multi-messenger era. We present the most comprehensive forecast of the 1–10 MeV sky to date. Building on Tsuji et al. (2021), which cross-matched the Swift/BAT and Fermi/LAT catalogs, we construct an updated joint catalog using the latest BAT and LAT releases, substantially expanding the sample. For each source, covering blazars, radio galaxies, pulsars, PWNe, SNRs, and globular clusters, we build phenomenological spectral models from hard X-ray to GeV energies and estimate the 1–10 MeV flux. We find ~200 promising targets exceeding $10^{-11} \ \rm{erg/cm^2/s}$, increasing from 87 sources reported in a previous study. In addition, combining these with Galactic diffuse and extragalactic background radiation, we produce predicted all-sky maps in the 1–10 MeV band. I will present the expanded catalog, the all-sky maps, and the impact on multi-messenger astronomy.

Speaker: Minami Urushihara (Kanagawa University)

30 From Photons to Neutrinos: Advancing Multi-Messenger Modeling with Gammapy

In recent years, several astrophysical objects have been identified as potential multi-messenger photon-neutrino emitters. New instruments dedicated to the study of these two messengers have been and are still being built, achieving unprecedented sensitivity. This increases the need for more sophisticated and precise analysis tools. Multi-wavelength and multi-messenger data fitting is crucial to constrain astrophysical models of particle acceleration and radiation mechanisms at work in these extreme environments.
In this contribution, we present a novel framework dedicated to multi-wavelength and multi-messenger modeling, built within the open-source software Gammapy. The framework enables the analysis of data from optical up to very-high-energy gamma rays, and the fitting of physical models directly on instrumental counts in these wavebands, without the need of computing flux points.
Focusing on blazars as a first example of a multi-messenger candidate, we showcase the performances of this tool in constraining multi-wavelength emission models, and how it reduces fitting biases. We show the results of a leptonic model that can be fitted on the fly, due to the short computing time needed. For more computing-intensive codes,we adopt a grid-fitting approach. We evaluate its performance on the leptonic model and present its first application to hadronic modeling of blazars. By fitting directly on photon and neutrino events, this novel approach enables, for the first time, statistically sound constraints on the physics of multi-messenger sources.

Speaker: Justin Albinet (CNRS, APC, Paris)

31 Gamma-ray emission towards Supergiant Shells in the Large Magellanic Cloud

Observations have shown that diffuse emissions relate to both the large- and small-scale structures of the LMC. Particularly, it has been suggested that some of the small-scale bright diffuse emissions coincide with Supergiant Shells (SGSs) previously identified based on Hα images. SGSs have been proposed to be powerful particle acceleration sites. We present a first study to unravel the correlation between the small-scale diffuse gamma-ray emissions and the Hα-selected SGSs, and connect them to the underlying cosmic ray properties in these SGS regions. We analyzed the gamma-ray emission from the LMC with more than 15 years of Fermi-LAT observations in the energy range of 300MeV-300GeV. A dedicated spatial and spectral analysis is done for the diffuse emission components of the LMC. Subsequently, we show results for the emissions from SGSs using phenomenological approaches and establishing their corresponding spatial templates for gamma-ray emission. Also, we show contributions of SGS emissions to the total diffuse emission across the LMC. Comparisons between our results in the GeV band and H.E.S.S. detections of relevant targets (e.g. 30 Dor C) are made as well.

Speaker: Qixin Yu (University of Innsbruck)

32 GRB Classification Beyond "T90"

Gamma-Ray Bursts (GRBs) are intense flashes of gamma radiation arising from catastrophic cosmic events such as stellar collapse and compact object mergers. They are historically divided into short and long classes based on the prompt emission duration T90, but this binary scheme does not fully capture the diversity revealed by recent observations. In this work, we investigate the structure of GRB prompt emission using a combination of simulations and unsupervised machine learning applied to Fermi–GBM light curves. The analysis is based on Fourier-domain representations of light curves, which are embedded in a latent space using dimensional reduction and clustering algorithms. Clustering of synthetic light curves with controlled power–law power spectral densities (PSDs) show that the method is sensitive to variability properties: single–component signals arrange in a clear sequence from white to red noise, while mixed–variability signals at fixed duration form largely distinct groups for different combinations of PSD components. When realistic T90 values are taken into account, however, the dominant structure in the embedded space reduces to a short–versus–long separation. The same behaviour is seen in the Fermi–GBM sample, where duration governs the large–scale geometry of the embedding and a steep–variability subgroup appears as an outlier population. These results provide a simple, simulation–anchored explanation for the persistence of duration–based classification in GRB studies, while also demonstrating that additional variability information is present but largely masked by the effects of differing time scales and signal superposition. They underline both the potential and the limitations of unsupervised methods for GRB taxonomy and point towards future work that combines duration control with variability–based features to obtain a more physically informative picture of the prompt emission.

Speaker: Amit Shukla (Indian Institute of Technology Indore)

33 High-Energy Electromagnetic Counterparts of Binary Black Hole Mergers in AGN Disks

The astrophysical origin of binary black hole (BBH) mergers remains one of the key open questions in gravitational-wave (GW) astronomy. Among the proposed formation channels, active galactic nucleus (AGN) disks provide a particularly promising environment, where stellar-mass black holes can migrate, dynamically pair up, and undergo repeated mergers. This process can produce BBH mergers with distinctive properties, including large component masses, characteristic spin distributions, and non-zero eccentricity.

In addition to their GW signatures, BBH mergers in AGN disks may generate high-energy electromagnetic (EM) emission. Recent studies suggest that gas-embedded binaries can launch magnetically powered outflows, with the resulting jet luminosity depending sensitively on the binary masses, separations, accretion rates, and surrounding gas properties. Such systems may therefore produce transient X-ray and gamma-ray counterparts accompanying the GW event.

We combine population-synthesis models of BBHs in AGN disks with prescriptions for magnetically driven outflows, linking the binary dynamics directly to potential high-energy observables. This framework enables us to predict both the GW properties of the merging population and the expected rates and luminosities of electromagnetic counterparts.

Our results provide the first population-level predictions for joint GW and high-energy EM detections from the AGN channel. These predictions are essential for assessing whether AGN disks can be observationally distinguished from competing BBH formation scenarios, and for guiding multi-messenger searches with future third-generation GW detectors and wide-field X-ray and gamma-ray facilities.

Speaker: Maria Paola Vaccaro (Heidelberg University, ITA-ZAH)

34 How to sparkle in gamma rays: stochastic variability of Fermi-LAT blazars

Blazars exhibit pronounced γ-ray variability, yet the physical processes driving this behavior remain uncertain. To investigate the statistical properties of this variability and their connection to acceleration mechanisms, we analyze a large, homogeneous sample of blazars from the Fermi-LAT Light Curve Repository. Light curves are studied at multiple cadences (3, 7, and 30 days), enabling a systematic comparison across timescales.
We combine power spectral density (PSD) analysis with stochastic time-series modeling based on exponentiated Ornstein–Uhlenbeck processes. This approach yields two key parameters: the variability amplitude (σₒᵤ) and the characteristic correlation timescale (θₒᵤ). Distinct trends emerge between blazar subclasses. Flat Spectrum Radio Quasars show steeper PSD slopes and shorter correlation timescales, indicating more rapidly evolving variability, while BL Lac objects display flatter PSDs and longer θₒᵤ, consistent with more persistent emission patterns.
These findings point to intrinsic differences in the variability regimes of the two classes and support a physical interpretation in terms of differing emission conditions. By extending the blazar sequence into the time domain, this work provides new constraints on the stochastic processes governing high-energy emission in relativistic jets.

Speaker: Dr Sarah Wagner (IFAE Barcelona)

35 iactsim: a CUDA-accelerated IACT simulation framework and its application to the ASTRI Mini-Array

Performance evaluation and data analysis of Imaging Atmospheric Cherenkov Telescopes (IACTs) rely on large-scale Monte Carlo simulations of air showers and of the response of telescope optics and Cherenkov camera electronics. The latter can be a computational bottleneck during the commissioning phase of new instruments, as it involves optical ray-tracing and electronics simulation for each pixel. Fortunately, these tasks consist of many independent calculations that can be run concurrently. This makes them ideal candidates for acceleration using Graphics Processing Units (GPUs), which have become common in both high-performance computing systems and consumer hardware.

In this contribution, we present iactsim, a CUDA-accelerated simulation framework for IACTs. Implemented in Python, C++, and CUDA C++, and utilizing CuPy for GPU offloading, iactsim is designed as a flexible toolkit to support instrument design and performance evaluation by enabling users to define custom optical systems and Cherenkov cameras.

To demonstrate the framework capabilities, we have been developing a simulator for the ASTRI Mini-Array telescopes using iactsim. It handles ray-tracing for the main shadowing elements, the camera window, and the Schwarzschild-Couder dual-mirror optics, where both mirrors are aspherical and the primary consists of 18 hexagonal panels. The ray-tracing algorithm also accounts for multiple reflections that occur between the protective window layers and the SiPM sensors. We also simulate the electronics of the ASTRI Cherenkov camera, which features 2368 Silicon PhotoMultiplier sensors arranged in 37 photon-detection modules, to reproduce the actual signal processing and camera trigger logic. We validate the simulations against observation and calibration data acquired by the first two operational telescopes (ASTRI-1 and ASTRI-3) of the ASTRI Mini-Array. Preliminary benchmarks indicate a speedup of two orders of magnitude compared to the current sim_telarray-based ASTRI simulator.

Speaker: Davide Mollica (INAF)

36 Impact of interstellar emission model on Fermi-LAT unassociated sources

The unassociated population represents about 30% of the sources in the latest Fermi-LAT catalog release (FL16Y after 16 years), with nearly one half lying at low (|b|<10°) Galactic latitudes. Those "Galactic unassociated" (GU) sources have distinct spectral and spatial characteristics, but their origin remains unclear.
The LAT interstellar emission model (IEM) accompanying the 4FGL catalog (and also used for the FL16Y catalog) was developed with 8 years of survey data and represents a major source of systematic uncertainty. The Fermi-LAT collaboration is striving to construct a new IEM for the next 5FGL catalog. Several approaches, based on advanced methods, make use of the LAT data themselves to modulate the gas templates and add uncharted components (like Loop I or the Fermi Bubbles).
Early tests have shown that GUs are particularly sensitive to the underlying IEM. This presentation will summarize the efforts toward a new IEM and focus specifically on the impact those improving IEMs have on the GUs, trying to quantify what fraction of the GUs could be missing interstellar emission (either missing gas or Inverse Compton emission).

Speaker: Jean Ballet (AIM, CEA Saclay)

JBallet_diffuseGUs_v4.pdf

37 Intergalactic magnetic field lower limits up to the redshift $z\approx3$

Large-scale intergalactic magnetic fields (IGMFs) may comprise both galactic and cosmogenic components, which can be probed via observations of delayed $\gamma$-ray emission from electromagnetic cascades initiated by the highest-energy photons emitted by distant sources. These components can, in principle, be distinguished through their redshift evolution; however, observational evidence for non-negligible magnetic fields has so far been largely limited to low redshifts.

This work extends constraints on the IGMF to redshifts $z \gtrsim 1$ using 17 years of all-sky observations of high-redshift active galactic nuclei with the Fermi/LAT $\gamma$-ray telescope. By combining Fermi/LAT data in the 0.1 GeV – 1 TeV energy range with Monte Carlo simulations of $\gamma$-ray-induced electromagnetic cascades, it is shown that the null hypothesis of zero magnetic field strength in the redshift interval $z \in [0.5,\,3]$ is disfavoured at the $\approx 8.6\sigma$ significance level. This corresponds to a lower bound of $B \gtrsim 1 \times 10^{-18}$ cG for magnetic field correlation lengths exceeding 1 Mpc.

The same dataset further constrains the volume-filling fraction of the IGMF to $f \gtrsim 90\%$ within the probed redshift range. The robustness of these results is verified against potential systematic effects, including source flux variability and uncertainties in the $\gamma$-ray attenuation model.

These results provide the first evidence to date for pervasive intergalactic magnetic fields at $z \gtrsim 1$, placing new constraints on their origin and evolution.

Speaker: Ievgen Vovk (ICRR, The University of Tokyo)

38 Interpreting the Gamma-Ray Emission from Westerlund 1: Hadronic or Leptonic?

High energy gamma-ray emission has been detected from a number of massive star clusters, indicating that these sources are sites of cosmic-ray acceleration. However, it is still unclear whether the gamma-ray emission is of hadronic or leptonic origin.

In this work, we present a comprehensive study of the gamma-ray emission from Westerlund 1, under the assumption that high energy protons and electrons are accelerated at the cluster wind termination shock. We investigate both the hadronic and leptonic scenarios by comparing the predicted SED with the gamma-ray measurements by Fermi-LAT and HESS and with the X-ray upper limit derived by eROSITA. The measurements of gamma-ray morphology by HESS and X-ray upper limit profile by eROSITA are used to constrain the environment properties and particle transport in the bubble surrounding Westerlund 1.

We find that both hadronic and leptonic models can reasonably account for the observed spectrum and morphology. In both cases, Bohm-like diffusion in the bubble is necessary to efficiently accelerate particles to energies consistent with the gamma-ray observations. For the leptonic scenario, the magnetic field immediately downstream of the shock can be constrained $\lesssim 4 \, \mu\mathrm{G}$ with the eROSITA upper limit. On the other hand, in order to explain the gamma-ray observations with the hadronic emission alone, the gas density inside the bubble should be $\gtrsim 2 \,\mathrm{cm}^{-3}$. A $\sim 3.6 \sigma$ detection of the corresponding neutrino emission is expected after 10 years of observation with KM3NeT.

Speaker: Ben Li (Gran Sasso Science Institute (GSSI))

39 Interstellar Gas and TeV Gamma Rays toward the Young Massive Cluster Westerlund 1: Implications for a Galactic PeVatron

Young massive clusters (YMCs) bright in gamma rays have received considerable attention as potential accelerators of cosmic rays up to PeV energies. In this context, investigating the interstellar medium (ISM) associated with such systems is crucial, as dense gas provides targets for cosmic-ray protons to produce gamma rays via hadronic interactions, thereby offering key constraints on acceleration and diffusion processes. We present CO and HI observations of molecular and atomic gas toward the extended TeV gamma-ray source HESS J1646$-$458, which is widely considered to be associated with the YMC Westerlund 1. Molecular clouds at $V_\mathrm{LSR} \sim -32$ km s$^{-1}$ coincide with arc-like structures seen at 8 $\mu$m, likely illuminated by strong far-ultraviolet radiation from Wd1. $^{12}$CO(3-2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of $\sim$5 km s$^{-1}$ toward the cluster center, suggesting a wind-blown bubble driven by the cluster activity. We also identify a complementary spatial distribution between the $V_\mathrm{LSR} \sim -55$ and $\sim -32$ km s$^{-1}$ clouds, connected by an intermediate-velocity component at $\sim -44$ km s$^{-1}$, consistent with signatures of cloud-cloud collision and triggered star formation. On larger scales, the total interstellar proton column density at $V_\mathrm{LSR} \sim -36$ to $-23$ km s$^{-1}$ shows a moderate spatial correspondence with the TeV gamma-ray shell. Combined with the large gas mass of $\sim 1.6 \times 10^6$ $M_\odot$ and the absence of bright synchrotron X-rays, the gamma-ray emission is consistent with a hadronic origin. The total energy of accelerated cosmic-ray protons is estimated to be $\sim 6 \times 10^{49}$ erg. In this presentation, we will discuss the physical connection between YMCs and the surrounding ISM and explore Westerlund 1 as a candidate Galactic PeVatron.

Speaker: Hidetoshi Sano (Gifu University)

40 JWST/MIRI Observations of the Nuclear Region of NGC 7582

Obscured active galactic nuclei (AGN) represent a crucial phase of supermassive black hole growth, in which dense circumnuclear dust hides the central engine and reprocesses its radiation into the infrared. Earlier mid-infrared observations with the Spitzer Space Telescope provided key constraints on dusty torus models but lacked the sensitivity and spectral resolution required to reveal subtle ice absorption features in nearby AGN. The galaxies in this study are part of the Galactic Activity, Torus, and Outflow Survey (GATOS), which focuses on investigating the structure of the dusty molecular torus and its connection to the host galaxy in local AGN. Recent JWST/MIRI mid-infrared spectra of the central region (~50 pc) of Seyfert 2 galaxy NGC 7582 have revealed prominent ice absorption feature. This is the first time we have detected the icy band at 6 μm in the local galaxies. Such an ice absorption band has not been included in existing AGN or starburst dust radiative transfer models. In particular, the template spectra from current AGN model libraries severely underestimate the mid-infrared flux when compared with the JWST data. To resolve this discrepancy, we propose a new class of frosty AGN models. These models incorporate icy grain mantles into clumpy dust torus configurations. Using Monte Carlo radiative transfer calculations of the spectral energy distribution, we account for both the imprint of embedded ice features and the additional heating contribution from Type-I quasar spectra, thereby extending and updating the AGN model library of Siebenmorgen et al. (2014). Our framework enables us to address several key questions: Under which physical conditions can water freeze onto dust grains in AGN environments? Is the ice reservoir primarily associated with the torus or with circumnuclear starburst regions? And more broadly, why do current AGN models fail to reproduce JWST/MIRI mid-infrared spectra? In this talk, we will present the physical conditions required to produce the 6 μm water-ice band in AGN, discuss the implications for the torus structure, and outline how the inclusion of icy dust fundamentally alters the interpretation of the spectral energy distribution (SED) of AGN.

Speaker: Ms Khushboo Khushboo (Ludwig Maximilian University of Munich/European Southern Observatory)

41 Lorentz invariance violation search with flaring active galactic nuclei observations of the first Large-Sized Telescope of CTAO

Rapid variability in very-high-energy (VHE) astrophysical sources—such as pulsars, gamma-ray bursts (GRBs), and flaring active galactic nuclei (AGN)—provides a way to probe potential violations of Lorentz invariance (LIV). These effects can be explored through measurements of time delays in the arrival of VHE photons. However, a significant source of uncertainty arises from delays unrelated to LIV that may be introduced by some intrinsic emission processes within the sources. To mitigate this limitation, we aim to combine observations from multiple sources, located at different redshifts. In this work, we present the results of a uniform analysis applied to all AGN observations obtained with the first Large-Sized Telescope of the forthcoming Cherenkov Telescope Array Observatory (CTAO). Our study includes a systematic search for intra-night variability in archival datasets corresponding to nights with significant excess detections among selected targets. By combining these observations, we place constraints on the characteristic energy scales at which deterministic or stochastic LIV effects may occur.

Speaker: Cyann Plard (LPNHE - CNRS)

20260901_gamma26_PLARD.pdf

42 Luminosity function of millisecond pulsars in globular clusters from their gamma-ray spectra

Millisecond pulsars (MSPs) are widely believed to be responsible for the gamma-ray emission of globular clusters (GCs), yet the underlying MSP gamma-ray luminosity function remains uncertain. Existing GC-based determinations of the latter often rely on external prescriptions for the number of MSPs in each cluster.

In this work, we constrain the MSP gamma-ray luminosity function in Milky-Way GCs directly from gamma-ray spectral energy distributions (SEDs), establishing an inference strategy that remains computationally viable for joint analyses of large GC samples.
We construct a forward model in which each GC hosts a population of MSPs whose luminosities are drawn from a universal log-normal luminosity function. Synthetic GC SEDs are generated by summing individual MSP spectra, and we infer posterior distributions using first a likelihood-based nested sampling and then a Bayesian implicit-likelihood inference in the neural ratio estimation (NRE) approach.

We show how the likelihood-based nested sampling approach becomes rapidly impractical as the number of jointly modelled clusters (and cluster-specific latent parameters) increases, whereas the NRE approach scales mildly with sample size and reproduces the likelihood-based posteriors in regimes where direct sampling is feasible.

We then apply the implicit-likelihood pipeline to Fermi-LAT SEDs from the 4FGL-DR4 catalog for a sample of 36 GC-associated sources, augmented by dedicated phase-resolved ON/OFF SEDs for two clusters hosting individually detected MSPs.

For the real data sample, we obtain informative constraints on the luminosity-function parameters consistent with previous analyses that constrain the cluster-specific MSP counts via complementary multi-wavelength information.

Speaker: Francesco Xotta (University of Nova Gorica)

XottaFrancesco_PosterGamma2026.pdf

43 Mapping the TeV Sky Around Pulsars: Catalogue, Correlations, and Predictions

Recent advances in $\gamma$-ray astronomy have led to the detection of a growing population of high-energy sources, many of which are associated with pulsars. These systems, including pulsar wind nebulae and halos, exhibit diverse morphological and spectral characteristics shaped by both intrinsic pulsar characteristics and interactions with the surrounding medium. Despite this progress, the mechanisms driving their formation and evolution remain poorly understood.

We present a comprehensive catalogue of TeV $\gamma$-ray properties of pulsar environments. The catalogue is constructed by collecting observations from all major operating TeV observatories and cross-matching them with pulsar properties from the ATNF Pulsar and Fermi-LAT 3PC catalogues. This provides a uniform database for validating population studies and constraining models of particle transport and energy losses.

Using this dataset, we investigate correlations between pulsar properties and the characteristics of their associated TeV emission. We further train a gradient-boosted decision tree model to predict the expected TeV surface brightness based on the properties of the central pulsar and the surrounding medium.
Applying this model to five energetic pulsars without detected TeV nebulae, we simulate observations with the Cherenkov Telescope Array Observatory. These simulations indicate that all five sources should produce detectable TeV nebulae if they follow established population trends. Future detections would therefore support these trends, while non-detections would point to additional factors inhibiting TeV surface brightness, making these sources key targets for CTAO observations.

Speaker: Tina Wach (MPIK)

Gamma26_poster_TinaWach.pdf

44 Methods for constraining hadronic interaction models with IACTs

Simulations of hadronic air showers play an important role in the data analysis chains of IACTs, used, among others, in the determination of instrument sensitivities and the measurement of cosmic-ray spectra.

The hadronic interaction models employed in these simulations suffer from intrinsic uncertainties that arise, in part, from our limited knowledge of hadronic physics. These models have primarily been constrained by data from collider experiments and have only been extrapolated to the ultra-high-energy regime. As such, the validation of these models in the IACT energy range remains limited.

Unlike particle detector arrays, which detect only an air shower’s footprint on the ground, IACTs provide a unique perspective on its longitudinal development, making it possible to compare different aspects of air shower development, such as multiplicity, lateral spread, and first interactions. This therefore allows for the testing of hadronic interaction models against real air shower data in energy and rapidity ranges that have not yet been directly probed.

We compare and contrast model predictions of hadronic air shower observables with simulated H.E.S.S. data, demonstrating the differences between models. This should then be measurable in real data, allowing for the validation of hadronic interaction models in the very-high-energy range. We expect such a study to strongly complement the results from the recent proton–oxygen run at the LHC, and serve as important input for model developers.

Speaker: Anita Deka Baruah (GNOI)

45 Modeling the X-ray and TeV light curves of gamma-ray binary PSR B1259−63

PSR B1259−63/LS 2883 is a gamma-ray binary consisting of a young pulsar orbiting a massive O-type star in a highly eccentric orbit with a period of 3.4 years. It has been extensively observed in radio, optics, X-rays, and gamma-rays for more than 20 years. All these observations, especially high-quality X-ray and TeV monitoring data from the last six periastron passages, require a model of the processes in the system that can explain the spectra in different energy bands over various orbital phases, as well as reproduce the observed light curves.
We present the results of modeling of the X-ray and TeV light curves of PSR B1259−63 taken around 2004-2024 periastra passages where the emission is produced via synchrotron radiation (for X-rays) and inverse Compton scattering (for TeV gamma-rays) from ultra-relativistic electrons accelerated at the intra-binary shock (IBS). The IBS forms due to the collision between the pulsar relativistic wind and the O-star non-symmetric (polar + disk) winds. The characteristic two-peak light curve in X-rays is explained by the disk pushing the IBS closer to the pulsar, which enhances the synchrotron emission. The relative peak heights and the overall LC behavior also require employing ideas previously used in the modeling of spider systems: Doppler boosting due to the bulk motion along the IBS and electron population advection and cooling.
A comparison of the model with the X-ray data shows excellent agreement, while the TeV model light curve, reproducing the general trend, leaves room for speculation about additional possible sources of VHE photons.
We also present, for the first time, the open-source code IBSEn (IntraBinary Shock Emission), which implements the modeling framework described above, including the IBS geometry, electron population evolution, and calculation of gamma–gamma absorbed non-thermal radiation with Doppler boosting.

Speaker: Gerd Puehlhofer

Poster_PSRB_model.pdf

46 Multi-Messenger Concordance for the Cygnus Region as the Source of the Cosmic-Ray Knee

The Large High Altitude Air Shower Observatory (LHAASO) has released the highest-energy measurements of the diffuse gamma-ray flux, offering the opportunity to study the spatial distribution and energy spectrum of Galactic cosmic rays (CRs) in the TeV-PeV sky through a multi-messenger approach. In addition, the high-precision CR observations by the same collaboration have revealed a pronounced feature in the proton spectrum at $\sim3-4$~PeV (associated to the CR 'knee'), while their observations of diffuse gamma-rays above $100$~TeV do not exhibit a corresponding spectral break. This persistent tension between the two observed components seems to challenge conventional scenarios, in which the local CR distribution is representative of the Galactic CR sea. In this work, we explore as a possible alternative, the scenario of a dominating contribution of a local PeVatron in the Cygnus region (an Ultra-High-Energy gamma-ray cocoon observed by LHAASO) to the CR 'knee'. We develop a two-population model of the Galactic CR flux, which we compare to the latest CR data available by satellite missions and ground-based observatories. We contrast the diffuse gamma-ray flux predicted from our model with recent observations and discuss the conditions of the CR populations that may alleviate the aforementioned tension. Finally, we also consider the contribution of our model to the diffuse neutrino flux and compare it with observations of the Galactic Plane by the IceCube Neutrino Observatory.
(Based on work in arXiv:2603.21665)

Speaker: Luis Enrique Espinosa Castro (Gran Sasso Science Institute)

47 MULTIWAVELENGTH VIEW OF A VERY HIGH-ENERGY FLARING MONSTER

The very-high-energy (VHE; E > 100 GeV) gamma-ray extragalactic sky probes some of the most extreme environments in the Universe, including active supermassive black holes, powerful relativistic jets, and regions of intense star formation. A significant fraction of gamma-ray sources exhibit strong flux variability in the form of flares. However, unveiling the physical processes powering these extreme events requires a panchromatic view of VHE emitters obtained as simultaneously as possible.
Here, we present preliminary results from a multiwavelength follow-up campaign of the flaring, high-redshift (z=0.56) VHE blazar PKS 1725+123. The source was detected by MAGIC, LST-1 and H.E.S.S. in August 2025 and promptly followed up in X-rays with XMM-Newton and NuSTAR over three consecutive days. These observations were complemented by a dense multiwavelength campaign spanning radio to optical frequencies, enabling us to track the spectral evolution of the source during its flaring state. This dataset provides valuable insights into the underlying emission mechanisms and particle acceleration processes driving the observed variability.

Speaker: Lea Marcotulli (GNOI)

48 Multiwavelngth observations of PSR B1259-63 over decades.

PSR B1259-63 is a classical gamma ray binary observed for more than a decade from radio up to TeV energies. In this system a young radio pulsar is orbiting around a massive Be star. The observed emission is believed to be due to the interaction of the relativistic electrons of the pulsar wind with the optical photons and massive outflows of the companion. In my talk I will review the multiwavelength properties of the source collected during more than a decade of broad band observations including the unpublished yet radio data from the 2024 periastron passage.

Speaker: Maria Chernyakova (DCU)

2026_gamma2026_PSRB1259_poster.pdf

49 Non-thermal emission from stellar bow shock nebulae: insights from a sample of 12 sources

In recent years, the role of the winds launched by massive stars in the production of high-energy particles has received significant attention. A very interesting case is that of massive runaway stars that move supersonially through the interstellar medium. In the interaction of the wind with the ambient gas a bow shock nebula is produced. The dust in the nebula is heated by the stellar photons and cools emitting in the infrared, often resulting in an observable extended structure. Many of these nebulae were reported to produce radio emission, and in 3 cases the non-thermal nature of the radiation is confirmed. In this work we analyse the cases of 12 massive runaway stars with confirmed radio observations. We aim to understand the physical conditions that determine the capability of the sources to produce relativistic particles and subsequently the non-thermal emission they produce at radio and gamma-ray energies. We analyse the current and future detectability of these sources at GeVs and TeVs. We conclude that the particles responsible for the synchrotron emission are likely accelerated in the wind shock. While at radio wavelengths the primary challenge is the separation between thermal and non-thermal contributions, at gamma rays, the main obstacle is the faint signal resulting from the lack of dense targets for the high-energy particles.

Speaker: Maria Victoria del Valle (University of São Paulo)

50 Not so Swift: 20 years of multi-wavelength observations of Mrk 421 and Mrk 501

In this work we aim to characterise the long-term temporal behaviour of the blazars Mrk 421 and Mrk 501 at synchrotron energies, namely in the optical, UV, and X-ray, in order to assess current models of these objects and their processes at higher (gamma-ray) energies. We investigated 20 years of data (2005–2025) from the Swift-UVOT and Swift-XRT telescopes. We examined spectral models, fractional variabilities, flux distributions, and X-ray photon index versus flux relations, as well as carrying out in-depth time series analysis using structure functions, Lomb-Scargle periodograms, and discrete correlation functions. Both sources showed intriguing variability at all studied wavelengths; this variability has been found to be energy-dependent, as has the trend of lognormality in flux distributions. X-ray photon indices fluctuated greatly throughout the entire period, showing an overall harder-when-brighter trend. Hints of a quasi-periodicity have been found in the X-ray data of Mrk 501 (host frame timescale of∼390 days, >3σ). No correlation at any time lag was found between the optical/UV and X-ray bands in either source.

Speaker: Gabrielle Louise Taylor (Landessternwarte, Universität Heidelberg)

GammaConference.pdf

51 Optical and Gamma-Ray Signatures of Blazar Flares: Building the Bridge between Rubin and CTAO

Blazars are among the most violent non-thermal sources in the universe, exhibiting dramatic variability across the electromagnetic spectrum. Yet, a fundamental question remains unanswered: how tightly coupled are their optical and gamma-ray emissions, and what does this tell us about the physical engines powering these objects?

In this talk, I will present a multi-year study of optical and gamma-ray correlations in blazars using data from Fermi-LAT and the ZTF optical survey. By developing a similarity metric to quantify cross-band correlations and a robust flare detection algorithm, we find that ~20% of blazars in our sample show >3σ correlation hints with remarkably short or absent time lags, a possible evidence for co-spatial emission. Moreover, we examine the possibility of distinguishing between emission processes from the optical-gamma-ray correlation signature.

We implement a real-time flare detection algorithm that achieves 72% purity in identifying gamma-ray flares and 98% in detecting optical low states, when compared with the gamma-ray high states detected following the former algorithm - demonstrating that optical surveys can serve as high-fidelity triggers for high-energy observations. This capability will be timely with the start of the Rubin Observatory, transmitting alerts through Fink. I will present the implementation of this methodology to detect and trigger observations on high or low blazar states in real time, providing a complement to the triggers from Fermi-LAT and, critically, enabling target-of-opportunity observations with the Cherenkov Telescope Array Observatory (CTAO).

Speaker: Julian Hamo (IJCLab - Université Paris-Saclay)

52 Performance Analysis and Spacing Optimization of SST-1M Telescopes for Candidate Sites

Two 4m-diameter Single-Mirror Small-Size Telescopes (SST-1Ms) - Imaging Atmospheric Cherenkov Telescopes (IACTs) - have been successfully commissioned at the Ondřejov Observatory in the Czech Republic. While these telescopes have successfully detected several Galactic and extragalactic sources within an optimal flux sensitivity range of several TeV to over 100 TeV, local atmospheric conditions significantly limit the annual observing hours. Consequently, relocation is being considered, with two primary candidate sites indetified: Malargüe, Argentina and Hanle, India. This presentation will provide a simulation-based performance analysis of the SST-1M telescopes for these two sites, including an optimization of intra-telescope spacing.

Speaker: Patrik Čechvala

53 Probing CO-dark Gas and Blazar Jet Power with ALMA Calibrators: Toward the Origin of High-Energy Neutrinos

Identifying the origin of TeV-PeV neutrinos requires understanding both how much target material is available for hadronic interactions and where high-energy emission originates in blazar jets. While excess neutrino emission toward the Galactic Center suggests that dense gas environments play an important role, the association of a high-energy neutrino with the blazar TXS 0506+056 indicates that relativistic jets can also accelerate cosmic rays. In both cases, the key quantities governing neutrino production, namely target density and particle acceleration power, remain poorly constrained.

We present a new observational approach based on the ALMA Calibrator Source Catalogue, a large and homogeneous dataset of bright blazars and background continuum sources, which allows us to probe both ingredients in a unified way. First, absorption-line measurements toward ALMA calibrators reveal diffuse molecular gas, including CO-dark components missed by conventional emission-line surveys, and thus provide direct constraints on the target density for cosmic-ray interactions. Second, using a statistically significant sample of ALMA-Fermi matched blazars, we find a strong correlation between millimeter and gamma-ray luminosities, indicating that millimeter emission traces the average jet power and particle acceleration efficiency. Taken together, these results demonstrate that ALMA calibrator observations provide a unique framework for linking target environments and jet energetics in the study of gamma rays and high-energy neutrinos.

Speaker: Kanako Narita (The Univerity of Tokyo)

54 Probing the Inner Engine of GRB 240825A with Its Broadband Spectral and Temporal Properties

GRB 240825A exhibits an unprecedented combination of temporal and spectral features rarely seen together in a single burst: a three-component prompt spectrum (a quasi-thermal photosphere, a Band-like non-thermal component, and a hard MeV tail), a well-defined afterglow, and a 6.37 Hz quasi-periodic oscillation (QPO) detected in the [2.07 s$-$3.25 s] interval. Using the latest multiwavelength analyses, we re-evaluate central-engine interpretations. We present a detailed comparison between (i) a newborn magnetar engine (spin-down + fallback, magnetically-driven jet, NS precession modes) and (ii) a Kerr black hole engine (fallback accretion + Blandford–Znajek, Lense–Thirring disk precession). We show both engines can, in principle, explain the QPO frequency, but they impose different constraints on energetics, jet composition, and multi-messenger signatures. We propose a prioritized observational test list and identify measurements from the existing studies that most strongly favor a magnetar interpretation while noting where ambiguity remains.

Speaker: DABAN SAEED (Institute of High Energy Physics)

55 Probing the Parameter Space of Axion-Like Particles Using Simulation-Based Inference

Axion-like particles (ALPs) appear in various extensions of the Standard Model and can interact with photons, leading to ALP-photon conversions in external magnetic fields. This phenomenon can introduce characteristic energy-dependent “wiggles” in gamma-ray spectra. The Cherenkov Telescope Array Observatory (CTAO) is the next-generation ground-based gamma-ray observatory, designed to provide enhanced sensitivity and energy coverage (20 GeV – 300 TeV) over current Imaging Atmospheric Cherenkov Telescopes (IACTs) and offers an excellent opportunity to study such effects.

In this work, we employ Simulation-Based Inference (SBI) to explore the parameter space of ALPs, targeting the flaring states of blazars, which are among the brightest gamma-ray sources and ideal candidates for probing ALP-induced spectral modulations. Additionally, we investigate whether this inference method can reproduce ALP exclusion limits comparable to those reported in previous studies using the classical likelihood-ratio approach. This study, therefore, provides an assessment of SBI as a tool for constraining ALP–photon interactions.

Speaker: Pooja Bhattacharjee (University of Nova Gorica)

56 Probing ultra-heavy dark matter with PANOSETI: prospects for dwarf spheroidal searches

Ultra-heavy dark matter (UHDM), with masses above 100 TeV, remains an under-explored region of parameter space in indirect detection searches. Self-annihilation or decay of these particles may produce very-high-energy gamma rays, motivating observations with instruments optimized at the highest energies. The Dark100 project will consist of an array of 7 telescopes, out of which 4 are currently deployed, using the Panoramic Search for Extraterrestrial Intelligence (PANOSETI) telescope system. It will operate as an imaging atmospheric Cherenkov telescope array, designed to probe gamma rays from tens of TeV up to the PeV scale.

We aim to evaluate the sensitivity of the Dark100 array to gamma-ray signals from UHDM annihilation in dwarf spheroidal galaxies, which are among the most promising targets due to their high dark matter content and low astrophysical backgrounds. The expected gamma-ray flux from dark matter annihilation is modeled using benchmark particle physics scenarios and astrophysical J-factors adopted from the literature for selected dwarf spheroidal galaxies. The Dark100 array’s instrument response is derived from air-shower simulations and a preliminary instrument model.

We present the first estimates of the Dark100 array’s sensitivity to dark matter annihilation signals in the ultra-heavy mass regime. We also discuss observational strategies for optimizing dwarf spheroidal galaxy observations with the array.

Speaker: Sruthiranjani Ravikularaman (Astronomisches Institut, RUB)

57 Prospects for high‑energy neutrinos from pulsating ultra‑luminous X‑ray sources

This study explores neutrino emission from neutron stars in binary systems, specifically pulsating ultra‑luminous X‑ray sources (PULXs), to clarify their role as particle accelerators. We calculate the expected neutrino flux from these sources and evaluate the detection potential with current and future instruments, including IceCube, KM3NeT, and TRIDENT. Our analysis compares a magnetospheric acceleration model with jet- or outflow-driven scenarios, to determine whether Galactic transient episodes can produce neutrino fluxes that are sufficiently high, over brief periods, to be observed. Furthermore, we establish an upper-limit on the neutrino flux from the PULX Swift J0243.6+6124 using data from the IceCube Observatory. These results provide a quantitative basis for future searches for transient neutrino emission from pulsating ULXs.

Speaker: Lorenzo Ducci (University of Tuebingen)

58 Revealing the population of Galactic Pulsar wind Nebulae

Pulsar wind nebulae (PWNe) are formed when a rotating neutron star powers a relativistic outflow that injects magnetised plasma into its surrounding supernova remnant. As the largest class of Galactic TeV sources, PWNe can provide key insights into particle acceleration processes in the Milky Way. In this work, we present a population synthesis of Galactic PWNe that describes both the spatial distribution of the sources in the Galaxy as well as the dynamical and spectral evolution of the individual sources.
We combine Monte Carlo population synthesis with time-dependent modelling of individual PWNe. Each simulated population contains approximately 1000 PWNe, with each source evolved from birth to a maximum age of 100 kyr. The spatial distribution follows a spiral arm structure. Properties of the progenitor supernova remnant, initial pulsar spin period, the magnetic field strength at the neutron star pole, ambient medium density, electron injection parameters and magnetic fraction, are drawn from observationally or theoretically motivated distributions. For each parameter set, we generate 100 independent realisations and compare the resulting synthetic populations with the H.E.S.S. Galactic Plane Survey (HGPS), in order to find parameters that are in best agreement with the HGPS limits. From the best-matching models, we can derive estimates of the unresolved Galactic PWN contribution to the diffuse gamma-ray emission in the Galaxy. The resulting framework may also provide predictions for the properties and detectability of PWNe in current and next-generation gamma ray surveys.

Speaker: Mohadeseh Ozlati Moghadam (University of Potsdam)

59 Sapphire++: a novel particle transport code

We present Sapphire++, an open-source code to simulate astrophysical plasmas and particles with highly relativistic energies written in C++.
Its distinguishing features are the inclusion of higher-order anisotropies in the particle distribution function $f(t, \mathbf{x}, \mathbf{p})$ and the application of the discontinuous Galerkin method to solve the underlying partial differential equations that model the evolution of $f$. To account for the higher-order anisotropies, a spherical harmonic expansion of the distribution function is used. [1, 2]

These techniques allowed us to discover new features in the particles’ energy spectrum produced by Fermi acceleration at oblique shocks. If the scattering frequency is not inversely proportional to the particle energy (Bohm scaling), the downstream spectrum may be curved. Furthermore, if the scattering frequency changes in the vicinity of the shock wave, there can be breaks in the spectrum. [3]

A recent addition to Sapphire++ offers the option to include the effect of radiation reaction forces in its computation of $f$. We show preliminary results exemplifying how radiation reaction forces modulate the particles’ energy spectrum at oblique shocks.

Future gamma-ray observations with the new generation of Imaging Atmospheric Cherenkov Telescopes may have the range and the resolution to detect the computed spectral features.

References:

[1] N. W. Schween and B. Reville. “Using spherical harmonics to solve the
Boltzmann equation: an operator-based approach”. In: Monthly Notices of
the Royal Astronomical Society (Feb. 2024).

[2] N. W. Schween, F. Schulze and B. Reville. “Sapphire++: A particle transport code combining a spherical harmonic expansion and the discontinuous Galerkin method”. In: Journal of Computational Physics 523 (15th Feb. 2025), p. 113690.

[3] A. Shirin T et al. “Spectral curvature and breaks from Fermi acceleration at oblique shocks”. In: Monthly Notices of the Royal Astronomical Society
(Nov. 2025).

Speaker: Nils Schween (MHKP)

60 Searches for Gamma-Ray Counterparts of a sample of IceCube Neutrino Events in the AGILE Public Data Archive

The search for gamma ray counterparts of IceCube neutrino events is a key item to understand the role of blazars as possible sources of cosmic high energy neutrinos. We have searched the counterparts of a selection of IceCube EHE, HESE GOLD neutrinos events in the AGILE gamma-ray satellite public data archive.
We report the candidate gamma-ray counterparts within the error regions centered on the detected neutrinos and report their multi-frequency light curves and Spectral Energy Distributions, together with the estimates of the gamma ray flux above 100 MeV for the AGILE detections. The possible association with different class of blazars is discussed.

Speaker: Prof. Rosa Poggiani (University of Pisa, Italy)

61 Short-Timescale GeV-Optical Variability for Flat Spectrum Radio Quasars

Using AGILE and Fermi data, we have produced gamma-ray lightcurves for almost all GeV flaring episodes in Flat Spectrum Radio Quasars (FSRQ) that are bright enough to probe variability down to at least 6 hour timescales. Past work typically focused on a few individual flares in a source, but between AGILE and Fermi, we now have almost 20 years of GeV time coverage available and this extended time coverage usually contains several flares (often 10 or more) per object. We can thus take a wider, ensemble look at gamma-ray flare properties. As we do this, we are finding a wider than expected variety of flaring behavior, even in just one source, implying we may have several distinct source regions and possibly emission mechanisms/geometries operating even in the same source. To complement the gamma-ray data and to help constrain better the underlying emission mechanisms, we have also collected the publicly available optical data for these flare events, of which there is also an increasing amount as wide-area optical variability surveys come online. In the standard external Compton (EC) models for FSRQ emission, the GeV (Compton) and optical (synchrotron) emission are produced by the same electrons and thus should be well-correlated. On short (sub-day) timescales, we are instead finding that this is not always the case.

Speaker: Paolo Coppi (Yale University)

62 Simulating CTAO Observations of the Black Widow Pulsar PSR J1959+2048: Constraints from Fermi-LAT Orbitally-Modulated Emission

Spider pulsars host an intrabinary shock (IBS) resulting from the interaction between the pulsar wind and the ablated material from the companion star, producing non-thermal emission through synchrotron and inverse Compton (IC) processes that make these systems promising targets for the Cherenkov Telescope Array Observatory (CTAO). In this work, we present preliminary results of an ongoing study of PSR J1959+2048, the original black widow pulsar, in which we search for orbitally-modulated gamma-ray emission in Fermi-LAT data to constrain IBS emission model predictions via a best-fit approach.  CTAO observations are simulated under Alpha Configuration IRFs, allowing us to assess the detectability of the source and characterise its expected very-high-energy spectral signature. Our results provide quantitative constraints on the IBS emission properties of PSR J1959+2048 and contribute to building a physically grounded framework for evaluating CTAO's sensitivity to spider pulsar systems.

Speaker: Livia Rocha (IFUSP - Brazil)

63 Study of a candidate pulsar halo in the region of 4HWC J1830-1006 with HAWC

The complex region of 4HWC J1830-1006 is linked to several sources, including HESS J1831-098 and 1LHAASO J1831-1007u, as well as the powerful pulsar PSR J1831-0952, which has a characteristic age of 128k years. This connection suggests a possibility of a candidate pulsar halo system. In this study, we present a spectral and morphological analysis of the complex region surrounding 4HWC J1830-1006, utilizing data from HAWC and other multi-wavelength observations. The association with a LHAASO source, along with the hard spectrum observed in HAWC data, indicates the presence of a potential PeVatron source. Additionally, we discuss possible emission scenarios associated with the powerful pulsar PSR J1831-0952 or with escaping cosmic rays interacting with a nearby dense molecular cloud.

Speaker: Dr Priyadarshini Bangale (Temple University)

PBangale_Gamma2026.pdf

64 Studying the Cosmic Ray Background of Atmospheric Cherenkov Telescopes with Hadron Colliders

Hadron induced cosmic ray showers make up the dominant component of the isotropic background events seen in ground based gamma-ray telescopes. Although it is possible to reject a very large fraction of these events a significant background of gamma-ray like proton showers still remains. Many of these gamma-like events consist of air showers where a large fraction of the initial particle energy is given to a small number of neutral pions in the first interaction, producing an air shower dominated by its electromagnetic component. However, due to the lack of hadronic model tuning for this event class the predicted number of background events in simulations is quite uncertain.

In this work we address the energy and pseudorapidity range of importance for this class of events, showing the that current collider experiments, specifically LHCf and RHICf, are able to probe interactions in this range. We demonstrate that using both current and future data these instruments should be able to probe the accuracy of hadronic model predictions and allow for more accurate tuning of the models.

Speaker: Robert Parsons (Humboldt-Universität zu Berlin)

65 The diffuse gamma-ray emission of low-energy neutron stars called electrospheres

Electrospheres are the environments of magnetized and rapidly rotating neutron stars. With their central star less powerful than the one of a pulsar, they do not produce electron-positron pairs with a high multiplicity. Curvature radiation by primary particles extracted from the surface of the neutron star is expected to play an important role. In a previous study, we have adapted the code Pulsar ARoMa (Mottez, 2024), computing self-consistent stationary solutions of the plasma dynamics of low-density electrospheres, to show that electrospheres have characteristic spectra slightly different than pulsars (Francez et al., 2026, in prep.). Their spectra peak above 10 GeV and can reach the TeV. The low luminosity of electrospheres, around 1029 erg/s, makes them hard to detect alone, but their population is much larger than the Galactic pulsar population. Qualitative estimates show that the total emission of electrospheres can reach, in the 0.1-100 GeV FERMI-LAT band, a luminosity of the same order of magnitude than the high-energy tail of the excess of diffuse gamma-ray emission in the Galactic Centre. Among the robust candidates to source the diffuse gamma-ray emission in the Galactic Centre at the GeV, is a population of millisecond pulsars, born after the recycling of “dead” neutron stars, i.e. electrospheres. In this work, we derive observables with simulations from Pulsar ARoMA and the Python package Gammapy to study the diffuse gamma-ray emission of an extended population of electrospheres, and we compare them to the high-energy tail of the Galactic Centre excess.

Speaker: Théo Francez (LUX - Observatoire de Paris - PSL)

66 The Intrabinary Shock in Spider Systems and the pulsar mass determination tension

Spider Systems are compact binary systems composed of a millisecond pulsar and a low-mass companion star. They are known for their cannibalistic nature, in which the pulsar emits relativistic winds that evaporate the companion star, ultimately leading to the observed isolated millisecond pulsars. One way to study how this wind is emitted and interacts with the companion star is through observations of the intrabinary shock -- the region where the incoming donor mass and the pulsar wind collide -- which is visible in X-rays and gamma rays. In this work, we review observations of the intrabinary shock in different Spider Systems and derive various properties from these observations, including the neutron star mass. We find a problematic discrepancy in the neutron star mass estimates derived from gamma-ray observations when compared to those obtained from optical counterparts. We further evaluate the possible causes for this discrepancy [this part is currently in preparation].

Speaker: Nathalia Pires (USP)

67 The KRA gamma model for the gamma-ray and neutrino diffuse emissions of the Galaxy up to the PeV

The KRA gamma model — based on the DRAGON and HERMES codes — has successfully been used by the IceCube collaboration as a template for its discovery of the neutrino diffuse emission of the Galaxy.
Remarkably, an updated version of the same model (KRA gamma Min) have been also found to correctly predict the gamma-ray counterpart of that emission as measured by LHAASO in the inner and outer Galactic Plane.
These findings favour the CR sea as the main origin of the gamma-ray and neutrino diffuse emissions as well as
a spatial dependent galactic CR spectra tracing CALET, DAMPE and KASCADE measurements at our local position.
In this contribution we will compare that scenario also with HAWC and other gamma-ray experimental results. Recent new analysis of IceCube and ANTARES data will be also discussed in that framework.

In this contribution we will compare that scenario also with HAWC and other gamma-ray experimental results. Recent new analysis of IceCube and ANTARES data will be also discussed in that framework.

Speaker: Dario Grasso (INFN)

68 The possible contribution of microquasars to the cosmic ray proton "knee"

LHAASO observations of SS433 and Cygnus X-3, well-known microquasars, have suggested that microquasars are likely galactic PeVatrons. We study the potential contribution of microquasars to the knee region of the Cosmic Ray (CR) spectrum, accounting for the stochastic nature of CR source ages and positions. We found that, in combination with SNRs with a maximum energy of $\approx$ 50 TeV, microquasars can reproduce the measured flux. We investigated a case in which large-scale magnetic turbulence does not cascade to smaller scales, resulting in energy-independent diffusion. We reproduced the flux with minor adjustments to the injected spectrum. The maximum energies of SNR in this case are quite constrained, as they would overproduce the fluxes at $\approx$ 300 TeV if they were to vary by a factor of 2. We found the dipole moment of CRs to be sensitive to the sources' positions and ages in each realisation. It is also larger (close to 10 times) than observed, not just due to the contribution from the youngest and nearest source, but also the large-scale non-uniform distribution of sources.

Speaker: chun khai loo (GSSI)

69 The Second H.E.S.S. GRB Catalogue: Results and Future Prospects

Detections of gamma-ray bursts (GRBs) in the very-high-energy regime (VHE; E > 100 GeV) have provided insights into the physical processes at the origin of their emission. Such detections were only possible thanks to dedicated strategies under the challenging conditions faced by ground-based instruments, requiring rapid follow-up observations triggered by external alerts and often resulting in non-detections. In this context, the High Energy Spectroscopic System (H.E.S.S.) has developed a comprehensive follow-up program resulting in the breakthrough VHE detections of GRB 180720B and GRB 190829A.

In this contribution, we present the results from the second H.E.S.S. GRB catalogue, based on a re-analysis of 15 years of observations from 2004 to late 2019. This catalogue constitutes the most extensive collection of VHE upper limits on GRBs to-date and provides constraints on their VHE emission. Comparisons with VHE-detected GRBs show that these events are consistent with being bright bursts at relatively low redshift, rather than representing a distinct population. This conclusion supports continued follow-up efforts and highlights the potential of next-generation facilities. Motivated by these results, we also present the current status of the H.E.S.S. GRB follow-up program, including recent developments in rapid-response capabilities, real-time alert handling, and observation scheduling. A particular focus is placed on the follow-up strategy and public communication of observation schedules in order to ensure coordination with the multi-messenger community.

Speaker: Bernardo CORNEJO (CEA - IRFU/DPhP)

70 The trigger system for the Small Sized Telescopes camera of CTAO

The Cherenkov Telescope Array Observatory (CTAO) will explore the very high energy gamma-ray sky with unprecedented sensitivity. The Small-Sized Telescopes (SSTs), focusing on the 2-300 TeV energy range, are equipped with dual-mirror Schwarzschild–Couder optics and highly pixelated cameras based on silicon photomultipliers (SiPMs).

The SST camera consists of 2048 SiPMs arranged in 32 modules, with each module comprising 64 pixels and equipped with dedicated readout electronics based on the TARGET technology, which allows the separation of signal digitization and first level triggering.
Indeed, these two processes are implemented into different Application Specific Integrated Circuits (ASICs): the latter is managed by the CT5TEA TARGET ASIC, which compares the analog sum of four adjacent pixels to a set trigger threshold. A camera-level trigger is generated by the backplane electronics when a coincidence between two neighbouring trigger patches is found.

The trigger performance was investigated in the laboratory by measuring the trigger rate under different settings and background light conditions. These measurements enabled the determination of the optimal trigger threshold for different Night Sky Background (NSB) conditions, with a conservative value of 40 mV covering rates up to 100 MHz p.e..

Measurements of the trigger rate as a function of increasing illumination were used to estimate the characteristic signal amplitude corresponding to 50% trigger efficiency. A value of about 14 p.e. was obtained for a trigger threshold set to 40 mV in dark conditions.

On-sky measurements at different zenith angles confirmed the expected behaviour of IACT systems, with the trigger rate following the characteristic cos(θ) dependence.

This contribution presents the design and implementation of the SST camera trigger system, together with the measurement methods used to study and characterize its performance.

Speaker: Isabella Sofia (INFN Torino, MPIK)

71 Three-dimensional calculation for nuclear de-excitation gamma-ray emissions driven by Galactic low-energy cosmic rays

Galactic cosmic rays are widely believed to originate from shock acceleration in supernova remnants; however, observations of ionization rates in the interstellar medium (ISM) in the inner Galaxy suggest the presence of additional, potentially unknown, acceleration sources. Low-energy cosmic rays (LECRs) excite nuclei in the ISM, producing MeV gamma-ray line emission through subsequent de-excitation, and observations of this gamma-ray emission enable robust constraints on the LECR flux and its spatial distribution.
We model the MeV gamma-ray spatial distribution using cosmic-ray spectra predicted by GALPROP, considering both a standard case and an LECR-enhanced case, the latter being inferred from ionization rate measurements. This allows us to evaluate its detectability relative to non-thermal leptonic emission components and to identify the locations where it can be detected by MeV gamma-ray missions. Furthermore, based on differences in nuclear reaction thresholds, we discuss constraints on the LECR spectral parameters using line-intensity ratios, such as the C-12 4.4 MeV to O-16 6.1 MeV ratio.

Speaker: Dr Tomohiko Oka (University of Würzburg)

72 Time-resolved & time-integrated spectroscopic study of Ultra-long GRB 220627A

Gamma-Ray Bursts (GRBs) produce ultra-relativistic, highly collimated jets, which offer valuable insights into particle acceleration and high-energy emission mechanisms. In this work, we present a detailed variability study, time-resolved and time-integrated spectroscopic study of the ultra-long GRB 220627A using observations from the Fermi Gamma-ray Space Telescope. The analysis covers a time interval exceeding 1200 s and focuses on two prompt emission episodes detected by the Fermi Gamma-ray Burst Monitor, separated by more than 600 s. The unusual temporal structure of GRB 220627A makes it an important candidate for investigating particle emission processes, small-scale variability, and the nature of the central engine powering the burst. To examine the possibility of gravitational lensing, correlations were explored between the time bins of the first and second emission episodes. A consistent relationship between flux and photon spectral distribution was observed across both episodes. The spectra were modelled using an exponentially cut-off power-law function, providing a satisfactory description of the emission in each episode. However, the ratio of MeV-to-GeV photons detected by the Fermi Large Area Telescope differs between the two episodes. In addition, high-energy gamma-ray emission detected by LAT persists only up to approximately 700 s. These results disfavour a gravitational lensing interpretation and instead suggest that the source represents an ultra-long burst exhibiting multiple prompt emission episodes. Time-integrated study supports the progenitor model involving the collapse of a blue supergiant star; however, the time-resolved study aligns towards a rapidly rotating newborn magnetar, providing further support for the classification of GRB 220627A as an ultra-long GRB.

Speaker: Mr Ayush Garg (Indian Institute of Technology, Indore)

73 Tracing the low activity in Mrk421 during the observations in 2015-2016

he Major Atmospheric Gamma Imaging Cherenkov (MAGIC) Telescopes, together with multiwavelength instruments, have conducted several observational campaigns to study Markarian 421. Thanks to its brightness and proximity, the emission properties of Mrk421 can be characterized on short timescales even during quiescent flux periods, which is not possible for fainter and farther TeV blazars. During the multiwavelength campaign in 2015–2016, exceptionally low flux states were observed in both the X-ray and TeV bands. This rich dataset provides a unique opportunity to investigate the baseline (or persistent) emission as opposed to the well-studied flaring states.

We study low-activity periods to constrain the physical parameters of the emission region that govern the baseline jet behavior. This allows us to investigate the underlying flow dynamics and the key processes shaping the jet emission, providing insight into the origin of variability in this active blazar and the factors driving its evolution. We further examine the relationships between observables, such as peak flux and peak position of synchrotron (a few keV) and SSC (~100 GeV), and emission-region parameters to gain insight into particle acceleration processes and the nature of the observed variability. These results indicate that a single dominant parameter does not govern the baseline emission, but instead reflects a regulated behavior arising from the interplay of multiple processes, including geometric effects and ongoing particle acceleration. This suggests that the observed emission may represent an effective average over inherently non-steady processes within the jet. The key results from this study will be presented at the conference.

Speaker: Chitranshi Bakshi (Saha Institute of Nuclear Physics)

74 Turbulence in Molecular Clouds

The bulk mass of gas in the interstellar medium (ISM) is located in so called Molecular Clouds (MCs), which are dense and cold environments known as nurseries of stars. Due to highly energetic Cosmic Rays (CRs) these dense gases are still weakly ionized even deep into their centers. We investigate the effect of this partial ionization by means of 3D two-fluid MHD turbulence simulations in which we model the neutral and ionized gas coupled by a collisional drag term explicitly. As the coupling is collisional we expect, on scales smaller than their collision frequencies, the gases to increasingly decouple while on larger scales the gases to move in unison. This has direct impacts on linear MHD waves and consequently on the turbulent cascade in these systems. We investigate the impact of the decoupling on the energy transfer over scales in the turbulent cascade, attempt to generalize the characteristics of turbulence in these weakly ionized environments and present the implications for star formation and CR transport.

Speaker: Christian Heppe (IPP)

75 Waiting for the Flare: Detecting GeV Sub-Flares to Understand PSR B1259−63

PSR B1259−63 is a gamma-ray binary system consisting of a 48 ms radio pulsar in a highly eccentric 3.4-year orbit around the massive O9.5Ve star LS 2883. The interaction between the highly relativistic pulsar wind and the outflow from the massive star generates complex multi-wavelength emissions, including radio, X-ray, and gamma-ray radiation. One of the most puzzling features of this system is a post-periastron GeV flare, which shows no obvious counterparts at any other wavelength.

Detailed analysis and modelling of the previous periastron passages indicate that this flare consists of several short (about 15 minutes long) very bright sub-flares. These events can be interpreted as bremsstrahlung radiation produced by the interaction of the weakly shocked pulsar wind outflow with the clumps of the massive star wind.

In this presentation, I will provide a detailed report on our systematic reanalysis of all available Fermi-LAT data over the last five periastron passages. Detecting gamma-ray flares is genuinely challenging. PSR B1259−63 is a faint source. Isolating individual sub-flares from weak, variable emission against the diffuse gamma-ray background requires careful statistical treatment and tailored analysis strategies. I will discuss how we addressed these difficulties and the limitations that remain. The central result of this analysis is that the distribution of sub-flare durations across different periastron passages directly constrains the size and density of the Be-star's disk.

Speaker: Iuliia Shebalkova (Dublin City University)

76 A Microquasar Candidate Powering 1LHAASO J2108+5153u

The unidentified ultra-high-energy source 1LHAASO J2108+5153u is a compelling PeVatron candidate, yet its nature remains unclear due to the lack of a confirmed counterpart at other wavelengths. Recent uGMRT observations revealing a double-jet radio structure within its positional uncertainty suggest a new physical interpretation. In this work, we investigate the possibility that this structure is associated with a Galactic microquasar capable of accelerating particles up to PeV energies. Under this assumption, we consider that particles are accelerated in the jet and escape into the interstellar medium, where they interact and produce the observed gamma-ray emission. We examine both leptonic and hadronic interpretations for the TeV-PeV range. We find that the microquasar scenario is energetically viable and can reproduce the observed emission using parameters consistent with those typically adopted for gamma-ray-emitting microquasars, supporting microquasars as potential Galactic PeVatron candidates. Future observations at radio, X-ray, and gamma-ray energies will be crucial to test this interpretation and confirm the nature of the uGMRT source.

Speaker: Ana Laura Müller (FZU - Institute of Physics of the Czech Academy of Sciences)

77 A novel catalog of hard gamma-ray sources observed by Fermi-LAT above 10 GeV

The Fermi Large Area Telescope (LAT) continuously surveys the gamma-ray sky since its launch in 2008. Through the years, the FHL catalog series has been produced, using the LAT data at the highest energies to characterize hard gamma-ray sources. The last published installment in this series is 3FHL, based on 7.5 years of data above 10 GeV and reporting the detection of 1556 hard gamma-ray sources.

In this contribution we present a novel catalog of hard Fermi-LAT sources, which will provide a comprehensive survey of the sky above 10 GeV using 16 years of LAT Pass8 data, corresponding to an improvement in statistics and sensitivity by a factor of 2, with respect to 3FHL. This catalog reports the detection and characterization of more than 2600 gamma-ray sources, nearly 100 of which are extended. Each source has been characterized in the spectral, spatial and temporal domain, thus finding that almost 500 sources show signs of significant temporal variability. The largest part of the detected sources is of extra-galactic origin ($\sim 85\%$), although galactic sources are reported as well. Tens of sources do not appear in any previous LAT catalog, making them particularly compelling targets for future studies. Moreover, the fraction of sources lacking a multi-wavelength counterpart is $\sim 15\%$ of the total, significantly less than the fraction reported in the latest full-band Fermi-LAT source list (FL16Y, with an unassociated fraction $\sim 30\%$).

This catalog will provide a fundamental reference for present and future ground-based facilities active above tens of GeV, such as the Cherenkov Telescope Array Observatory (CTAO) and many others. It will also be a key resource for the design of future multi-wavelength and multi-messenger studies.

Speaker: Pietro Monti-Guarnieri (University of Trieste & INFN Trieste)

gamma2026_poster_5FHL_v3.pdf

78 Data-driven improvement of HE to VHE extrapolation schemes of AGN spectra

Over the past three decades, more than seventy blazars have been
detected at very high energies (VHE; E > 100 GeV) by the current
generation of imaging atmospheric Cherenkov telescopes (IACTs). Since
these instruments require precise pointing for observations, potential new
sources must be identified in advance of detection. A common method for
this identification is the extrapolation of spectra from sources observed at
high energies (HE; E > 100 MeV) by the large area telescope of the Fermi
satellite. However, the assumptions underlying these extrapolation
schemes must be carefully evaluated to yield reliable predictions. In this
study, we compare different extrapolation frameworks used to derive VHE
spectra starting from HE observations. We evaluate these frameworks
using long-term archival observations from H.E.S.S., MAGIC and VERITAS
gathered in STeVECat, the largest collection of VHE extragalactic spectra
from IACTs to date. Finally, we propose a reliable and robust extrapolation
scheme, aimed at improving source identification in the VHE regime.

Speaker: Lucas Gréaux (AIRUB)

gamma26_poster_greaux_extrapolation.pdf

79 Divergent Pointing with CTAO: Performance and Science Prospects

The next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), currently the most sensitive ground-based instruments for very-high-energy gamma-ray astronomy, will be represented by the Cherenkov Telescope Array Observatory (CTAO). With two arrays located in the Northern and Southern hemispheres, CTAO will provide full-sky coverage with unprecedented sensitivity over a broad energy range, from about 20 GeV up to 300 TeV.
The standard observation mode for IACTs is the parallel pointing configuration, in which all telescopes are aligned toward the same sky region. This approach maximizes event reconstruction quality, enabling high angular resolution with an accurate background rejection. Thanks to the large number of telescopes deployed at both sites, CTAO will enable the exploration of complementary pointing strategies. Among these, the divergent pointing mode consists of individual telescopes directed slightly offset from a common central direction, effectively increasing the instantaneous field of view (FOV). This configuration is well-suited for wide-area surveys and the follow-up of poorly localized transient events, including gravitational-wave events, gamma-ray bursts (GRBs), and high-energy neutrino alerts. This gain in FOV, however, comes at the cost of reduced on-axis performance, with degraded sensitivity, angular resolution, and background rejection compared to the standard parallel configuration.
In this work, we present a performance study of the divergent pointing mode based on dedicated Monte Carlo simulations for both CTAO arrays. We assess performance with the increased field of view, including sensitivity, angular and energy resolution, and discuss its potential role in future CTAO observing strategies. Our preliminary results indicate that the off-axis sensitivity of the divergent pointing configuration can surpass that of the parallel configuration, and its effective area is enhanced at low energies for large offset angles, while the on-axis performance remains better in the parallel pointing mode, thereby reinforcing the suitability of this approach for wide-field and transient-driven observations.

Speaker: Jahanvi Jahanvi (University of Udine-INFN Trieste)

80 Gamma-ray signatures from anisotropic cosmic ray transport in the Milky Way

The magnetic field of the Milky Way is composed of a large-scale coherent background and a turbulent component. The spatial diffusion of cosmic rays (CRs) in this composed magnetic field configuration is expected to become anisotropic. The diffusion coefficient along the field line $\kappa_\parallel$ is greater than in the degenerated perpendicular directions $\kappa_\perp$.

In this work, we utilize the open-source framework CRPropa to investigate the anisotropic diffusion of CRs within the Milky Way. We phenomenologically vary the ratio $\epsilon = \kappa_\perp / \kappa_\parallel$, ranging from nearly parallel diffusion ($\epsilon = 10^{-3}$) to purely isotropic diffusion ($\epsilon = 1$). The source parameters are optimized to match the observed CR data at Earth. Afterwards, the line-of-sight integration framework HERMES is used to calculate the all-sky gamma-ray emission. Based on the resulting all-sky maps, the feasibility of gamma-ray observations to constrain not only the parameters of such anisotropic transport but also the structure of the Galactic magnetic field will be discussed.

Speaker: Julien Dörner (Ruhr University Bochum)

81 The broadband spectral energy distributions of candidate neutrino blazars

There is mounting evidence that blazars are potential sources of high energy neutrinos. Such candidate neutrino blazars are ideal targets to investigate the high energy emission processes and to understand their role as neutrino sources. We report results on four candidate neutrino blazars, PKS 0446+112, TXS 0506+056, PKS 1424−418 and PKS 1502+106. We carried out a gamma-ray spectral and timing analysis on three time periods that comprise a quiescent epoch, an epoch that corresponds to neutrino detection and a flaring epoch. We also carried out modeling of the broadband pectral energy distribution (SED) on those three epochs. We found that the gamma-ray spectra of the BL Lac TXS 0506+056 can be adequately described by a power-law, while the spectra of the other three FSRQs require a log-parabola model. The shortest doubling/halving timescales in their gamma-ray variability are 4.70 hrs, 9.24 hrs, 30.76 hrs and 15.42 hrs for PKS 0446+112, TXS 0506+056, PKS 1424−418 and PKS 1502+106, respectively. The SEDs of most of the epochs for the sources are well explained by a leptonic scenario. However, the quiescent epoch of PKS 1502+106 and the neutrino-emission epoch of PKS 0446+112 required an additional hadronic component to reproduce the observed SEDs. Our analysis reveals a complex interplay of leptonic and hadronic processes. While certain neutrino-associated epochs are well reproduced with a leptonic model, others necessitate a hadronic component to explain the SEDs.

Speaker: Markus Boettcher (North-West University)

82 Spectral variations between different CR species caused by abundancy gradients in the CSM of massive stars

It has been a long-standing paradigm that the bulk of Galactic comic-ray gets accelerated at supernova remnant (SNR) shocks via diffusive shock acceleration. As observations – both direct and indirect – became better, many features appeared in the cosmic-ray spectra at Earth and in the emission spectra of remnants, that require deviations from the expectation of a simple power-law of accelerated particles with a power-law index of s=-2.
Here, we demonstrate how the presence of a abundancy-gradients in the circumstellar medium (CSM) around core-collapse SNR can cause slight differences in spectral indices of different CR-species. We use RATPaC to simultaneously model the acceleration of various CR species at an SNR shock expanding in a CSM bubble that shows abundancy gradients.
The burning of hydrogen over time leads to an increasing hydrogen-fraction from the vicinity of the star towards the edge of the wind-bubble, while the helium fraction is usually decreasing with distance. Such a situation causes a softening of the hydrogen spectrum with respect to helium - a feature observed for CRs measured at Earth. The relative spectral difference is dependent on the strength of the abundancy gradient and the acceleration time of CRs and provides an alternative channel for constraining the turbulence conditions at SNR shocks.

Speaker: Robert Brose (Universität Potsdam)

Tuesday 1 September

Plenary: IV

Convener: Martin Pohl (University of Potsdam, Deutsches Elektronen-Synchrotron DESY)

83 Cosmic Rays from AGN Jets & Lobes

Speaker: Andrew Taylor (DESY)

GAMMA26_AT.pdf

GAMMA26_AT.pptx

84 Particle acceleration and high-energy emission in blazars

Speaker: Fabrizio Tavecchio (GNOI)

Tavecchio.pdf

85 Particle Acceleration in Astrophysics

Speaker: Brian Reville (MPIK)

Gamma26_talk.pdf

10:30 Coffee break

Plenary: V

Convener: Lars Mohrmann (MPIK)

86 Origin and Evolution of Galactic Magnetic Fields

Magnetic fields play a crucial role in high-energy gamma-ray astronomy, as they regulate the acceleration, confinement, cooling, and emission of the relativistic particles that produce gamma rays. In this talk, I will give an overview of the origin and evolution of magnetic fields in galaxies, with a particular focus on dynamo processes. Galactic dynamos convert kinetic energy from turbulent motions into magnetic energy through random stretching, twisting, and folding of magnetic field lines. I will discuss both small-scale and large-scale dynamos and highlight recent insights from analytical theory and numerical simulations. The talk will connect these theoretical developments to the broader question of how magnetic fields shape the non-thermal Universe.

Speaker: Prof. Jennifer Schober (University Bonn)

Schober_gamma26.pdf

87 Tracing Cosmic-Ray Origin with the Interstellar Gas and Gamma Rays

Under the hadronic scenario, gamma rays are produced when cosmic rays interact with surrounding interstellar gas (neutral and/or ionised). The interstellar gas thus plays a critical role in assessing the likelihood of a hadronic production origin for any gamma-ray emission and in revealing the underlying particle accelerator. This is all done in comparison to gamma-ray emission resulting from accelerated electrons (via leptonic processes).

In this talk, I will review the current status of our knowledge in observationally and theoretical linking the interstellar gas with various GeV to PeV gamma-ray sources with their potential particle accelerators. I will then look towards the future potential to be exploited with various facilities such as the CTAO, LACT, ASTRI mini-array, LHAASO and SWGO.

Speaker: Gavin Rowell (Adelaide University)

gpr-radio-gamma.pdf

88 The Physics of the Fermi Bubbles

The last decade and a half of studying the diffuse gamma-ray emission from our Galaxy using the Fermi Telescope has advanced our understanding of the high-energy processes in galactic winds, particularly the Fermi Bubbles. Observations in X-ray, radio, and UV bands have complemented the gamma-ray observations. Together, they represent a complex view of the energetic processes at the Galactic Center. Several theoretical models have been proposed to explain these features, but with no consensus about their origin. In this talk, I will review the physics of these bubbles as far as we understand it. I will also discuss some future directions to solidify our understanding of these bubbles.

Speaker: Kartick Sarkar

12:30 Lunch

Parallel A: Theory I

Convener: Laura Olivera-Nieto

89 Microquasar remnants as sources of very-high-energy gamma rays

Microquasars are X-ray binaries with relativistic jets that transport matter and energy from the central engine into the interstellar medium. These jets can inflate large cavities filled with relativistic particles accelerated in interaction regions along the flow. As the system evolves, these cavities develop into extended cocoons analogous to those observed in extragalactic radio galaxies. After the jets switch off, the cocoon continues to evolve over long timescales. High-energy protons confined within these structures gradually diffuse into the surrounding medium, where they illuminate nearby clouds and generate very-high-energy gamma-ray sources lacking clear counterparts. The remnant itself is expected to be faint and difficult to detect due to its low surface brightness. In this presentation, I discuss detailed modeling of these systems and assess their detectability across the electromagnetic spectrum.

Speaker: Gustavo E. Romero (Instituto Argentino de Radioastronomía (IAR))

Microquasar remnants- Romero .pdf

Microquasar remnants- Romero .pdf

90 Gamma-ray emitting binaries with extreme non-thermal phenomena

Extreme non-thermal phenomena shown by accreting and non-accreting systems hosting a massive star and a compact object are linked to the interaction between the stellar and the compact object outflows, and between the latter and the large-scale environment. In this talk, I will describe some important physical processes to keep in mind when trying to understand the high-energy phenomenology of these systems.

Speaker: Valenti Bosch-Ramon (Universitat de Barcelona, ICCUB)

vbosch.pdf

91 Particle acceleration at recollimation shocks in sub-relativistic jets

Growing observational evidence suggests that sub-relativistic astrophysical jets may accelerate particles at slowly evolving standing shocks. In this context, recollimation shocks are expected to arise naturally as jets propagate through dense environments, where the interaction with the ambient medium and the pressure of the surrounding cocoon can mediate their formation while maintaining a quasi-stationary structure. Despite their large inclination relative to the jet axis, these shocks can be sufficiently strong to sustain efficient particle acceleration. In this work, we develop a general theoretical framework to investigate particle acceleration at recollimation shocks, combining an analytic description of jet hydrodynamics extended to the sub-relativistic regime with a semi-analytic treatment of particle acceleration and transport. By solving the spatially dependent transport equation, we derive particle energy distributions and constrain the maximum achievable energies as a function of the system’s physical parameters. Our results highlight the potential importance of recollimation shocks as efficient particle accelerators in sub-relativistic jets. Finally, we discuss applications of this framework in the context of jets in Seyfert galaxies, microquasars, and protostellar systems.

Speaker: Enrico Peretti (INAF-OAA)

Peretti_Gamma_2026.pdf

92 Spatially Dependent Multi-wavelength Modeling of the SS 433 Jets

The microquasar SS 433 drives mildly relativistic jets extending over tens of parsecs, and the recent LHAASO detection of gamma rays above 100 TeV has established it as one of the most compelling Galactic PeVatron candidates. Utilizing deep Chandra observations, we have also resolved the spatial and spectral structure of the nonthermal X-ray knots along the jets, and a subsequent proper-motion analysis has shown that the innermost knots are consistent with standing recollimation shocks, pointing to near-Bohm particle acceleration. Motivated by these advances, we revisit and substantially extend our earlier one-zone leptonic modeling. We present a spatially dependent, one-dimensional cosmic-ray transport model that self-consistently computes the nonthermal radiation along the jet axis from radio to ultra-high-energy gamma rays. Despite adopting a steady jet, the model reproduces a broad range of observations, including the energy-dependent morphology of the emission regions in both the X-ray and gamma-ray bands. We discuss the implications for the required acceleration efficiency and its relation to the Bohm limit, and show that a spatially dependent treatment is essential for understanding extreme Galactic particle accelerators such as SS 433.

Speaker: Yoshiyuki Inoue (Shibaura Institute of Technology)

Gamma2026_SS433_Inoue.pdf

Gamma2026_SS433_Inoue.pptx

93 Origin of orbitally modulated PeV emission from Cyg X-3: evidence for counterjet dominance

We investigate the origin of the orbitally modulated PeV emission from the microquasar Cygnus X-3 recently detected by the LHAASO collaboration. We develop a multi-zone hadronic scenario in which nuclei are accelerated in a compact, strongly magnetized region at the jet base and advect downstream to regions of lower magnetic field, enabling their escape. These relativistic hadrons interact with the intense radiation field and dense stellar wind of the Wolf-Rayet companion, producing gamma-rays via both photo-hadronic (p-$\gamma$) and hadro-nuclear (p-He) processes.

We find that the interaction optical depths across the binary are less than unity, implying that the emission is controlled by column densities along particle trajectories. For the low inclination of the system ($i \sim 26 - 28^{\circ}$), this implies that the counterjet dominates the observed PeV emission, since path lengths through target fields are significantly larger. This geometry naturally explains the observed phase shift of the TeV-PeV light curve relative to superior conjunction, as well as its mirrored symmetry with respect to the GeV emission modulation pattern: while the PeV emission is dominated by the counterjet, the GeV emission, produced in the optically thick regime via inverse Compton scattering, originates in the approaching jet.

Our model reproduces both the spectral energy distribution measured by LHAASO and the full orbital modulation profile of the TeV-PeV emission. The spectrum is dominated by p-$\gamma$ interactions above $\sim$1 PeV and by hadro-nuclear interactions at lower energies. We include a realistic, non-blackbody radiation field shaped by radiative transfer in the stellar wind, as well as $\gamma$-$\gamma$ pair absorption, which significantly affects the spectrum and modulation below $\sim$1 PeV.

Speaker: Anton Dmytriiev (University of the Witwatersrand)

Gamma_2026_talk_A_Dmytriiev_CygX3.pdf

94 Hydrodynamic Evolution and Enhanced Particle Acceleration in the Recurrent Nova T CrB

The outburst of the recurrent nova T Coronae Borealis (T CrB) will provide an opportunity for studying high-energy particle acceleration in dense, evolving environments. Standard diffusive shock acceleration models often assume uniform flow profiles downstream of the shock. However, in compact systems like novae, hydrodynamic evolution creates significant velocity gradients on the length scales involved into the cosmic-ray acceleration process that modify the spectra of particles and their emission. We present a semi-analytical solution for the momentum distribution of particles accelerated at strong, non-relativistic shocks, specifically accounting for the post-shock velocity gradients driven by hydrodynamic evolution. We demonstrate that these structures could increase acceleration efficiency and the maximum particle momentum. We apply this theoretical framework to the 3D HPC numerical simulations of the recurrent nova T Coronae Borealis, modeling the interaction between the fast ejecta and circumbinary medium. We estimated its visibility for gamma-ray and neutrino observatories CTAO, ASTRI, LHASSO, IceCube, KM3NeT. Our results show that the tight coupling between HD structures and particle spectra offers predictive insights into high-energy gamma-ray emission and the associated neutrino signatures, highlighting the important role of non-uniform flow in shaping the multi-zone spectra of interacting aspherical transients.

Speaker: Oleh Petruk (Istituto Nazionale di Astrofisica (INAF) - Palermo)

TCrB_gamma_heidelberg.pptx

Parallel B: Instrumentation/Analysis I

Convener: Richard White (MHKP)

95 Simulating the Southern Wide-field Gamma-ray Observatory

The Southern Wide-field Gamma-ray Observatory (SWGO) is a future gamma-ray observatory to be built in Chile. SWGO will feature several thousand water Cherenkov detector units that cover an area of a square kilometer. To make critical design choices for the detector it is essential to understand their influence on the science capabilities of the instrument. To this end it is necessary to have a sophisticated simulation and reconstruction framework.

This talk will highlight the recent improvements made by the collaboration in the simulation of gamma ray events in SWGO and their subsequent reconstruction. In particular, this includes advances in the realism of the simulation by including effects of background cosmic ray events, properties of the used photomultiplier tubes and characteristics of the readout electronics. Additionally, an overview of the reconstruction chain combining well verified reconstruction techniques as well as cutting edge algorithms will be presented.

Speaker: Johannes Bennemann (Max-Planck-Institut für Kernphysik)

Simulating the Southern Wide-field Gamma-ray Observatory.pdf

96 Gammapy: open source tool for gamma-ray astronomy

Gammapy is an open-source Python framework designed for gamma-ray astronomy data analysis. Built on scientific Python ecosystem and using community-supported open data standards, Gammapy offers a uniform platform for reducing and modeling data from different gamma-ray instruments. It greatly facilitates interoperability between observatories, enabling comprehensive joint analyses.

Initiated in 2014 as a toolbox for TeV analysis, it has evolved into a robust and versatile tool backed by an active and steadily expanding user community, and provides analysis support for major gamma-ray telescopes. Gammapy is routinely used by all Imaging Atmospheric Cherenkov Telescopes (IACTs) and Water Cherenkov Detectors, and now supports joint analysis with Fermi-IACTs.

In this contribution, we provide an overview of the project history, highlighting its significant milestones and achievements as well as its current status. The main concepts and features of the library are presented along with a variety of scientific use cases supported by Gammapy. We highlight the features of the latest stable release and provide future perspectives and planned developments that further enhance the library's functionalities and improve its performance. Gammapy highlights the importance of open-source collaboration in the gamma-ray astronomy community and beyond.

Speaker: Régis Terrier (APC, CNRS)

Terrier gamma 2026.pdf

97 Simulating an imaging atmospheric radio telescope

Currently imaging atmospheric Cherenkov telescopes provide the most precise TeV gamma-ray measurements, but are limited to a duty cycle of about 15\% due to their reliance on clear, moonless nights. Building on this idea, a novel imaging atmospheric radio telescope could combine radio detection with powerful imaging-based reconstruction while enabling observations with a duty cycle close to 100\%.

We present a simulation based on the Huygens–Fresnel principle, which models radio-wave propagation beyond the far-field approximation while reproducing aberrations predicted by geometric optics.

Our simulation enables the formation of radio images from arbitrary electric field inputs. Applied to extensive air showers, simulated with CORSIKA–CoREAS, it produces images that preserve key physical properties such as arrival direction and shower morphology. Notably, gamma- and hadron-induced showers exhibit distinct image structures, closely resembling those known from Cherenkov telescopes.

Furthermore, frequency-dependent imaging provides sensitivity to shower geometry, highlighting the potential of multi-band observations. Our results demonstrate the feasibility of radio-based imaging of air showers and establish a proof of principle for imaging atmospheric radio telescopes under idealized conditions.

We will present our simulation, discuss characteristic image features of air showers, and outline next steps towards incorporating detector effects and realistic noise.

Speaker: Anne Elise Timmermans (Max-Planck-Institut für Kernphysik)

GAMMA_ IART_presentation.pdf

98 Pointing, Calibration, and Optical Astronomy: Differentiable NSB Modeling for IACTs

Imaging Atmospheric Cherenkov Telescopes (IACTs) record gamma-ray events against an irreducible night sky background (NSB) from stars, airglow, zodiacal light, and moonlight. Using this light for calibration, pointing, or optical astronomy is difficult for IACTs because of large pixels, unusual pixel shapes, and the asymmetric PSFs produced by the mirror dish design, which make standard optical astronomy techniques not directly applicable to most IACT types.

We present a differentiable forward model that predicts the pixel-wise NSB, enabling gradient-based recovery of atmospheric and instrumental parameters directly from science camera data. Applied to pointing reconstruction across multiple IACT designs, the method matches or surpasses methods using dedicated calibration hardware without any additional instrumentation or observation time requirements.

Beyond pointing, the framework extends to instrument calibration, atmospheric monitoring and optical astronomy, unlocking the unique aperture size and timing resolution of IACTs for science beyond gamma rays.

Speaker: Gerrit Roellinghoff (ECAP, FAU Erlangen-Nürnberg)

2026_Gamma_Roellinghoff.pdf

2026_Gamma_Roellinghoff.pptx

99 Satellite-based gamma-ray astrophysics with a highly compact, polarization-sensitive detector composed of oriented crystals

High-density and high-Z crystals are a key element of most detectors used to observe High Energy (HE) $\gamma$-rays from space, such as Fermi-LAT. The lattice structure is usually ignored in the instrument design, simulation and calibration, but recent studies have shown that this is a rough approximation, since photons with an energy above few GeV impinging along the axis of an oriented crystals interact differently from the ordinary. Specifically, if the angle between the photon trajectory and the crystal axis is smaller than $\sim 0.1^\circ$, a large enhancement of the pair-production cross-section is observed. The e$^\pm$ bremsstrahlung cross-section is enhanced as well, thus an acceleration of the electromagnetic shower development can be observed. The intensity of these effects grows for energies up to few TeV and then saturates; smaller enhancements can be observed even for incidence angles as large as $1^\circ$.

Notably, for photon energies above a few GeV and incidence angles up to several degrees, the pair-production cross-section exhibits a pronounced dependence on the crystal orientation with respect to the photon polarization vector.

In this contribution we discuss how oriented crystals could be used to develop a novel class of light-weight pointing space-borne $\gamma$-ray telescopes, capable of achieving an improved sensitivity and resolution while coping with the strict requirements in terms of mass and volume of space detectors, thanks to a better shower containment in a smaller volume with respect to non-oriented crystalline detectors. We will also discuss how an oriented tracker-converter system could be used to measure the polarization of a $\gamma$-ray source above few GeV, in a regime that remains unexplorable through any other detection technique. This novel detector concept could open new pathways in the study of the physics of extreme astrophysical environments and potentially improve the detector sensitivity for indirect Dark Matter searches in space.

Speaker: Pietro Monti-Guarnieri (University of Trieste & INFN Trieste)

gamma2026 Monti-Guarnieri.pdf

100 Enhancing Low-Energy Sensitivity in IACT: A Deep Learning Approach with GammaLearn and LST-1

The Cherenkov Telescope Array Observatory (CTAO) represents the next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs), offering a sensitivity improvement of up to a factor of 10 over current instruments. The first Large-Sized Telescope (LST-1) is already operational at La Palma, providing unprecedented low-energy sensitivity (~20 GeV). This capability enables studies of distant extragalactic sources such as Active Galactic Nuclei (AGN). However, at these energies, the combination of low-intensity Cherenkov light and background dominance presents significant challenges, limiting the performance of traditional analysis methods.
Deep learning provides a powerful solution by leveraging the full temporal and charge information of IACT events, improving the reconstruction of key observables - energy, arrival direction, and particle type. In this work, we utilize GammaLearn, a deep learning framework tailored for IACT data analysis that employs a Convolutional Neural Network (CNN) called GammaPhysNet. We present a performance comparison between GammaPhysNet and the standard analysis using real LST-1 observations, focusing on the low-energy sensitivity improvement, which is critical for studying faint and distant sources.

Speaker: Guillaume Grolleron (LAPP (CNRS))

Gamma2026 - Real data analysis with gammaLearn-3.pdf

15:30 Coffee break

Parallel A: Galactic I

Convener: Armelle Jardin-Blicq (LP2i Bordeaux)

101 Highlights on gamma-loud binary systems with the LST-1 and MAGIC telescopes

Gamma-loud binaries represent unique laboratories for studying particle acceleration, radiation mechanisms, and photon-photon absorption under varying geometrical conditions. Four classes have been identified as gamma-ray emitters: the historically known as gamma-ray binaries, powered by pulsar rotation; microquasars, powered by accretion onto a compact object (black hole or neutron star); novae, powered by the thermonuclear runaway on a white dwarf; and colliding wind binaries, powered by stellar winds from massive stars. In this presentation, we will review the latest results by the first Large-Sized Telescope (LST-1) of Cherenkov Telescope Array Observatory (CTAO) and MAGIC, located at the Roque de los Muchachos Observatory, La Palma, Spain.

Speaker: Alicia López-Oramas (Instituto de Astrofísica de Canarias (IAC))

Binaries_Gamma-Heidelberg_Lopez-Oramas_Final.pdf

102 On the GeV Emission from Nova Eruptions

Novae are an unexpected source of $\gamma$-rays, discovered by Fermi in 2010 and formally declared a source class by the Fermi-LAT collaboration in 2014. There are only 7 cataloged novae in the most recent catalog (FL16Y). Theoretically, novae contain all the ingredients to be prolific $\gamma$-ray emitters, but for every $\gamma$-ray nova detected, as many as 10 optical novae are observed. It has been 8 years since a comprehensive survey on the nature of $\gamma$-ray novae was published. Over that time, the Fermi mission has provided several data releases, improved background models, and advances in data analysis software. Also in that time, dozens of optical novae have erupted. It has been suggested that the $\gamma$-rays from these systems are generated in collisionless non-relativistic shocks. In this theory, the observed correlation between optical and $\gamma$-ray nova lightcurves naturally arises if a portion of the optical luminosity is reprocessed shock power. In this work, we investigate this scenario by analyzing Fermi-LAT-detected novae in varied time bins and then correlating the bin size that maximizes the nova's significance ($t_{\gamma}^{*}$) with the nova's optical eruption data. Furthermore, we investigate whether there is a common magnitude to which the novae decay in their respective measured $t_{\gamma}^{*}$. We find that across our population, a nova's $t_3$ appears to be closest to the analysis bin size that optimizes the Fermi-LAT detection significance. We also aim to perform likelihood stacks of the undetected population of novae that have erupted since the beginning of the Fermi mission.

Speaker: Owen Henry (City University of New York, American Museum of Natural History)

OwenH_gamma2026_slides.pdf

103 Study of the very-high-energy emission of Cygnus X-3 with the MAGIC Telescopes

Cygnus X-3 is a microquasar consisting of a compact object of unknown nature and a Wolf-Rayet star in a tight 4.8-hour orbit. The compact object launches powerful jets that are an excellent site for particle acceleration up to relativistic energies and subsequent emission of non-thermal radiation. Cygnus X-3 has been detected across a broad frequency range, from radio up to, very recently, gamma rays above 1 PeV. However, it has not yet been firmly established as a very-high-energy (VHE) gamma-ray emitter at TeV energies. The recent LHAASO detection of this source from 60 TeV up to 3.7 PeV establishes it as a PeV particle accelerator and the origin of the most energetic gamma rays recorded up to date.

Due to its long-standing scientific interest, Cygnus X-3 has been monitored by the MAGIC telescopes in the VHE band since the beginning of operations. In this contribution, we will present a long-term analysis of 130 h of data collected with MAGIC between 2013 and 2024. This represents the largest available dataset (in both exposure and time coverage) at VHE to date, resulting in the most constraining VHE upper limits between 100 GeV and a few TeV. Both the temporal and spectral features of Cygnus X-3 during this 11-year period will be interpreted and discussed within the broad multi-wavelength context of the source.

Speaker: Luis Barrios-Jiménez (Instituto de Astrofísica de Canarias, Universidad de la Laguna)

Cyg_X-3_GAMMA2026.pdf

104 Multiwavelength spectral study of the gamma ray binary LS I +61 303 around its entire orbit

Gamma-ray binaries are binary star systems which emit most of their energy above 100 MeV. A small number of these objects are detected as variable point sources in very-high-energy gamma rays (VHE, E>100 GeV). Many of these systems are composed of a rapidly-rotating Be star orbited by a compact object, usually a neutron star. The high-energy nonthermal emission is thought to originate from the shock that forms between the pulsar and stellar winds, and VHE emission typically peaks near the system’s periastron.

The binary LS I +61 303 was identified as a MeV gamma ray emitter nearly fifty years ago and has since been the subject of extensive observations across the electromagnetic spectrum. Composed of a primary Be star and radio-detected pulsar in a 26.5 day orbit, this gamma-ray binary exhibits complex periodic behavior and variability from radio wavelengths to VHE gamma rays. Unlike other VHE-detected Be/pulsar binaries, the peak VHE emission from LS I +61 303 is observed near its radial-velocity-measured apastron,. Whether this indicates a misunderstanding of the binary’s orbit or additional mechanisms at play is an area of active research.

Over the past 18 years, VERITAS has observed LS I +61 303 to monitor for flaring behaviour and to characterize the binary’s emission around its entire orbit. We present 340 hours of VERITAS observations with complementary observations by Swift-XRT and Fermi-LAT. We explore the binary’s keV to TeV emission around its orbit, searching for signatures of absorption and pulsar-disk interactions in the high-energy nonthermal spectrum of LS I +61 303 to better characterize this enigmatic system.

Speaker: Anne Duerr (GNOI)

lsi_gamma2026.pdf

lsi_gamma2026.pdf

105 HESS observations of the 2024 periastron passage of PSR B1259-63/LS 2883 gamma-ray binary

PSR B1259-63 is a known representative of gamma-ray binaries – high-mass binaries emitting most of their non-thermal energy in the gamma-ray range. The system hosts a pulsar in an eccentric orbit, with a 3.4 yr period, around an O9.5Ve star (LS 2883). At orbital phases close to periastron passages, the system radiates bright and variable non-thermal emission, for which the temporal and spectral properties are, for now, poorly understood. In this regard, very high-energy (VHE) emission is especially useful to study and constrain radiation processes and particle acceleration in the system.

In this talk we present the results of the monitoring of PSR B1259-63 with the High Energy Stereoscopic System (H.E.S.S.) around the most recent periastron in July 2024. The observations consist of 47 hours of data and cover the period from -36 to +38 days relative to the periastron.

We report a firm >20 sigma significance detection of the system around the periastron in the 0.4-80 TeV band. For the first time, we also report a hint of a low-energy spectral break at ~1 TeV. The spectral index after the break is roughly consistent with the one reported by H.E.S.S. during the previous periastra.

The observations around 2024 periastron allowed us to fill the gaps in the TeV light curves obtained during previous periastron passages making the light curve more complete. Based on the obtained lightcurve we report an apparent absence of correlation of the TeV flux with the X-ray flux as observed by Swift/XRT, contrary to the 2021 periastron passage findings.

Speaker: Aleksei Kuzin (University of Tübingen)

slides_kuzin_psrb24.pdf

106 Probing the periastron passage of WR 140 at very high energies with MAGIC and LST-1

Colliding-wind binaries (CWBs) are a promising but still largely unexplored class of very-high-energy (VHE) gamma-ray emitters. Among the gamma-loud binaries detected above 100 GeV—including gamma-ray binaries powered by non-accreting pulsars, microquasars, and novae—only one colliding-wind system, Eta Carinae, has been firmly detected to date. Establishing whether additional CWBs can accelerate particles up to VHE is therefore crucial to understanding particle acceleration and hadronic processes in massive stellar environments.

WR 140 is a colliding-wind binary composed of a Wolf–Rayet star (∼10M⊙​) and an O-type companion (∼30M⊙​), orbiting each other with a period of 7.9 years and a high eccentricity (e∼0.9). Its strong supersonic winds form a wind–wind collision region where efficient particle acceleration is expected, making it a compelling candidate for VHE gamma-ray emission, particularly around periastron.

We present an observational campaign on WR 140 carried out with the MAGIC telescopes and the LST-1 to cover the key orbital phases of the 2024 periastron passage, from the first conjunction through periastron and shortly beyond the second conjunction. We report on three complementary analyses corresponding to the different telescope configurations: MAGIC-only, LST-1-only, and joint MAGIC+LST-1 observations. This multi-instrument approach provides a comprehensive view of the source evolution across its most relevant orbital phases and a unique opportunity to investigate potential VHE gamma-ray emission and its variability.

Speaker: Fernando Frias Garcia-Lago (Instituto de Astrofísica de Canarias (IAC))

GAMMA_WR140.pdf

107 Microquasar Jets Imprinting the ISM: Sub-Parsec CO Filaments in HESS J1023−575

The TeV $\gamma$-ray source HESS J1023$−$575 is one of the brightest H.E.S.S. sources toward the young massive cluster Westerlund2 and has been proposed as a candidate site of Galactic cosmic-ray acceleration. However, the physical connection between the $\gamma$-ray emission and the surrounding ISM remains unclear. Our ALMA observations reveal that the molecular gas is composed of numerous thin filamentary structures with widths of $\sim$0.5pc and lengths of 10$–$20pc. These filaments exhibit a remarkable linear alignment along the Jet–Arc axis over a projected length of $\sim$170pc. A cavity structure with a diameter of $\sim$2pc, corresponding to the jet width, is identified, with filaments preferentially distributed along its boundary.

These structures provide direct morphological evidence of Jet$–$ISM interaction. Comparison with magnetohydrodynamical models of microquasar jets suggests that the filamentary clouds are fossil imprints of jets propagating through ambient HI gas. Asymmetry between the Jet and Arc clouds is explained by differences in the surrounding HI density. The position-velocity structure toward the Arc indicates an expanding motion. Assuming an expansion velocity of 3$–$4km/s and a radius of $\sim$20pc, the formation timescale is estimated to be 5$-$7Myr, consistent with simulations.

Using CO and HI gas distributions together with the $\gamma$-ray luminosity, the total cosmic-ray proton energy is estimated to be 7×10$^{48}$ erg under the hadronic scenario, approximately an order of magnitude larger than that of TeV $\gamma$-ray SNRs such as RX J1713$-$3946 and RX J0852$-$4622. The activity timescale is inferred to be 1–10 Myr, significantly longer than the typical duration of cosmic-ray acceleration in SNRs. These results indicate that HESS J1023$-$575 is a long-lived and powerful Galactic cosmic-ray accelerator, capable of supplying energy comparable to more than 10$^3$ SNRs, and that CO filaments provide a new observational tracer of Jet–ISM interaction, highlighting microquasars as an important and previously underexplored population of Galactic cosmic-ray sources.

Speaker: Kisetsu Tsuge (Gifu University/NAOJ)

gamma2026_Tsuge_v2.pdf

108 Discovery of Extended X-ray emission from the Pevatron microquasar V4641 Sgr

Microquasars are increasingly recognized as powerful cosmic accelerators, and in systems such as SS 433 X-ray observations have proven a key tool for tracing synchrotron emission from relativistic particles. We present new results from a deep XMM-Newton observation campaign targeting the region surrounding the microquasar V4641 Sgr. In recent LHAASO observations, V4641 Sgr was identified as a rare class of extreme galactic particle accelerator, the so-called super-PeVatrons, with significant emission detected up to 0.8 PeV and no indication of a spectral turn-off. We report the detection of extended X-ray emission spatially coincident with the centroids of the TeV emission observed by H.E.S.S., HAWC, and LHAASO and aligned with the known radio jet axis. The morphology of the X-ray emission reveals elongated features suggestive of large-scale jet–environment interaction. The spectral properties of these regions are consistent with a non-thermal origin and are compatible with a synchrotron scenario for the X-ray emission. We also detect hints of two additional extended X-ray features, underlining the complexity of the system. These findings pave the way for combined X-ray and gamma-ray analysis of the physics of relativistic outflows and their interaction with the surrounding medium, ultimately helping to constrain the contribution of microquasars to the observed cosmic-ray spectrum.

Speaker: Caterina Tresoldi (INAF-OAB)

Tresoldi_gamma2026_DEF2.pdf

Parallel B: Extragalactic I

Convener: Wystan Benbow (Center for Astrophysics | Harvard & Smithsonian)

109 High-energy polarisation: a new frontier in blazar jets

Multiwavelength polarimetric studies are a powerful tool used to probe the structure and physics of blazar jets. In particular, high-energy polarisation measurements allow us to discriminate between hadronic and leptonic emission scenarios. However, current polarimetric instruments are limited in sensitivity, restricting such measurements to only the brightest sources and highest flux states. A new generation of X-ray and gamma-ray polarimeters is currently under development and will significantly improve these capabilities, enabling systematic studies of blazar polarisation across a larger population of sources. In this work, we investigate the detectability of blazar polarisation as a function of flux and polarisation degree, and we estimate the corresponding duty cycle expected for future instruments, i.e. the probability to detect a given source in a blind survey. We further explore how the SED class, typical variability levels, and the shape of the synchrotron component can impact the duty cycle of the instruments. This method unifies how theoretical expectations are compared to observational capabilities and aids to assess which sources, physical models, and flux states should be prioritised for polarimetric studies. We offer practical guidelines for planning future multiwavelength polarisation campaigns that aim to conclusively differentiate between competing theories.

Speaker: sara capecchiacci (IA-FORTH)

Heidelberg.key

Heidelberg.pdf

110 Investigating blazar variability using flux distributions for CTAO long-term monitoring

The long-term monitoring (LTM) program of blazars is an essential part of the Key Science Project on Active Galactic Nuclei (AGN) with the Cherenkov Telescope Array Observatory (CTAO). Extracting the probability distribution function (PDF) of the flux from unbiased blazar light curves provides important insight into the physical processes taking place in relativistic jets and their connection to the observed variability. This work aims to determine an optimal observational strategy for the CTAO LTM program using flux distributions as a quantitative metric. To accomplish this, a simulation-based analysis framework was developed, enabling a systematic comparison of different observational strategies.
For a representative set of AGN sources, light curves were simulated as would be observed with the future CTAO, following four different observational strategies within a fixed time budget. For each simulated light curve, several PDF models were fitted and their goodness of fit was evaluated. Then, the best-fit parameters were used to generate Monte Carlo simulations in order to assess the ability of the method to retrieve the underlying PDF model.
The algorithm has the potential to discriminate between different models for the flux distributions (Gaussian, lognormal, and alpha stable). Once validated on simulations, the method is applied to archival light curves from existing gamma-ray instruments to extract flux PDFs and study the physical processes driving blazar variability.

Speaker: Anastasiia Mikhno (LPNHE, IJCLab)

Mikhno_Gamma_2026_final.pptx.pdf

111 TeV Variability and Spectral Evolution of Mrk 421 and Mrk 501: Insights from LHAASO’s Unbiased Long-Term Monitoring

The Large High Altitude Air Shower Observatory (LHAASO) provides a unique window into blazar astrophysics through its wide field of view and >98% duty cycle, enabling unbiased, continuous monitoring of active galactic nuclei. We present comprehensive TeV (0.4–20 TeV) observations of the archetypal blazars Mrk 421 (z=0.031) and Mrk 501 (z=0.034) from 2021 to 2025.

For Mrk 421, three years of LHAASO-WCDA data yield a total significance of 215 σ. We robustly identify 23 distinct outbursts with durations of 1–19 days. Critically, the measured flaring duty cycle of ~14.6% is significantly lower than previous IACT estimates, highlighting the impact of trigger bias in historical samples. The time-averaged spectrum extends to ~13 TeV and exhibits a definitive "harder-when-brighter" trend. Multi-wavelength analysis reveals a strong, zero-lag correlation (>3σ) between TeV and X-ray fluxes, supporting co-spatial emission, while the GeV–TeV connection remains comparatively weak.

For Mrk 501, 4.5 years of monitoring (90 σ) resolve eight high states—including a year-long active episode—interrupted by a >400-day quiescent period. Fractional variability peaks in the TeV band (F_var ~ 0.64), and flux distributions shift from log-normal in high states to Gaussian in quiescence, suggesting distinct physical origins. The intrinsic spectrum hardens from α ≈ 2.36 (low state) to 2.10 (high state), with an exponential cutoff near ~9 TeV.

Broadband SEDs across all states are well-described by one-zone synchrotron self-Compton (SSC) models. These results demonstrate LHAASO’s capability to capture the full dynamical range of blazar behavior, providing powerful new constraints on particle acceleration and cooling in relativistic jets.

Speakers: LHAASO Collaboration, min zha (Institute of High Energy Physics, CAS, Beijing, China)

gamma26_zham_20260901.pdf

112 Very-High-Energy and Neutrino-Selected Blazars at Parsec Scales

The population of blazars selected by their strong very-high-energy (VHE) gamma-ray emission or association with high-energy neutrinos continues to challenge astrophysics. The difficulty arises from a long-standing mismatch between the low apparent jet speeds measured with VLBI and the high Doppler factors inferred from VHE and neutrino observations. This discrepancy is known as the "Doppler factor crisis." In this talk, we present recent observational results from our analysis of multi-year, parsec-scale polarization and kinematics studies of VHE- and neutrino-selected blazars obtained with the VLBA at 15 GHz within the MOJAVE program. Our findings point to two observational resolutions to this problem. The first involves extremely small jet viewing angles (less than 1 degree), supported by toroidal magnetic field detection in polarization VLBI data. The second invokes a stratified jet structure in which a fast-moving spine coexists with a slower sheath, arising from our parsec-scale brightness distribution analysis. Together, these results provide a new observational method to identify promising VHE gamma-ray and neutrino blazar candidates with extreme Doppler boosting and constrain physics of proton acceleration and VHE emission.

Speaker: Yuri Kovalev (MPIfR)

Kovalev_VHE_Heidelberg2026.pdf

113 Kink-Driven Magnetic Reconnection in Relativistic Jets: A Mechanism for Rapid gamma-ray Variability in Blazars

Rapid, high-amplitude $\gamma$-ray flares in blazars pose a fundamental challenge to conventional shock-based acceleration models, motivating the exploration of alternative dissipation mechanisms such as magnetic reconnection. In this presentation, we discuss results from three-dimensional relativistic magnetohydrodynamic (RMHD) and resistive RMHD simulations of turbulent, magnetized jet plasma columns, performed with the PLUTO code, with a focus on current-driven kink instability. Our simulations show that kink-induced helical distortions naturally trigger magnetic reconnection, forming filamentary current sheets that fragment into chains of merging plasmoids. This process provides an efficient pathway for converting magnetic energy into particle acceleration and plasma heating. To analyze reconnection across different scales, we introduce a novel framework that combines hierarchical structure identification with reconnection diagnostics, enabling robust detection of active current sheets. Statistical analysis of their geometry and orientation reveals a subset aligned with the jet axis. These structures act as embedded “jets-in-a-jet,” producing relativistically moving plasmoids subject to strong Doppler boosting. This mechanism naturally explains the superposition of rapid flares on longer-timescale variability in blazar $\gamma$-ray light curves. Importantly, the hierarchical and turbulent nature of reconnection also predicts fast variability at large jet viewing angles, consistent with observations of misaligned radio galaxies such as M87 or IC 310. Overall, these results provide new insight into the plasma dynamics of relativistic jets and strengthen the case for magnetic reconnection as a key driver of blazar $\gamma$-ray variability.

Speaker: Chandan Kumar Das (Indian Institute of Technology Indore)

Gamma26_Chandan.pptx

114 Gamma Ray Constraints on Heavy Axion-Like-Particle Decays from Fermi-LAT and H.E.S.S. Blazar Spectra

The propagation of very-high-energy (VHE, Eγ ≳ 100 GeV) gamma rays from extragalactic sources is strongly affected by interactions with the extragalactic background light (EBL), leading to pair production and an energy- and redshift-dependent attenuation of the intrinsic gamma-ray flux. This process makes the Universe increasingly opaque to VHE photons.

New physics scenarios involving axion-like particles (ALPs) can modify this picture. In particular, heavy ALPs with masses of order ~ 10 eV may decay into two photons on cosmological timescales, thereby contributing to the diffuse EBL. If such ALPs constitute a significant fraction of the dark matter density, their decay would enhance the EBL intensity and increase the gamma-ray optical depth.

In this talk, I will present a study of this scenario using a large sample of extragalactic gamma-ray spectra observed with H.E.S.S. and the Fermi Large Area Telescope. We model the contribution of decaying ALPs to the EBL and assess their impact on blazar spectra over a wide redshift range. By comparing the observations with standard EBL models, we derive constraints on the mass and photon coupling of heavy ALPs and evaluate their viability as a dark matter candidate.

Speaker: Atreya Acharyya (IJCLab, Université Paris-Saclay)

ALP_EBL_Gamma.pdf

115 Laboratory test of the stability of TeV γ-ray-induced pair cascades in cosmic voids - implications for the intergalactic magnetic field

We have generated a dense electron–positron pair beam at CERN using 440 GeV/c protons and studied its propagation through a meter-length plasma, analogous to TeV γ-ray-induced pair cascades in the intergalactic medium. This was to test if pair beam instabilities disrupt the cascade, thus accounting for the observed lack of reprocessed GeV γ-rays from blazars. We find that the instability is suppressed if the beam is not perfectly collimated or monochromatic. Thus a moderate strength intergalactic magnetic field is indeed required to explain the observations. It is likely a relic of the early universe, rather than of astrophysical origin.

Speaker: Gianluca Gregori

Heidelberg_2026_GG.pdf

116 Studying Seyfert Galaxies with CTAO

Seyfert galaxies represent a distinctive subclass of Active Galactic Nuclei and are emerging as compelling multi-messenger sources in the high-energy universe. They are capable of accelerating cosmic rays to extreme energies, thereby producing both gamma rays and high-energy neutrinos. Notably, some of the nearest Seyfert galaxies, such as NGC 1068 and NGC 4151, have been identified among the brightest neutrino-emitting candidates by the IceCube Neutrino Observatory, exhibiting significant excesses.

The possibility of studying these objects with the Cherenkov Telescope Array Observatory (CTAO) in the very-high-energy (VHE) domain represents a major opportunity to investigate the different cosmic-ray acceleration sites within Seyfert galaxies. These include star-forming regions, collimated radio jets, wide-angle outflows, and hot coronae, each associated with distinct multi-messenger signatures. In this contribution, we will critically assess these environments and the underlying physical mechanisms, focusing on their observability with CTAO and their implications for high-energy neutrino production.

Particular emphasis will be placed on constraining the location of the neutrino-emitting regions, highlighting how VHE gamma-ray observations with CTAO can provide key insights into these environments.

Speaker: Mr Justin Albinet (CNRS)

Gamma_Seyfert_2026.pdf

18:30 Early Career Researcher Event

117 Public Talk

Öffentlicher Vortrag
Prof. Dr. Werner Hofmann
MPI für Kernphysik

Kosmische Teilchenbeschleuniger
Auf Spurensuche mit Gammastrahlen

Abstract

Wednesday 2 September

Plenary: VI

Convener: Lucia Haerer

118 Cosmic Rays at Earth

Speaker: Stefano Gabici (APC)

gabici.pdf

119 Galactic transport of cosmic rays and diffuse emission

Speaker: Phillipp Mertsch (RWTH Aachen)

Mertsch_Gamma26.pdf

120 X-ray Observations of Galactic PeVatrons: Probing the Most Energetic Particle Accelerators in the Milky Way

The recent discovery of ~50 ultra-high-energy (UHE; E > 100 TeV) gamma-ray sources, together with neutrino emission along the Galactic Plane, provides compelling evidence for the existence of Galactic PeVatrons. Previous studies of TeV gamma-ray sources have demonstrated that X-ray observations play a crucial role in identifying these extreme particle accelerators by detecting synchrotron emission from primary and secondary TeV-PeV electrons. In particular, combining broadband spectral energy distributions (SEDs), morphological data and multi-epoch observations in the X-ray and TeV bands offers a powerful tool for investigating particle acceleration, transport and energy-loss mechanisms in these UHE sources. In this talk, I will present recent X-ray observations of Galactic PeVatron candidates, including young supernova remnants, pulsar wind nebulae, microquasars, the Galactic Center, and so-called dark PeVatrons. I will highlight how multi-wavelength observations and detailed SED modeling can constrain the spatial and energy distributions of TeV-PeV particles, as well as the environmental parameters such as ambient magnetic fields. I will also discuss future prospects for X-ray observations of Galactic PeVatrons.

Speaker: Dr Kaya Mori (Columbia University)

Gamma2026_KMori.pdf

10:30 Coffee break

Parallel A: Future Synergies

Convener: David Green (CTAO)

121 CTAO+SWGO: Status and Performance

Speakers: Jim Hinton (MPIK), Dr Roberta Zanin (CTAO)

SynergiesIntro.pdf

122 Surveys of the Galactic Plane, source populations and diffuse emission

Speaker: Quentin Remy (MPIK)

CTAO_SWGO_gamma26_remy.pdf

123 Gamma-ray transients

Speaker: Dr Edna Ruiz Velasco (LAPP)

transientssyn_gamma2026_pdf.pdf

124 Dark matter searches and BSM Physics

Speaker: Michele Doro (University of Padova)

doro_2026_CTAO_Gamma.pdf

125 Final remarks

Speaker: Jim Hinton (MPIK)

Parallel B: Theory II

Convener: Anabella Araudo (GNOI)

126 A Leptonic Model for Neutrino Emission from Blazars

I present a leptonic model for neutrino emission from blazars. This model includes two major emission components: a small blob and a large blob. The large blob can produce optical, X-ray, and gamma-ray emission through the synchrotron and synchrotron self-Compton processes. The small blob produces gamma-rays by Compton scattering of photons from the nearby part of an extended jet. The extended jet photons absorb gamma-rays from the small blob, initiating a cascade through the electron pair production process. Neutrinos are produced through the absorption of gamma-rays by extended jet photons through the muon and pion pair production process and their subsequent decay. We show that this model can explain the spectral energy distributions of three blazars associate with neutrino emission: TXS 0506+056, PKS0735+178, and 5BZB J0630-2406, including the inferred neutrino emission from these sources as found by the IceCube detector. Our models have jet powers below the Eddington limit, unlike many lepto-hadronic models. Our model may be distinguished from lepto-hadronic models for blazar emission through breaks in gamma-ray spectra, the ratio of the gamma-ray break energy to the neutrino energy, and the flavor ratio of neutrinos.

Speaker: Justin Finke (US Naval Research Laboratory)

finke_neutrino_heidelberg_sept2026.pdf

127 High Energy Particle Acceleration in Shearing Flows

Stochastic Fermi-type particle acceleration has emerged as a compelling mechanism for the efficient energization of particles in supermassive BH systems. I will review recent advances, and exemplarily highlight: (i) the role of shear acceleration in sustaining ultra-relativistic electrons and protons in large-scale jets such as in Centaurus A, (ii) its implications for the production of extended (semi-steady) VHE emission in M87, and (iii) the efficiency of turbulent CR acceleration processes in neutrino-candidate Seyfert galaxies.

Ref:
Wang, Reville, Rieger & Aharonian 2024, ApJ 977, L20
Rieger & Duffy 2025, ApJ 988, 245
Lemoine & Rieger 2025, A&A 697, 124
Wang et al. 2026, A&A in press

Collaborators: Peter Duffy, Martin Lemoine, Felix Aharonian, Brian Reville, Xiaona Sun, Jieshuang Wang, Jia-Xing Wang

Speaker: Frank Rieger (IPP)

Shear_Acceleration.pdf

128 Linking relativistic jet dynamics to high-energy emission: the role of recollimation and instabilities

Relativistic jets, from microquasars to active galactic nuclei, power some of the most luminous high-energy emission in the Universe. However, linking their large-scale dynamics to localized particle acceleration and radiation sites remains a fundamental challenge. We present high-resolution 2D and 3D relativistic magnetohydrodynamic simulations using the PLUTO code that follow the propagation of relativistic jets in stratified environments. We show that external confinement and ambient pressure gradients naturally induce recollimation shocks and complex, time-dependent flow structures. By tracking the nonlinear evolution of these features, we demonstrate that energy dissipation is systematically concentrated in dynamically evolving regions associated with recollimation and jet instabilities. These sites provide a physically grounded origin for high-energy emission, directly tied to the global jet dynamics. Our results establish a framework connecting macroscopic jet evolution to variability, offering predictive insight for current and next-generation gamma-ray observations.

Speaker: Stella Boula (INAF-OAB)

Gamma2026_Boula

Gamma2026_Boula.pptx

129 Gamma-ray signatures of proton acceleration in extragalactic sources

The presence of non-thermal protons, which are required to produce astrophysical neutrinos, leads to spectral signatures in the electromagnetic spectrum. We discuss with several examples (such as AGN blazars and Gamma-Ray Bursts) where this "hadronic-induced" electromagnetic cascade would be expected in the gamma-ray bands,
how it relates to other wavelength bands, and how the cascade can be described in general terms. Our discussion is relevant to identify the origin of cosmic rays at the highest energies with gamma-ray instruments, from Fermi to CTAO.

Speaker: Walter Winter (DESY)

2608_GAMMA_Winter_W.pdf

130 Cosmic Rays from the Galactic Center: Simulating the Fermi and eROSITA Bubbles

The Milky Way hosts large-scale high-energy structures such as the Fermi Bubble and the eROSITA bubble, which provide a unique laboratory to study the interplay between galactic center activity, stellar feedback, cosmic rays (CRs), and the circumgalactic medium. Their origin, dynamical evolution, and non-thermal emission mechanisms remain actively debated.

We present high-resolution magnetohydrodynamical simulations of Milky Way-like galaxies with a detailed galactic center model that self-consistently follow CR injection, transport, and coupling to the multi-phase interstellar medium. Our models naturally produce large bipolar outflows and bubble-like structures resembling the observed Fermi/eRosita bubbles. This allows us to directly connect physical launching scenarios with observable signatures.

We identify the dominant driving mechanisms of the bubbles, quantify their expansion history and shock properties, and assess the relative importance of thermal gas, magnetic fields, and CR pressure support. In particular, we investigate how CR diffusion, streaming, and advection shape the CR distribution and determine the resulting gamma-ray morphology and spectra. With the given shock properties and magnetic alignments we can directly address the particle acceleration in shock fronts. Using synthetic observables, we test hadronic and leptonic emission models and discuss how shock acceleration, transport physics, and line-of-sight projection effects constrain the origin of the observed emission.

Speaker: Philipp Girichidis (Heidelberg University)

agama-CRMHD-inv.mp4

flat-CRMHD-inv.mp4

Girichidis-erosita-bubbles.pdf

131 Mutual Effects of Radiation Field and Cosmic Rays on Spectroscopic Tracers of Molecular Gas in the Milky Way

A widely used technique for inferring the flux of cosmic rays at different positions in the Milky Way is to measure gamma-ray emission arising from the decay of pions produced in the interaction of cosmic rays with molecular gas in dense cold clouds. This however presupposes that the column density of gas throughout the cloud is independently known. This condition is hard to realise because the molecular hydrogen column must typically be deduced from line emission from tracer molecules, such as the emission from the rotational transitions of CO.

In this contribution we utilise a simple thermal and chemical model for the cold interstellar medium to predict the emissivity of the principal coolants of neutral and molecular gas as a function of ambient conditions defined by pressure, metallicity, intensity and colour of the ambient interstellar radiation field, as well as cosmic ray ionisation rate in the gas. We show that gas heating via optical photons dominates heating over UV heating in molecular gas in passive (ie non-star-forming) clouds in galactic environments like the central molecular zone of the Milky Way, a region with highly elevated diffuse optical radiation fields from the galactic bulge and elevated mid-plane thermal pressure. We also show that there is a sharp transition between optically-dominated heating and CR-dominated gas heating with increasing CR flux in this region, with the cross-over point dependent on ambient pressure. Because the CO molecule is a principal gas coolant, the optical and/or CR heating of molecular gas directly affects the emissivity in the CO molecule. Using simple semi-analytic hydrostatic models of gas clouds we predict the emergent CO emission per Hydrogen molecule from molecular clouds as a function of the incident low energy CR flux at different galactic locations and discuss implications for the interpretation of gamma-ray observations of passive clouds.

Speaker: Richard Tuffs (MPIK)

R_Tuffs_Gamma.pdf

rtuffs.pptx

12:30 Conference Photo

12:40 Lunch

Parallel A: Galactic II

Convener: Marianne Lemoine-Goumard

132 The TeV gamma-ray detection of 4FGL0822.8-4207 with H.E.S.S.: powered by cosmic rays from a YSO or from Puppis A?

4FGL 0822.8-4207 is a Fermi-LAT detected gamma-ray source of whose nature remains debated: it could either be caused by cosmic rays escaping the nearby supernova remnant Puppis A irradiating a compact molecular cloud at a projected distance of 20 pc from the center of Puppis A, or it could be powered by a local cosmic-ray accelerator in the cloud/starforming region. In particular, the massive young stellar object (YSO)/Herbig Haro object IRAS 08211-4158 is in the latter scenario the prime candidate cosmic-ray accelerator. The YSO is located at a distance of 1.5 kpc, which is also consistent with the distance of Puppis A.

In this presentation we will report and discuss the discovery of 4FGL 0822.8-4207 at very-high-energy gamma rays, with the H.E.S.S. Imaging Atmospheric Cherenkov Telescope. The source is mildly extended (~4 arcmin) and is spatially coincident with the position of the YSO and the core of the compact molecular cloud hosting this YSO. We discuss the implications of this detection in the framework of both the escaping cosmic-ray scenario and the alternative interpretation in which young stellar objects may, in some cases, accelerate particles up to multi-TeV energies.

Speaker: Jacco Vink (Anton Pannekoek Institute & GRAPPA, University of Amsterdam)

vink_hess_j0822_gamma_2026_v1.pdf

133 A gamma-ray view of protostars: A new class of cosmic-ray accelerators.

Massive young stellar objects (YSOs) are active engines driving powerful outflows. Theoretical models predict that a fraction of the accreted material is ejected in the form of highly collimated, fast jets and outflows. The injection of mechanical energy by these jets into the interstellar medium (ISM) produces strong shocks that can accelerate non-thermal particles and trigger feedback processes that regulate star formation.

In this talk, we present the first detected population of protostellar jets emitting gamma rays as a byproduct of efficient cosmic-ray (CR) acceleration. The characteristics observed among the sample of 33 Gamma-Loud Protostars allow us to probe the energetic feedback injected by protostellar jets into the ISM and strongly favor a hadronic scenario, in which high-energy gamma rays (up to ~TeV energies) are produced through interactions of accelerated protons.

We conclude that protostellar jets constitute a previously unrecognized population of Galactic CR accelerators and that gamma-ray power is closely related to the bolometric luminosity of the driving YSO, where the CR production scales with the mechanical power of the jet.

Speaker: Javier Méndez-Gallego (IAA-CSIC)

gamma_slides.pdf

134 Constraints on proton acceleration in massive star clusters: a broadband perspective from radio to gamma rays

Thanks to fast mass losses due to the collective stellar winds, the environment around SCs is potentially suitable for particle acceleration up to PeV energies. While probing the energetic limit for acceleration in these environments will be a task for next generation ground-based observatories like the CTAO, space-based observatories like Fermi-LAT can be already used to disentangle the main emission mechanism and to constrain the acceleration properties (such as efficiency and electron-to-proton ratio) of stellar winds shocks.

The number of SCs with Fermi-LAT has been constantly increasing in recent years, and now counts at least a dozen objects. Moreover, a strong correlation between unidentified Fermi-LAT sources and Galactic HII regions, powered by young massive stellar systems has been demonstrated, suggesting that this number will further increase.

Among these, the very young super star cluster RCW38, embedded in the Vela molecular cloud ridge, is of particular interest. The dense environment in which it is located allows its hadronic emission component to stand out.

We present here the results from a new morphological and spectral analysis encompassing broadband data from gamma-ray data down to the low frequencies measured by SKAO precursors (e.g. MWA $\lesssim$ 100 MHz ). The improved modeling and the recently delivered data allow us to put strong constraints on the hadron production and in general on the acceleration properties which can be found at SC shocks. We discuss this in the context of Galactic cosmic rays and compare it with results of gamma ray analysis of other young SCs. We finally give some prospects for future exploitation of the upcoming ground-based facilities, especially the CTAO, and its possible synergies with the SKAO.

Speaker: Giada Peron (Inaf Osservatorio Astrofisico di Arcetri)

peron_heidelberg2026.pdf

135 Energy-Dependent Extended Gamma-Ray Emission in the Westerlund 1 Region seen by the Fermi Large Area Telescope

Gamma-ray observations suggest that winds, shocks, and turbulence around massive star clusters accelerate particles and drive the processes by which they merge with the large-scale Galactic cosmic-ray population. H.E.S.S. data have established that Westerlund 1 is a massive star cluster accelerating particles to at least several tens of TeV. In this contribution, we present a characterization of extended gamma-ray emission in the vicinity of Westerlund 1 using more than 16 years of data from the Fermi Large Area Telescope. The morphology of the gamma-ray emission is strongly energy dependent. At energies above ~3 GeV, the emission is dominated by a ~150 pc diameter source in the direction of a low-density region in the interstellar medium at the edge of the Galactic disk. This source is offset from the TeV gamma-ray emission surrounding the cluster and suggests the presence of a cosmic-ray loaded nascent outflow. At lower energies, we detect extended emission components overlapping with neutral matter in the Galactic disk located within ~100 pc of the Westerlund 1 superbubble edge. We discuss their potential relation to particles escaping the system.

Speaker: Luigi TIBALDO (IRAP/Université de Toulouse)

tibaldo_gamma_wd1_v3.pdf

136 Cosmic ray feedback and gamma-ray signatures in Milky Way-like galaxies

Cosmic ray protons (CRs), with their substantial energy density, play a key role in galaxy evolution, and shaping the interstellar medium (ISM). Accelerated in supernova shocks, CRs propagate along magnetic field lines, distributing energy and momentum throughout the galaxy. This process ionizes and heats the gas, drives large galactic outflows by creating pressure gradients, and regulates star formation. Diffuse gamma-ray emission provides a crucial observational tracer of the distribution of CRs, encoding information about their transport, energetics, and interaction with the ISM. High-resolution observations are, however, limited to the Milky Way, whose observed emission is shaped by our local environment within the Local Bubble.

We perform high-resolution magnetohydrodynamical simulations of Milky Way-like galaxies, where we follow individual massive stars and include self-consistent stellar feedback such as supernovae and CRs, dynamically coupled to the MHD equations. We model the ISM using a non-equilibrium chemical network that includes hydrogen and carbon species, allowing us to account for relevant cooling and heating processes. Additionally, we back up our simulations with post-processed, multi-wavelength gamma-ray emission from CR protons, enabling us to analyze luminosities, spectra, full-sky emission maps, and angular power spectra for many observer positions, including those located inside Local Bubble-like environments.

We first present the effects of CR feedback on the galactic structure and gas dynamics across different ISM phases. We find that CRs convert fountain flows into sustained galactic outflows - absent in the MHD-counterparts - driving mass loss from the entire disk and magnetizing the CGM. We show that the simulations naturally reproduce key observational properties of the Milky Way, including gamma-ray luminosities and spectral slopes. We also demonstrate that the local environment plays a significant role in shaping the simulated gamma-ray sky, highlighting the importance of understanding the observer’s local surroundings when using gamma rays to trace Galactic CR physics.

Speaker: Karin Kjellgren (Heidelberg University)

gamma2026.pdf

137 The TeV Diffuse Emission of the Milky Way as seen by the HAWC Observatory

After escape from their sources, Galactic cosmic rays propagate through the Milky Way and interact with the interstellar gas and radiation fields. The Galactic diffuse emission of gamma rays resulting from these interactions traces the global distributions of cosmic rays and matter in the Milky Way.

Here, we present the results from a study of the TeV Galactic diffuse emission with the HAWC observatory. This study, based on 8 years of Pass 5 HAWC data, features a data selection and background model specifically adapted for the study of large-scale emission and a detailed treatment of systematic uncertainties. The emission from individual sources is subtracted using an algorithm developed for the foreseen CTAO Galactic plane survey.

We show the measured emission spectra in multiple regions of the Galactic plane and find consistency with recent results from the LHAASO experiment, but a flux deficit compared to Tibet ASγ measurements. We also compare our findings to the IceCube measurements of the Galactic Neutrino flux.

A decomposition of the measured flux in Galactocentric radius reveals a rise of the cosmic ray density towards the Galactic Center, in excess of the predictions by conventional models, but in line with an extrapolation from GeV measurements.

Speaker: Georg Schwefer (Max-Planck-Institut für Kernphysik)

HAWC_GDE_GAMMA_26_pdf.pdf

Parallel B: Extragalactic II

Convener: Prof. Reshmi Mukherjee

138 Exploring Perseus Radio Galaxies IC 310 and NGC 1275 with LST-1 observations and first Swift-XRT 3D Analysis

Radio galaxies are a key target source class for Cherenkov telescopes, although only six have been detected so far at very high-energy (VHE; E > 100 GeV). Contrary to the vast majority of TeV-detected active galactic nuclei, radio galaxies have misaligned jets (θ >10°), which offer unique views across multiple wavelengths to explore particle acceleration processes and regions, as well as tools to test fundamental physics and cosmology. To accurately probe connections with lower wavelengths, the source needs to be separated from diffuse emission commonly surrounding radio galaxies.

Presented here is a broadband, multi-wavelength study of radio galaxies led by the first Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array Observatory (CTAO), supported by a novel Swift-XRT 3D analysis method. Traditional Swift-XRT analysis for bright sources suffers from source saturation and requires corrections that exclude the possibility to account for complex diffuse environments, thereby yielding biased flux estimates. The development and application of the novel 3D (2D spatial coordinates & energy) analysis developed with gammapy presented here will help provide unbiased X-ray fluxes and benefit multi-wavelength analyses.

Flaring and quiescent states are studied with ~50 hours of LST-1 observations from 2022-2025, for two nearby (z<0.019) radio galaxies in the Perseus cluster:

IC 310 is a transitional object halfway between a blazar and a radio galaxy, with one of the fastest variabilities recorded in the VHE domain. We find a long quiescent state briefly broken by a minor flare.

NGC 1275 is the bright central galaxy of the Perseus cluster and an active VHE source explored by multiple instruments. Two major bright flares were detected around the turn of 2022-2023, for which we discuss intranight variability, followed by longer quiescent states. Collected contemporary multi-wavelength data reveal broadband evolution, and allow us to explore potential mechanisms of the AGN activity.

Speaker: Tora Therese Høiland Arnesen (nstituto de Astrofísica de Canarias, Universidad de La Laguna)

26GammaHeidelbergv5.pdf

139 Unveiling cosmological structures from their imprints on gamma-ray spectra

During their multi-billion-year journey to Earth, extragalactic very-high-energy gamma rays deeply interact with cosmological fields. The main player is the extragalactic background light (EBL), over which gamma-rays are absorbed through photon-photon pair production. Secondary players are the intergalactic magnetic field (IGMF) and the CMB, which respectively deflect the charged pairs and convert them back to lower energy gamma-rays (~GeV) through inverse-Compton interaction.
EBL and IGMF strengths are expected to be somehow correlated with the large-scale filamentary structure of the Universe. In this presentation, I will show how the large gamma-ray spectra catalogs from Fermi-LAT and Ground-based Cherenkov telescopes allow us to probe for anisotropies in cosmological fields. I will mostly focus on probing for EBL large-scale anisotropies and extend to the potential effects of cosmic voids on the observed gamma-ray spectra.

Speaker: Olivier Hervet (UC Santa Cruz)

2026_08_Gamma_EBLmap_Hervet_v2.pdf

140 Recent Highlights from the VERITAS AGN Program

Since 2007, VERITAS has operated as one of the world’s most sensitive very-high-energy (VHE; E > 100 GeV) gamma-ray observatories. Studies of active galactic nuclei (AGN) are a major component of the VERITAS science program and more than 9,300 hours (~50%) of its good-weather observations were targeted on these jet-powered objects. VERITAS AGN efforts include a comprehensive program to discover new VHE emitters, multi-wavelength campaigns on known VHE AGN, and target-of-opportunity observations of AGN flares. The VERITAS spectral and variability measurements probe a variety of astrophysical processes in AGN and their jets.
Recent scientific highlights from the VERITAS AGN program, including the discovery of VHE emission from 3C 273 and any new announcements, will be presented.

Speaker: Wystan Benbow (Center for Astrophysics | Harvard & Smithsonian)

Benbow_Gamma2026.pdf

141 ASTRI Mini-Array preliminary results on extra-galactic sources

The ASTRI Mini-Array Project consists of nine dual-mirror small-sized Cherenkov telescopes, being deployed at the Observatorio del Teide (Spain) and currently in the commissioning phase of each telescope. The ASTRI Mini-Array has a large field of view of about 10 degrees, a wide energy range from 1 TeV to 200 TeV, an angular resolution of 3 arc-minutes, and an energy resolution of 10 percent at 10 TeV. In this contribution we present the preliminary results on a few extra-galactic sources such as Markarian 421, Markarian 501, BL Lacertae and the Perseus Cluster performed with the first ASTRI telescopes, demonstrating the capability of the system to detect variable extra-galactic sources and to contribute to multi-wavelength campaigns.

Speaker: Stefano Vercellone (INAF - Osservatorio Astronomico di Brera)

Vercellone_ASTRI_Gamma2026.pdf

142 Monitoring the TeV sky with LHAASO-WCDA

With its wide field of view and high duty cycle, the Water Cherenkov Detector Array (WCDA) of the Large High Altitude Air Shower Observatory (LHAASO) is uniquely suited for continuous monitoring of the TeV sky, enabling unbiased and long-term observations of variable and transient phenomena. At the end of 2023, we established a real-time monitoring system and created a sample of known TeV-emitting AGNs and hard Fermi-LAT sources. We constructed light curves with various temporal binnings. In this talk, I will report the detection of multiple significant flaring events from several sources, with timescales ranging from days to months. For selected bright flares, we investigated their temporal profiles and compared them with observations at lower energies when available, aiming to constrain emission mechanisms and particle acceleration processes in relativistic jets. For bright sources like Markarian 421 and Markarian 501, we explored the potential quasi-periodic oscillations in the long-term light curves. Our results demonstrate the strong capability of LHAASO-WCDA as a powerful survey instrument for time-domain gamma-ray astronomy. Continuous monitoring of AGNs at TeV energies provides valuable insights into jet physics and complements observations from imaging atmospheric Cherenkov telescopes and space-based instruments.

Speaker: Jianeng Zhou (Shanghai Astronomical Observatory, CAS)

gamma2026_wcda_monitoring.pdf

143 Characterisation of H.E.S.S. AGN flares

Blazars exhibit strong variability across the entire electromagnetic spectrum on timescales of minutes to years, especially at high energy (HE, >100 MeV) and very high energy (VHE, >100 GeV) 𝛾-rays, where amplitudes and durations of outbursts are the most extreme. Despite decades of observations with the Imaging Atmospheric Cherenkov arrays up to TeV and multi-TeV energies and the continuous survey of the HE 𝛾-ray sky by Fermi-LAT, revealing an abundance of such flaring events, their physical origin remains debated. Thus, a model-independent statistical characterisation of variability signatures can provide meaningful insights into the underlying particle acceleration and radiation processes, put constraints on the size and location of emitting zone(s) in the jet. In this work, we present the first collection of VHE 𝛾-ray flaring active galactic nuclei observed with H.E.S.S. For the found VHE flares, we perform morphology fits, and characterise the amplitude, duration and asymmetry. These flares are further examined by showing how their HE properties compare to those of a large public sample of Fermi flares. We also investigate the spectral variability for the brightest sub-set of flares, providing further constraints on the evolution of the emitting particle population.

Speaker: Anna Luashvili (North-West University, Centre for Space Research)

Luashvili_Gamma2026_final.pdf

15:30 Coffee break

Parallel A: Galactic III

Convener: Tina Wach (MPIK)

144 Unmasking a Galactic Mystery: A joint H.E.S.S. and HAWC Analysis of HESS J1843-033

The unidentified source class represent a large fraction of Galactic very-high-energy (VHE) gamma-ray emitters, among them is HESS J1843-033. This complex gamma-ray source has extended emission reaching up to several hundreds of TeV. The detection of this source by both the HAWC and LHAASO experiments indicates acceleration of particles up to the highest energies (PeV energies). Despite its detection by multiple instruments, it remains an unidentified source, as no clear counterpart has yet been identified.

To unveil the extreme particle accelerators in the Milky Way, the highest energy event observations are crucial. The region surrounding HESS J1843-033 has a number of plausible counterparts, including Supernova Remnants (SNRs), a pulsar and various Fermi-LAT objects, each holding the potential to offer insight into the origin of the gamma rays.

In this contribution, we present the first joint analysis of this source with both the H.E.S.S. and HAWC observatories. By leveraging the complementary strengths of the two instruments, we aim to characterise the morphology and spectral properties of the source across a broad energy range. The open-source Python software Gammapy is utilised to model the complex emission towards this source. We provide a comprehensive look into HESS J1843-033 across multiple wavebands, including X-rays and gamma-rays spanning the TeV-PeV range, to understand its complex structure.

Speaker: Kirsty Feijen (APC, CNRS)

Gamma26_HESSJ1843_KF.pdf

145 Direct Observational Constraint on Upstream Magnetic Field Amplification in RX J1713.7-3946

The supernova remnant RX J1713.7−3946 has attracted significant attention from the viewpoint of particle acceleration, due to the detection of TeV gamma rays (e.g., H.E.S.S. Collaboration 2018) and the dominance of synchrotron X-ray emission (e.g., Koyama et al. 1997). Recently, point-like X-ray sources (hotspots) were discovered in the northwestern region (Higurashi et al. 2020), and their association with molecular cloud cores was reported by Sano et al. (2020). We performed a comprehensive analysis of X-ray data obtained with Chandra, and discovered X-ray hotspots across the remnant. We found that the number density of hotspots not only downstream but also upstream of the shock is higher than in surrounding regions unrelated to the remnant, indicating a physical association between the SNR and the hotspots. One possible emission mechanism of the hotspots is synchrotron X-ray emission from secondary electrons produced via the decay of charged pions generated by p-p interactions between molecular cloud cores and protons diffused into the shock upstream. Based on this scenario, we constrained the spatial distribution of diffused protons from the hotspot distribution, allowing us to estimate the upstream magnetic field, which has been difficult to determine observationally. While downstream magnetic field amplification of order $\mathrm{\sim mG}$ has been suggested (e.g., Uchiyama et al. 2007), we have derived an upstream field of $\mathrm{10~\mu G}$. This value is consistent with the weak amplification predicted by simulations of the Bell instability (Bell 2004; Inoue et al. 2024), which is a plasible mechanism for upstream magnetic field amplification. From diffusive shock acceleration theory, such a field implies a maximum proton energy of $\mathrm{\sim10~TeV}$, consistent with the gamma-ray cutoff reported by H.E.S.S. Collaboration (2018).

Speaker: Yuya Kawabata (Konan University)

gamma2026-kawabata.pdf

146 Is G1.9+0.3, the youngest Galactic supernova remnant, a PeVatron?

G1.9+0.3, the youngest (< 150 yrs old) Galactic supernova remnant (SNR), offers a rare opportunity to probe particle acceleration in the early stage of an SNR when it is most energetic, and hence, the strongest candidate for a PeVatron. Despite its extremely fast shock speed (> 10,000 km/s), however, G1.9+0.3 has not been detected in the gamma-ray band. We instead use hard X-rays--synchrotron radiation from the most energetic electrons--to study this dynamic and quickly evolvying cosmic-ray accelerator. We present multi-epoch NuSTAR observations of G1.9+0.3. We interpret the observed NuSTAR flux variability by combining it with radio flux variability and applying time-dependent spectral energy distribution modeling. We discuss the potential for G1.9+0.3 as a PeVatron, and the prospect for gamma-ray detection of this source with current and future observatories.

Speaker: Dr Jooyun Woo (Columbia University)

Gamma2026_G1.9.pdf

147 Detection of the Gamma-Ray source LHAASO J0534+3533

We report the observation of an extended TeV source, LHAASO J0534+3535, with a significance exceeding 10σ in the energy range from TeV to tens of TeV. The source is spatially coincident with the center of the old supernova remnant (SNR) candidate G172.8+1.5, which resides in a major star-forming region in the outer Galaxy at a distance of 1.8 kpc. The gamma-ray emission from LHAASO J0534+3535 shows an extension of approximately 0.5°, with some GeV sources nearby. We present the data analysis results for LHAASO J0534+3535 and discuss its likely association with SNR G172.8+1.5, and also the probability of one gamma ray source or two sources.

Speaker: Xiurong Li (IHEP)

gamma2026-lixr.pptx

148 Discovery of Very-High-Energy gamma-ray emission from supernova remnant G108.2-0.6 by LHAASO

An extended Very-High-Energy (VHE) gamma-ray source coincident with the location of the large radio shell-type SNR G108.2-0.6 is newly discovered by LHAASO. With no excess gamma-ray emission above 100 TeV, the source energy spectrum is well fitted by a power-law function, implying no obvious cutoff. The VHE gamma-ray observation of this extended source has revealed a large shell-type structure with similar position and extension as SNR G108.2-0.6, thereby confirming their association. CO observations by MWISP indicate little spatial correspondence between MCs and SNR G108.2-0.6. Based on preliminary results from LHAASO and multi-wavelength observations, SNR G108.2-0.6 may belong to a class of TeV SNRs, where three prototypes are RX J1713.7-3946, RX J0852.0-4622 and SN 1006.

Speaker: Qin-Yi Cheng (IHEP CAS)

149 GeV detection of SN 2017egm: the gamma-ray signature of superluminous supernovae with Fermi-LAT

Superluminous supernovae (SLSNe) are a recent class of astronomical transients whose luminosities exceed those of typical core-collapse supernovae by 10 to 100 times. What makes SLSNe so different from regular core-collapse SNe is still in debate. There are mainly four different energy sources being considered to explain the high peak luminosity of SLSNe: ejecta fallback accretion onto a black hole, radioactive decay of 56Ni, circumstellar interaction, or a magnetar wind nebula. Gamma-ray observations can help to constrain the emission models, and it has been predicted that gamma-ray emission from a magnetar wind nebula could be visible approximately 100 days after the explosion when the opacity decreases. To test this hypothesis, we studied SLSNe gamma-ray light curves using 16 years of Fermi-LAT data for a sample of 6 nearby sources (< 200 Mpc) SLSNe. From this sample, we confirm a significant detection for SN 2017egm that coincides with the temporal and spectral predictions of the magnetar-dominated models. In particular, comparison of the models with the observed Fermi gamma-ray spectrum allows us to constrain the magnetization parameter of the nebula, which is critical for the TeV counterpart of the magnetar nebula. In contrast, while the magnetar model is the most tempting model, the interaction of the shock with multiple CSM shells appears to be inconsistent with the data. Based on our best model, we explore predictions for the detectability of such objects with the CTAO and the horizon of detectability. The discovery of SN 2017egm as a gamma-ray emitter establishes a new category of gamma-ray sources and could provide a novel opportunity to investigate extreme magnetars, which possess millisecond rotational periods and magnetic fields of 10^14 G.

Speaker: Guillem Marti-Devesa (ICE-CSIC)

Gamma2026_Fermi_SLSNe.pdf

150 Studies of the LHAASO Peanut region with VERITAS

The LHAASO “Peanut” is a composite and off-Galactic plane ultra-high-energy gamma-ray source that is among the most intriguing sources released in the 1LHAASO catalogue. The region, which includes a diffuse strip and point-like sources LHAASO J0216+4239, LHAASO J0207+4300, spans nearly five degrees in extension. Currently, there exists no identified multiwavelength counterpart to the Peanut, likely due to both its large extent and the lack of coverage of off-Galactic-plane regions in many existing archives. Diverse models have attempted to describe the Peanut region, such as a “mirage” pulsar wind nebula driven by the nearby millisecond pulsar (PSR J0218+4232), a microquasar outflow from an unidentified central engine, or perhaps a new source class altogether. Although many of these proposed models provide a plausible description of the LHAASO TeV data, the engine and acceleration mechanisms can only be isolated by identifying a multiwavelength counterpart at lower energies or deeply constraining a lack thereof. Imaging Atmospheric Cherenkov Telescopes (IACTs), such as VERITAS, are well-suited to search for this lower energy counterpart due to their large field of view, finer spatial resolution, and overlapping energy range with LHAASO. Here, we present the first IACT results on the Peanut region and its components, including a comprehensive VERITAS spectromorphological analysis of the full Peanut region. In addition, we present a deep study of the putatively associated PSR J0218+4232 at TeV energies and a detailed Fermi-LAT analysis of the Peanut region. We also expand on one-zone leptonic and hadronic spectral modelling of the source using LHAASO, VERITAS and Fermi-LAT data. Together, these analyses supplement the physical interpretations beyond what is possible with LHAASO data alone, in an effort to understand the nature and acceleration mechanisms of this mysterious source and inform the prospects for detection with CTAO.

Speaker: Samantha Wong (McGill University)

gamma_peanut_2026.pdf

151 Unveiling the PeVatron potential of the plerionic supernova remnant G0.9+0.1 through NuSTAR and broadband SED modeling

Pulsar wind nebulae (PWNe) are among the most promising candidates for Galactic leptonic PeVatrons. This hypothesis is further supported by the growing number of ultra-high-energy (UHE, E>100 TeV) PWN detections reported in recent years.
X-ray synchrotron radiation from TeV-PeV electrons plays a crucial role in constraining the maximum particle energy, and recent X-ray studies of TeV PWNe suggest that some of these systems can power acceleration up to PeV energies. As such, their multi-wavelength observations and broadband spectral energy distribution (SED) modelling are essential tools for both localising and characterising the properties of the relativistic particle populations.
We present a new study of the second brightest TeV source in the Galactic Center region: the PWN powered by the energetic pulsar PSR J1747−2809 and located inside the composite-type supernova remnant (SNR) G0.9+0.1. Its X-ray to TeV gamma-ray properties, including a nearby UHE source observed by HAWC, are suggestive of a PeVatron candidate. For the first time NuSTAR has detected hard X-ray emission up to 30 keV, revealing the synchrotron burn-off signature of relativistic electrons. Our radio to TeV SED modelling suggests that PWN G0.9+0.1 is ~2.2 kyr old and has not yet interacted with the reverse shock of its parent SNR. We find that this PWN has the potential to be a PeVatron PWN, with an estimated maximum electron energy of ~2 PeV. The source therefore offers a valuable case study for next-generation X-ray and gamma-ray observatories such as NewAthena and the Cherenkov Telescope Array Observatory.

Speaker: hongjun an

gamma2026_G0.9+0.1_HAn.pptx

Parallel B: Extragalactic III

Convener: Stefano Vercellone (INAF - Osservatorio Astronomico di Brera)

152 A fully angle- and polarization-dependent synchrotron + SSC model for blazars with realistic magnetic-field configurations

Blazars, a subclass of radio-loud, active galactic nuclei (AGNs) with relativistic jets oriented close to our line of sight (LoS), exhibit highly polarized emission from radio through optical bands, and more recently, in the X-rays. Polarization measurements provide one of the few direct probes of the order and geometry of magnetic fields within
AGN jets.

We develop a polarization-sensitive synchrotron+SSC blazar model that incorporates the observer’s viewing direction and complex magnetic field geometries, including toroidal and helical configurations relevant to relativistic jets and compact emission regions. The model explicitly resolves angular dependencies and self-consistently evolves the full Stokes parameters (I, Q, U) through synchrotron and SSC mechanisms.

We explore the imprint of magnetic field geometries on the SED and linear PD of a generic blazar. For toroidal fields, certain orientations produce maximal observed flux in an almost unpolarized state, while others yield minimum flux accompanied by the maximum polarization state. For helical fields, specific orientations yield an increase in degrees of polarization with photon frequency. This has direct implications for the interpretation of current and future polarimetric observations, providing a pathway toward more physically grounded constraints on particle distributions and jet magnetization in blazars and gamma-ray bursts.

Speaker: Markus Boettcher (North-West University)

Boettcher_Gamma2026_Blazar_Polarization.pdf

Boettcher_Gamma2026_Blazar_Polarization.pptx

153 Gamma-Ray Flaring Statistics of Blazars using 18 Years of Fermi –LAT Monitoring Data

Due to the continuous all-sky monitoring by the Fermi Large Area Telescope (LAT) gamma-ray light curves for several thousand sources, including a large population of blazars, are now at our disposal. Blazars are typically associated with pronounced variability and episodic flaring activity, which has been extensively studied across the electromagnetic spectrum. However, detailed investigations have largely concentrated on objects exhibiting frequent and high-amplitude flares, which biases our understanding of the broader population.
Using publicly available LAT light curves, we perform a systematic study of gamma-ray flaring activity across the full blazar sample over 18 years of observations. We find that, while variability is common, prominent flares are comparatively rare: the majority of blazars exhibit few, if any, significant flaring episodes, and only a small fraction shows more than ten gamma-ray flares over the full monitoring period. This highlights a strong discrepancy between well-studied, highly active sources and the typical behavior of the population.
In our presentation, we will discuss the flaring properties, such as flare occurrence rates, amplitudes, and (a)symmetry for a large blazar population, thereby providing a representative view of the gamma-ray flaring statistics of blazars.

Speaker: Andrea Gokus

talk_blazar_flaring_behaviour.pdf

154 The Origin of Year-Scale QPOs in GeV Blazars through Geometric Masking: Revealing the Intrinsic Duty Cycle of Particle Acceleration

The physical origin of quasi-periodic oscillations (QPOs) in blazars remains a central debate in high-energy astrophysics, with competing scenarios attributing the year-scale modulation to either intrinsic plasma-driven instabilities or geometric effects such as jet precession. We present a comprehensive spectral analysis of two benchmark high-synchrotron-peaked (HSP) blazars, PG 1553+113 and PKS 2155-304, using approximately 18 years of Fermi-LAT data to resolve this ambiguity. By employing Singular Spectrum Analysis to isolate oscillatory components from red noise and Block Bootstrap methods for robust correlation testing, we uncover a unified phenomenological picture. Both sources display a statistically significant "softer-when-brighter" chromatic trend, a behavior atypical for HSP blazars, alongside a QPO modulation that is effectively achromatic in spectral phase. This decoupling, where fast flares are chromatic but the long-term modulation is not, strongly disfavors intrinsic emissivity models for the QPO and supports a geometric origin, where Doppler factor variations modulate the flux without altering the intrinsic electron distribution. Furthermore, in PKS 2155-304, we identify a rare spectral hardening event that is strictly phase-locked to the QPO trough. We propose a "Geometric Masking" scenario to explain this phenomenon: relativistic boosting during QPO maxima amplifies the soft spectral component (likely the jet sheath or cooled populations), effectively masking intrinsic acceleration signatures. Only during geometric troughs, when this boosting is minimized, rendering the hard spectral signatures of processes such as shocks or magnetic reconnection observable. These findings imply that extreme particle acceleration events in blazars may be nearly ubiquitous but systematically obscured by geometric viewing angles.

Speaker: Adithiya Dinesh (Universidad Complutense de Madrid & IPARCOS)

Heidelberg_gamma_Dinesh_2026.pdf

155 A blazar is a blazar is a blazar: Comparing long-term variability properties of PKS 1510-089 and PKS 2155-304

The flat spectrum radio quasar PKS 1510-089 (z=0.361) is one of the brightest sources as seen with Fermi-LAT and a regular target for Swift. This provides an extremely rich, long-term data set that sheds light on the long-term evoltuion of blazars. We have analyzed the data from the mentioned satellites spanning the years 2005-2024 to study the multiwavelength properties of this surprsing source. We have derived correlation plots between fluxes and between fluxes and spectral parameters to search for any correlated behavior. While several tracks are visible in such plots, no global correlations persist. This is also underlined by a detailed DCCF study that does not reveal any global correlation at any lag. This is surprising and implies that correlations change from event to event revealing a turbulent behavior of the jet. Furthermore, we analyse flux distributions and find that the gamma-ray and X-ray distributions are compatible with log-normal functions, while the optical distributions are compatible with two well-separated distributions. One describes the disk-dominated state, while the other describes the highly variable jet-dominated state. The fractional variability of the light curves suggest a decline in variability over the years with a first drop in 2016 and a much more pronounced one in 2021. All these findings point to a very complx long-term behavior, including that it cannot readily be explained within a single zone scenario.

Speaker: Michael Zacharias (LSW Heidelberg)

Zacharias_GAMMA_2026.pdf

156 Multi-wavelength view of 3C 279 during the 2017-2018 EHT campaigns including an unprecedented $\gamma$-ray flare

Spinning supermassive black holes at the centers of galaxies can launch powerful magnetized jets. When these jets are oriented within a few degrees of our line of sight, they are classified as blazars-active galactic nuclei that exhibit variable, non-thermal emission across the entire electromagnetic spectrum, from radio to $\gamma$-rays.

3C 279 is an archetypal blazar with a prominent radio jet showing broadband flux density variability. In April 2017 and April 2018, the Event Horizon Telescope (EHT) observed 3C 279 with an unprecedented angular resolution of $\sim$20 $\mu$as. In parallel, an extensive quasi-simultaneous multi-wavelength (MWL) campaign was carried out using both ground- and space-based observatories, covering frequencies from radio to the TeV range.

Here, we present preliminary results from the first two EHT-MWL campaigns, including the detection of a record-breaking $\gamma$-ray flare observed by Fermi-LAT. We also provide initial interpretations based on modeling the time-variable broadband emission and polarization of 3C 279.

Speaker: Ermes Aviano (University of Trieste and INFN Trieste)

EHT_MWL_3C279_γ_2026.pdf

157 Rapid gamma-ray spectral variability during an exceptional flare of PKS 1510-089

Blazars are some of the brightest objects observed in the gamma‑ray sky, but the physical processes and particle populations responsible for their emission are still not well constrained. The most powerful blazars are flat spectrum radio quasars (FSRQs), which are, however, rarely detected at very high energies (VHE ; E > 0.1 TeV) due to their usually large redshifts. PKS 1510-089 (z=0.361) is the only FSRQ regularly detected at VHE gamma-rays, even during its low state.
We report on the detection of an exceptional gamma-ray outburst in July 2019. This flare was detected at VHE with H.E.S.S. (High Energy Stereoscopic System) and is accompanied by a rich multi-wavelength dataset (optical/UV, X-rays and high-energy gamma-rays). The H.E.S.S. observations reveal pronounced intra-night variability down to a ~20 min timescale and the combination of H.E.S.S. and Fermi-LAT data shows significant gamma-ray spectral variability within a single night.
These characteristics make this flare an excellent and unique laboratory to probe the physical conditions in the emitting region and the properties of the radiating particle distribution within the jet. In this contribution, we present the details of the observational campaign together with a time-dependent leptonic modeling of the multi-wavelength dataset, allowing us to constrain properties of the environment and follow the evolution of the underlying electron population.

Speaker: Pierre Pichard (Laboratoire AstroParticule et Cosmologie - Université Paris Cité)

PICHARD_PKS1510_GAMMA26.pdf

PICHARD_PKS1510_GAMMA26.pptx

158 Highlights from VERITAS Starburst Galaxy Observations

The rising population of starburst galaxies detected by ground- and space-based gamma-ray observatories has further positioned these objects as significant contributors to cosmic-ray physics. As part of the effort to increase this population, the VERITAS Collaboration conducted a long-term program of very-high-energy (VHE; E>100 GeV) gamma-ray observations of starburst galaxies. The target selection for the sample survey is based on either high star-formation rates and central matter densities, or prior detections in the MeV–GeV gamma-ray band by Fermi-LAT.
The VERITAS program has accumulated hundreds of hours of observational data from more than ten starburst galaxies, including the prominent VHE emitter M82. Highlights from these extensive observation campaigns will be presented. By combining VERITAS measurements with multiwavelength data from instruments across the electromagnetic spectrum, we aim to provide a comprehensive understanding of the underlying emission mechanisms and cosmic-ray transport processes. These findings also provide valuable guidance for optimizing observational strategies for next generation gamma-ray observatories.

Speaker: Lab Saha (Center for Astrophysics, Harvard & Smithsonian Institution)

SBGs_Gamma_2026_Lab_Saha.pdf

159 VERITAS Observations Contemporaneous with the LHAASO Detection of NGC 4278

Significant gamma-ray emission between 1 TeV and 20 TeV from a point source, 1LHAASO J1219+2915, consistent with the location of the LINER/LLAGN galaxy NGC 4278 was recently reported by the LHAASO collaboration.
These data were later split into active and quasi-quiet states, with most of the LHAASO significance coming from the active state (MJD 59449-59589). Subsequent analysis of \textit{Fermi}-LAT and \textit{Swift}-XRT observations have been used to explore the double-peaked broad-band emission.
Models of the spectral energy distribution (SED) are currently unconstrained due to the lack of contemporaneous multi-wavelength data at either peak. Here we report serendipitous observations of NGC 4278 with VERITAS, made possible by the contemporaneous observations of the nearby blazars 1ES 1218+304, 1ES 1215+303, and W Comae, each of which are located within $2^\circ$ of NGC 4278. VERITAS did not detect any gamma-ray emission and a flux upper limit was calculated. The
flux upper limits constrain the photon spectrum of the quasi-quiet period, and together with \textit{Fermi}-LAT, indicate a peak in the SED between 100 GeV and 2 TeV.
We present an interpretation of the broadband SED that is based on acceleration of protons in the corona of the AGN, followed by p-$\gamma$ interactions and optically thin $\gamma$-ray emission. Within this framework, the implied neutrino signal is slightly below the current sensitivity of IceCube.

Speaker: Martin Pohl (GNOI)

NGC4278.pdf

Poster Session II

160 Fermi-LAT Discovery of a Unexpected Gamma-ray Outburst from the Peculiar Compact Steep Spectrum Radiogalaxy 3C 216

Only a handful of compact steep spectrum (CSS) and GHz Peaked Spectrum (GPS) radiogalaxies have been detected individually at high energy, MeV-GeV gamma-ray bands. Even a stacking analysis has not revealed a collective gamma-ray signal from CSS/GPS population. 3C 216 is a interesting CSS radiogalaxy, with relatively compact radio-band morphology and a inner relativistic radio jet, detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. Gamma-ray photons are more easily detected in blazars, where the beamed jet is closely aligned with the line of sight. Fermi-LAT observed increased gamma-ray activity from 3C 216, culminating in a unexpected, strong, rapid GeV gamma-ray outburst discovered in May 2023, during a shift of LAT gamma-ray sky watcher and flare advocate service. ToO follow-up X-ray observations are obtained by Neil Gehrels Swift space telescope. The spectral energy distribution (SED) is found dominated by a single emission zone with synchrotron self-Compton (SSC) emission, evolving coherently in time. The discovery of an intense SSC-radiation flare in 3C 216, is indicating that on sub-galactic scales, both the presence of recently injected relativistic particles, and abundant photon fields from central regions of their hosts, are present. Radio features include an upturn in the integrated spectrum around a few GHz, a compact central component with flat spectrum and a strong bend, and superluminal motions on parsec scales. Along with the non-relativistic steep spectrum radio lobes seen in projection, 3C 216 has an evident blazar core, bend and seen under a small viewing angle.

Speaker: Stefano Ciprini

161 Gamma-ray counterparts of the Fast X-ray Transients detected by Einstein Probe

The Einstein Probe mission is rapidly increasing the number of known fast X-ray transients (FXTs), opening a new window on short-lived high-energy phenomena in the Universe. A major open question is whether these FXTs represent the softer and lower-luminosity extension of the classical long gamma-ray burst (GRB) population, or whether they include events belonging to a physically distinct class of transients.

With the growing sample of Einstein Probe detections, systematic comparisons with previously known GRB populations are now becoming possible. In this work, I present a search for MeV and GeV counterparts to FXTs, aimed at assessing whether a significant fraction of these events produces detectable high-energy emission and how this emission relates to their X-ray properties. In particular, I use Fermi-GBM data to search for prompt gamma-ray counterparts, and Fermi-LAT data to place upper limits in cases with no significant detection.

These results provide a direct comparison between FXTs and the known GRB population in terms of high-energy detectability and broad-band emission properties within the synchrotron and synchrotron self-Compton (SSC) framework. If some of these transients are powered by relativistic jets, a non-detection in the LAT band can help in constraining the intrinsic jet parameters like the bulk Lorentz factor and the magnetic field.

Speaker: Ansh Chopra (Gran Sasso Science Institute)

162 Projected performance of the upgraded prototype Schwarzschild-Couder Telescope

The Schwarzschild-Couder Telescope (SCT) is a medium-sized telescope candidate for the Cherenkov Telescope Array Observatory (CTAO) featuring novel dual-mirror optics and a finely pixelated silicon photomultiplier (SIPM) camera. The prototype SCT (pSCT) with a 9.7 meter mirror was inaugurated in 2019 at the Fred Lawrence Whipple Observatory in southern Arizona and detected the Crab Nebula with a partially instrumented camera with 1536 pixels. Currently, the pSCT camera is being upgraded with improved camera electronics and achieved first light in April 2026 with a partially instrumented focal plane. The fully instrumented camera will have an 8-degree field of view and a total number of 11,328 pixels. In this talk, I will present preliminary results on performance metrics (angular and energy resolution, sensitivity) of the fully instrumented pSCT using Monte Carlo simulations analyzed using ctapipe, the official Python framework for CTAO data processing.

Speaker: Miguel Escobar Godoy (University of California Santa Cruz)

163 Probing Dark Matter and Cosmic Rays in X-COP Galaxy Clusters with CTAO

Galaxy clusters act as unique cosmological laboratories for studying cosmic-ray (CR) acceleration and Dark Matter (DM) signatures. Until now, the sensitivity of gamma-ray telescopes has been insufficient to detect their diffuse emission, but the upcoming Cherenkov Telescope Array Observatory (CTAO) offers the potential to achieve this milestone. Classical targets like Perseus cluster, featured in the CTAO Key Science Project, present complicated gamma-ray profiling challenges. The presence of a bright central active galactic nucleus (AGN) in these targets severely complicates the extraction of the underlying faint very-high-energy (VHE) signals.

In this contribution, we will present a comprehensive CTAO feasibility study targeting the XMM Cluster Outskirts Project (X-COP) sample. Like Perseus, X-COP clusters are low-redshift objects that is critical for detecting faint VHE signals. They also offer distinct observational advantages over classical targets (like Perseus): they lack a dominant, VHE bright central AGN, enabling a much cleaner spatial and spectral disentanglement of the diffuse emission. Furthermore, their intracluster medium thermal properties are exceptionally well reconstructed from joint X-ray and Sunyaev-Zel’dovich data, and their underlying mass distributions are tightly constrained through precise hydrostatic mass profiling analysis.

To fully exploit these targets, we perform detailed spatial and spectral modeling of the expected gamma-ray emission originating from hadronic interactions and potential DM annihilation or decay. We employ semi-analytic code MINOT, to accurately model the complex thermal and non-thermal components. Using the package alongside gammapy KESACCO and ctools, we simulate tailored CTAO observation setups under realistic sky background conditions.

We will present the projected CTAO sensitivity for the most promising X-COP candidates, demonstrating the observatory's capability to establish robust constraints on both CR populations and DM particle models. Our results provide detailed gamma-ray predictions for these objects, highlighting the unique value of the X-COP sample for exploring cluster physics in the upcoming CTAO era.

Speaker: Y. Emre Bahar (INAF OAS Bologna, Italy)

164 A new spectral framework for Lorentz Invariance Violation searches in blazars

Lorentz Invariance Violation (LIV) is predicted by some Quantum Gravity scenarios. LIV may lead to energy-dependent modifications to particle kinematics, affecting the threshold and cross section of $e^+e^-$ pair production, resulting in observable deviations in the absorption of very-high-energy (VHE) gamma-ray by the Extragalactic Background Light (EBL). This effect can be exploited to search for LIV signatures in the spectra of blazars by incorporating LIV-modified pair production kinematics into the computation of EBL absorption. We developed a spectral analysis framework based on broadband modelling of the intrinsic source emission using a synchrotron self-Compton (SSC) scenario, combined with an LIV-dependent EBL attenuation model. This methodology was applied for the first time to a set of multiwavelength (MWL) observations, including Fermi and LST gamma-ray data, of BL Lacertae during its flaring activity in 2022. By performing joint broadband fits, we derive constraints on LIV parameters and show the potential of this method as a complementary approach to time-of-flight analyses for searching for LIV signatures at VHE energies.

Speaker: Alberto Rosales (University of Lodz)

LIV-Spectral-Framework-Poster_v3.pdf

165 A VHE all-sky survey with H.E.S.S.

Imaging atmospheric Cherenkov telescopes have so far explored the 100 GeV–1 TeV band primarily through targeted observations and limited surveys, leaving the VHE extragalactic sky inhomogeneously sampled. In this work, we use 14 years of Fermi-LAT data to construct a homogeneous sample of candidates emitting above 100 GeV. From this sample, we select candidates that are neither associated with known TeV emitters nor have significant prior H.E.S.S. exposure.

Among the candidates, we detect VHE emission with H.E.S.S. (High Energy Stereoscopic System) at a high success rate, demonstrating the high efficiency of the Fermi-LAT–based preselection. For each detected source, we perform morphological and spectral analysis in the 0.1–100 TeV energy range using standard H.E.S.S. reconstruction and background-estimation procedures. The identified counterparts and measured spectral properties are consistent with AGN, in agreement with the blazar-dominated VHE extragalactic population. In this contribution, we present four previously unpublished VHE-emitting blazars and discuss their broadband spectral and variability characteristics.

Speaker: Dr Pranjupriya Goswami (Landessternwarte, Universität Heidelberg)

HESS_VHE-survey_.pdf

166 A Water Cherenkov Detector Array (WCDA) for cosmic-ray detection in Lake Geneva

The Lake telescope (LACTEL) project proposes the development of a Water Cherenkov Detector Array (WCDA) for the detection of high energy cosmic electron and gamma-ray induced Extensive Air Showers (EAS). A significant challenge in the characterization of such showers is the suppression of the hadronic background which is several orders of magnitude higher than the desired flux of electrons and gamma-rays. LACTEL aims to effectively suppress this hadronic background to the irreducible level of electron EAS above 10 TeV by using a two-layer detector configuration, allowing muon tagging to identify hadronic showers.

The detector array consists of several light-tight water tanks floating on the lake. Each tank has a photomultiplier tube to detect the Cherenkov light produced by the charged particles of the EAS. An elementary cell of the array is made up of two such tanks that are stacked one on top of the other. Tanks in the top layer serve to detect the electromagnetic component of the showers while the lower tanks act as a muon and hadron detector. For the photomultiplier tubes, repurposed optical modules from the ANTARES neutrino telescope (decommissioned in 2022) will be used. The design and testing of water tanks and electronics are ongoing with three prototype iterations installed in Lake Geneva, within the LéXPLORE research platform.

In this contribution results from detailed simulations of the detector array and first results of the tests conducted with the ANTARES optical modules will be presented.

Speaker: Ronald Scaria (EPFL)

LACTEL-poster-gamma2026.pdf

167 Astro-COLIBRI: An innovative tool for real-time multi-messenger transient astrophysics

Time-domain astrophysics is a rapidly growing field focused on the study of transient phenomena such as Gamma-Ray Bursts (GRBs), Fast Radio Bursts (FRBs), supernovae, novae, and AGN flares. Their characterization increasingly relies on a multi-messenger and multi-wavelength approach, combining gravitational waves, high-energy neutrinos, and electromagnetic observations across the spectrum. Such a coordinated strategy requires efficient information sharing and thus tools capable of rapidly compiling and contextualizing key data for each new event. We present Astro-COLIBRI, a well-established platform designed to meet this challenge.

Astro-COLIBRI is an advanced platform combining a public RESTful API, real-time databases, and a cloud-based alert system. It continuously listens to multiple real-time alert streams, applies user-defined filters, and places each event within its multi-messenger and multi-wavelength context. Through its user-friendly interfaces, including a web application and mobile apps for iOS and Android, the platform provides clear data visualization as well as concise summaries of key event properties and observing conditions for user-defined locations.

In this contribution, we present the main functionalities of Astro-COLIBRI, together with its architecture and data structure, which have enabled the construction of a comprehensive transient event database. We highlight recent developments driven by community needs and showcase real-world applications focused on the follow-up of transient events, demonstrating the impact of this tool in time-domain astrophysics.

Speaker: Bernardo CORNEJO (CEA - IRFU/DPhP)

168 Bounding anisotropic Lorentz invariance violation from measurements of the effective energy scale of quantum gravity

Observations of very-high-energy gamma rays provide strong constraints on Lorentz invariance violation (LIV). These constraints are typically obtained by measuring energy-dependent time delays from distant flaring events. Such effects arise from modified vacuum dispersion relations, leading to differences in the propagation times of photons emitted simultaneously from gamma-ray bursts, active galactic nuclei, or pulsars. These modifications are commonly parametrized within a phenomenological framework by an effective quantum-gravity energy scale $E_{QG,n}$. While such constraints are well established in the LIV literature, their translation into specific coefficients within alternative theoretical frameworks, such as the Standard-Model Extension (SME), is rarely carried out. In this work, we show that existing bounds on the quadratic case ($n=2$) of $E_{QG,n}$ can be systematically converted into constraints on the non-birefringent, CPT-conserving SME coefficients $c^{(6)}_{(I)jm}$. After applying the method to the most stringent limits on $E_{QG,n}$ available, we derive new limits on $c^{(6)}_{(I)jm}$ improved by one order of magnitude w.r.t. previous results.

Speaker: Merce Guerrero (University of Tartu)

169 Classification of blazar flares and their characteristic emission signatures

Blazars are one of the most extreme objects in the universe: powerful AGNs with their jets pointing towards Earth. However, their electromagnetic multiwavelength emission is not yet fully understood. While the lower energy emission is believed to be produced by relativistic leptons, the origin of the gamma-rays detected from these sources is still uncertain. In many cases, a variety of different models can fit the data equally well. Additionally, blazars are often highly variable objects.
In this work, we are investigating how the behaviour of blazar flares could help to distinguish different leptonic and lepto-hadronic emission scenarios. Assuming that all of the nonthermal emission originates from the blazar jet, we consider different physical flare origins, such as flares caused by enhancements of external radiation fields, clouds traversing the jet, changes in particle injection and magnetic fields, single zones dominating the emission, and also continuous jet models. Then we explore their observational signatures in different wavelength bands and their potential neutrino emission, compare them to each other, and investigate which characteristic features could be used to discern different scenarios and emission mechanisms. While the identification of the underlying physical scenario is often not unique and different physical mechanisms can lead to similar flare behaviour, important information can be extracted in certain cases, and the modeling of flares can help to discern the location of emission regions, the photon production mechanisms, and the jet dynamics.

Speaker: Mischa Breuhaus (Max-Planck-Institut für Radioastronomie)

heidelberg_gamma_2026_Breuhaus.pdf

170 Constraints on axion-like particles from a combined analysis of blazars observed with MAGIC and LST-1

The conversion of very-high-energy (VHE) photons into axion-like particles (ALPs) in external magnetic fields can leave observable imprints on the gamma-ray spectra of astrophysical sources, generally appearing as energy-dependent irregularities or as a spectral hardening at TeV energies. In this talk, we present one of the most extensive ALP searches to date, analyzing data from four blazars (Mrk 421, Mrk 501, BL Lac, 1ES 1959+650) for a total of over 900 hours of observations. The sources were observed over a 10-year time period, from 2014 to 2024, with the Major Atmospheric Gamma-ray Cherenkov (MAGIC) telescopes and with the first Large-Sized Telescope (LST-1) of the Cherenkov Telescope Array Observatory (CTAO). Taking advantage of the large amount of data, we are able to characterize each source during various activity states, identifying periods of approximately steady VHE emission with the Bayesian block algorithm. We perform broadband multi-wavelength fits of quasi-simultaneous spectral data for each block, aided by the Markarian Multiwavelength Data Center (MMDC) framework, from which we infer the properties of the emitting region. We then estimate the VHE photon survival probability for each source, assuming ALP-photon conversions to take place in the blazar jets and in the magnetic field of the Milky Way. Finally, we develop a modular statistical framework designed to combine data batches from different instruments and sources at the likelihood level. We use this approach to search for statistically significant evidence of ALPs; as we find none, we report our results as upper limits on the ALP-photon coupling constant over the ALP mass range between 1 and 1000 neV.

Speaker: Francesco Schiavone (INFN Bari)

171 Contribution of dark matter annihilation to the diffusive extragalactic gamma-ray background

Observations from the $\textit{Fermi}$ Large Area Telescope have constrained the diffuse extragalactic gamma-ray background (EGB) across the 0.1 -- 820 GeV energy range. While approximately half of this emission is attributed to resolved individual sources, the origin of the unresolved EGB remains an open question. Potential contributors include star-forming galaxies, active galactic nuclei, millisecond pulsars, and dark matter (DM) annihilation. Because the astrophysical baseline is highly uncertain, the DM contribution is usually bounded by the residual emission rather than evaluated directly. In this work, we calculate the expected intensity of the extragalactic DM annihilation gamma-ray background radiation based on the 20 GeV excess detected in the Galactic halo and discuss its consistency with the Fermi data, taking into account contributions from other astrophysical sources.

Speaker: Mr Junling Chen (University of Tokyo)

172 Cosmic ray acceleration by turbulent magnetic reconnection in relativistic jets

Magnetic reconnection is recognized as a promising mechanism for particle ac-
celeration in magnetically dominated astrophysical sources and may play a key
role in explaining the origin of ultra-high-energy cosmic rays (UHECRs) and the
associated very-high-energy (VHE) γ-ray emission. Particles can be energized
in reconnection layers via a first-order Fermi process, resulting from repeated
stochastic interactions with converging magnetized flows. In the presence of turbulence, which is ubiquitous in astrophysical systems, reconnection can proceed
rapidly, with rates reaching a substantial fraction of the Alfvén speed, leading
to very efficient particle acceleration. In this work, we perform 3D RMHD-PIC
simulations of relativistic jets to investigate the spectrum of high-energy par-
ticles in realistic astrophysical environments. We discuss how the properties of the
turbulent driven reconnection, the CR feedback to fluid, the diffusion of parti-
cles in the system and other related factors may impact the acceleration and
shape the particle energy spectrum and the resulting γ-ray emission.

Speaker: Petros Stefanou (IAG-USP)

173 Cosmic ray propagation across multiple scales in NGC 1333

Cosmic ray propagation within molecular clouds encompasses both diffusion in turbulent environments and ballistic streaming within dense cores, and is strongly influenced by environmental conditions. Estimates for the CR diffusion derived from gamma-ray observations typically correspond to large-scale properties, without a direct link to the CR transport on small-scales, tied to the gyro-scale microphysics. In this study, we apply a new empirical method towards obtaining CR diffusion coefficients in molecular clouds based on small-scale magnetic field properties. NGC 1333 is an active star-forming region within the nearby Perseus molecular cloud. Radio/submillimeter polarisation data obtained from both JCMT and ALMA is used to estimate the magnetic field strength and consequently derive the magnetic power spectrum, from which the diffusion coefficient is obtained. We hence compare the CR diffusion coefficient across the region and on multiple scales, as probed by both radio and Fermi-LAT gamma-ray data.

Speaker: Alison Mitchell (ECAP, FAU Erlangen-Nürnberg)

Poster_DiffusionNGC1333_Gamma2026.pdf

174 Cosmogenic Origin of KM3-230213A: Delayed Gamma-Ray Emission from A Cosmic-Ray Transient

The highest-energy cosmic neutrino detected by the ARCA detector of KM3NeT has reignited the quest to pinpoint the sources of ultrahigh-energy cosmic rays (UHECRs; $E\gtrsim 0.1$ EeV). By uncovering the associated multimessenger signals, we investigate the origin of the 220 PeV $\nu_\mu$ event KM3-230213A from an unknown transient that accelerated cosmic rays to $\sim 10$ EeV. Unlike an astrophysical origin, where the $\nu_\mu$ is produced inside the source, here we consider UHECR protons that escape the source interact with the cosmic background radiation, producing a PeV-EeV cosmogenic neutrino spectrum. The secondary $e^\pm$ and $\gamma$-rays initiate an electromagnetic cascade, resulting in a cosmogenic $\gamma$-ray spectrum. The latter peaks at a delayed time of $\gtrsim 10^4$ years compared to the light travel time from the transient to observer, due to deflection of charged particles in the extragalactic magnetic field (EGMF). Our results shed light on the nature of the UHECR source for the $\nu_\mu$ event and provide crucial insights into the detection of multi-TeV $\gamma$-rays of cosmogenic origin from similar past cosmological transients. Using the $\gamma$-ray sensitivity of currently operating and next-generation imaging atmospheric Cherenkov telescopes, the flux and time-delay distribution can constrain the source distance. We further show that the detection of such a $\gamma$-ray signal above the background depends on the EGMF strength. Together with the non-detection of coincident spatial or temporal photon counterparts at the current epoch, this detection is the first compelling candidate for a sub-EeV cosmogenic neutrino.

Speaker: Sovan Boxi (Raman Research Institute)

175 Data-driven modelling of Fermi-LAT PSF using pulsed emission from pulsars

Blazars emit high-energy photons at the TeV scale that travel through the intergalactic medium toward the observer. These primary gamma rays interact with the Extragalactic Background Light (EBL) to create electron-positron pairs, which then upscatter Cosmic Microwave Background (CMB) photons to GeV energies. Because these pairs are deflected by the intergalactic magnetic field along the line-of-sight, the secondary photons arrive at a non-zero angle, potentially creating an observable extended emission called "halo" around the point source. Detecting these halos requires extremely precise modelling of the telescope's Point Spread Function (PSF) to correctly distinguish the faint extended emission from the bright central point source.
In this talk, we present a new data-driven model of the Point Spread Function (PSF) of the Fermi Large Area Telescope (LAT) using pulsar observations. We compare our model to the current Fermi-LAT PSF and apply it to study the possible presence of a halo around one of the most energetic blazars, Mrk 501. While the standard PSF model favours the detection of an extended emission, we find no evidence of a halo around Mrk 501 when using the revised PSF model.

Speaker: Jeffrey Blunier (APC Paris)

176 Deep learning detection of gamma-ray sources in Fermi-LAT data

We present a novel end-to-end deep-learning framework for gamma-ray source detection and characterization that addresses key limitations of traditional likelihood-based analyses, which are strongly affected by large and uncertain diffuse gamma-ray backgrounds, particularly near the Galactic plane. Our approach replaces explicit background modeling with a data-driven strategy, combining a U-Net-based segmentation network with dedicated deep neural networks for source localization, flux estimation, and probabilistic validation. Trained on realistic simulations of ten years of Fermi-LAT observations, the method demonstrates robust performance across different interstellar emission models, achieving completeness and precision comparable to established likelihood-based catalogs. Moreover, when applied to real data, the resulting gamma-ray source catalog is consistent with the Fermi-LAT catalog for the same observational period.This work establishes a scalable and model-independent framework, paving the way for next-generation applications of deep learning in gamma-ray source detection and characterization.

Speaker: Giacomo Principe (University of Trieste - INFN Trieste)

Poster_DNN_for_Fermi_source_detection_Principe.pdf

177 Design and Simulation of RubikSat: A CubeSat-Based 3D Gamma-Ray Detector

Space observatories such as Fermi Gamma-ray Space Telescope and AGILE have revolutionized gamma-ray astronomy, enabling major discoveries in both steady and transient phenomena. At the same time, advances in CubeSat technologies are significantly increasing access to space, opening new opportunities for cost-effective high-energy astrophysics. In this context, we present RubikSat, a modular payload concept based on a three-dimensional array of scintillating elements (“cube of cubes”), designed for the detection of high-energy gamma radiation and charged cosmic particles and compatible with a 3U CubeSat platform. A preliminary design is currently under investigation, consisting of 27 cubic scintillators (30 × 30 × 30 mm³ each) arranged in a compact 3D geometry and individually read out by Silicon Photomultipliers (SiPMs, 6 × 6 × 1.3 mm³). The scintillating core is surrounded by six plastic scintillator tiles (120 × 120 × 10 mm³), enabling particle discrimination through coincidence and anticoincidence logic. We employed Geant4 simulations to investigate the detector response and to guide the optimization of the design. Different scintillator configurations, including both CsI(Tl), NaI(Tl) and CeBr3, are currently under study, and their performances are being systematically compared. Preliminary results on the point spread function indicate an angular resolution on the order of tens of arcseconds at 662 keV. This configuration represents a reference starting point for further optimization. Ongoing work focuses on extending the simulations to the veto system, exploring a broader energy range, refining the position-reconstruction pipeline, and timing performances. This approach enables systematic design-space exploration and paves the way for optimized CubeSat-based instruments and, ultimately, constellations capable of continuous monitoring of steady and transient gamma-ray phenomena, complementing existing large-scale observatories.

Speaker: ANTONIO ZACCARO (Università degli Studi di Bari Aldo Moro - INFN Bari)

ZACCARO_Rubiksat_Poster.pdf

178 ExHaLe-jet: Modeling blazar jets with an extended hadro-leptonic radiation code

Blazars emit across all electromagnetic wavelengths. While the so-called one-zone model has described well both quiescent and flaring states, it cannot explain the radio emission and fails in more complex data sets, such as AP Librae. In order to self-consistently describe the entire electromagnetic spectrum emitted by the jet, extended radiation models are necessary. Notably, kinetic descriptions of extended jets can provide the temporal and spatial evolution of the particle species and the full electromagnetic output. Here, we present the initial results of a newly developed hadro-leptonic extended-jet code: ExHaLe-jet. As protons take much longer than electrons to lose their energy, they can transport energy over much larger distances than electrons and are therefore essential for the energy transport in the jet. Furthermore, protons induce injection of additional pairs through pion and Bethe-Heitler pair production, which can explain a dominant leptonic radiation signal while still producing neutrinos. In this talk, we discuss the differences between leptonic and hadronic dominated SED solutions, the SED shapes, evolution along the jet flow, and jet powers. We also highlight the important role of external photon fields, such as the accretion disk and the BLR.

Speaker: Michael Zacharias (LSW Heidelberg)

ExHaLe_poster_Gamma26.pdf

179 Exploring the Magnetic Field in Relativistic Jets with TeV Gamma rays and X-ray Polarization

The Imaging X-ray Polarimetry Explorer (IXPE) is an X-ray telescope able to constrain linear polarization in the 2-8 keV band. Particularly good targets for IXPE are blazars whose synchrotron emission peaks in or around the X-ray band (called high-synchrotron peaked blazars, or HSPs). Polarization measurements of the synchrotron radiation from HSPs can be used to infer the magnetic field structure in jets. Moreover, in HSPs, the same electrons responsible for the synchrotron emission observed in the X-ray band can also cool via inverse-Compton scattering. This produces TeV gamma-ray emission detectable with observatories like the Very Energetic Radiation Imaging Telescope Array System (VERITAS). Modeling the resulting broadband spectral energy distribution of HSPs provides an additional test for the magnetic field, specifically providing an estimate of the magnetic field strength. Simultaneous IXPE and TeV observations of HSPs, such as Mrk 421, provide tight constraints on the jet magnetic field intensity and geometry, probing the particle acceleration and cooling mechanisms. Using particle-in-cell simulations, we show that the observed dynamics of the X-ray polarization can be explained as the radiative output of multiple emitting cells in the jet, powered by magnetic reconnection. Then, we investigate how reconnection can produce a spectral energy distribution consistent with the observed X-ray and TeV flux.

Speaker: Benjamin de Jonge (Washington University in St. Louis)

180 Exploring Very-High-Energy Emission in the Perseus Cluster: Multiwavelength Studies of NGC 1275 and IC 310

In contrast to blazars, which dominate the Very-High-Energy (VHE; E>100 GeV) sky, radio galaxies (RG), observed at large jet-viewing angles, are difficult to detect at VHE, leading to a bias toward nearby sources. Their rapid variability and strong flaring activity are difficult to reconcile with modest Doppler boosting, suggesting alternative emission mechanisms. We present a multi-wavelength study of NGC 1275 and IC 310, two VHE-emitting RG in the Perseus cluster, focusing on recent flaring states.

NGC 1275 (3C 84; z = 0.01756), the Brightest Cluster Galaxy in Perseus, has a history of multiwavelength observations and evolving parsec-scale radio morphology. We construct broadband SEDs of the December 2022 and January 2023 flares using contemporaneous VERITAS, Fermi-LAT, NuSTAR, Swift-XRT and Swift-UVOT observations. We model the Compton SED peak with a log-parabola and a power-law with exponential cutoff during each of the flaring states to probe spectral curvature evolution. The observed hour-scale VHE variability and broad Compton peak disfavor a single-zone synchrotron self Compton (SSC) scenario, motivating a multi-zone SSC framework.

IC 310 (z = 0.0188), located at the outskirts of the Perseus cluster, exhibits extreme TeV variability on minute timescales. Its classification remains unclear between a blazar and a narrow-angle tail radio galaxy with jets bent back via ram pressure. On March 8th, 2024, LHASSO observed IC 310 in an extreme flaring state. VERITAS conducted follow-up observations through April 4th, 2024 and during this period initiated a multi-wavelength campaign with NuSTAR and the VLBA. Along with the radio observations which reveal the emergence of two new knots, our results suggest complexity beyond a strictly single-zone interpretation.

Speaker: Lucy Fortson (GNOI)

181 Flat-spectrum radio quasars as high-energy neutrino sources

The astrophysical sources responsible for high energy neutrino production remain a major puzzle, with AGN standing out as primary candidates. However, the specific emission sites within these accreting supermassive black hole systems, such as relativistic jets versus the accretion disk or corona, are still debated.
To investigate this, we cross-matched well localized neutrino events from the IceCat-1 catalog with a large, optically selected sample of quasars from the SDSS. Using radio data from the CLASS survey, we separated this population into flat-spectrum radio quasars (FSRQs) and radio-quiet (RQ) quasars. Our results show that while positional matches for RQ quasars are consistent with chance coincidences, FSRQs exhibit a significant correlation.
Notably, for events at declinations above 20 degrees, the correlation reaches a significance of 4 sigma. For the most promising candidates, we extracted the neutrino spectra from IceCube data and determined their multiwavelength SEDs from public observations.
These data are compared with theoretical models obtained through numerical simulations of the radiative interactions of cosmic-ray electrons and protons accelerated within the relativistic jet, employing a consistent one-zone framework. These findings indicate that over 60% of IceCube’s astrophysical neutrinos could originate from FSRQs, providing possible evidence that the emission is mainly driven by relativistic jets rather than the accretion flow.

Speaker: alberto moretti (INAF)

182 From a force-free, Poynting dominated jet solution to blazar emission: application to Markarian 421

According to the leading theoretical paradigm for the launching of Active Galactic Nuclei (AGN) jets, outflows of relativistic plasma are produced near the black hole as highly magnetized and remain Poynting-flux dominated out to the distances where optical and X-ray emission are generated.

I will present an axisymmetric, stationary model of a Poynting-dominated jet in which the electromagnetic field configuration is fully ordered (i.e., non-turbulent) and determined by the jet shape, which in turn depends solely on the pressure profile of the external confining medium.

I will show that this model can reproduce the multifrequency polarization properties of high-synchrotron-peaked (HSP) blazars, where both the optical and X-ray emission originate from synchrotron radiation by relativistic electrons.

I will extend this model to compute the full broadband spectral energy distribution (SED), assuming synchrotron self-Compton (SSC) scattering as the mechanism responsible for the high-energy component of the SED.

The novelty of my approach lies in the self-consistent computation of the SED for a structured jet. I will assume a stationary, multi-zone emission region in which the jet electromagnetic fields are derived self-consistently from the analytical model of magnetically dominated outflows introduced above. This framework reduces the number of free parameters required to fit observational data, as key quantities such as the bulk Lorentz factor and Doppler factor are directly determined by the jet electromagnetic field.

I will apply this scenario to a specific source, the HSP blazar Markarian 421, and investigate how the best-fit parameters differ between a multi-zone emission region and a standard single-zone scenario, in which the emission originates from a compact feature (i.e., blob) propagating within the jet electromagnetic field.

Speaker: Filippo Bolis (INAF-OAB & Università degli Studi dell'Insubria)

183 Galactic Cosmic Ray Transport in the Giant Circumgalactic Medium Halo

Recent observations revealed a substantial reservoir of gas in the circumgalactic medium (CGM), extending out to hundreds of kiloparsecs. This discovery has sparked growing interest in how cosmic rays(CR) propagate throughout the Galactic halo and the consequence of their interactions with this extended gas component. In this work, we investigate the possibility of diffusive CR transport in the extended Galactic halo and show the resulting differences from the traditional leaky-box and slab models. We find that when the halo size is sufficiently large($\rm \gtrsim 40 \, kpc$), the results become insensitive to the halo size. Moreover, such large-halo models remain compatible with current observational constraints for reasonable values of the diffusion coefficient.

Speaker: Chaoming Li (DESY)

184 Gamma-ray halos around blazars through the eyes of CTAO

In a near future, the Cherenkov Telescope Array Observatory (CTAO) will be operational to map the sky in very-high-energy gamma rays with unprecedented sensitivity. Amongst its source targets are blazars. In particular, we are looking for the observational evidence of the presence of a gamma-ray halo around blazars also called "pair halos". These halos would be due to the development of electromagnetic cascades in the intergalactic space, because the primary gamma rays from the blazars produce e+/− pairs when interacting with EBL photons and these particles produce further gamma rays via inverse Compton scattering with ambiant photons, such as CMB photons. In this contribution, I present the results of simulations of such pair halos using the CRPropa software, and some prospects for their detectability with CTAO using gammapy simulations.

Speaker: Armelle Jardin-Blicq (LP2i Bordeaux)

185 GeV γ-ray emission in the field of the shell-type supernova remnant Vela Jr revisited

We present an updated analysis of the GeV γ-ray emission from the shell-type supernova remnant RX J0852.0−4622, also known as Vela Jr, using 15 years of Fermi Large Area Telescope data. We quantitatively model the GeV morphology and find that it is best described by a masked H.E.S.S. shell template, indicating that the embedded pulsar wind nebula contributes little to the GeV flux. The 0.1–500 GeV spectrum is well fitted by a hard power law with a photon index of 1.77 ± 0.03 and connects smoothly to the TeV spectrum, confirming previous results with improved precision. We further construct an independent eROSITA shell template and derive the 1–5 keV X-ray spectral energy distribution of the whole remnant, which provides new constraints on the synchrotron emission. We model the multiwavelength spectral energy distribution with both a pure leptonic model and a hybrid lepton–hadron model. While the pure leptonic model reproduces the overall broadband shape, the hybrid model provides a better statistical description of the same data set, supporting a mixed-origin picture in which the hadronic contribution is mainly relevant in the GeV band, whereas the TeV emission remains predominantly leptonic.

Speaker: Mr Tingting Ge (Sun Yat-sen University)

186 How can M87 flare so brightly at TeV energies?

Super-fast (~day), very-high-energy (VHE; >0.1 TeV) gamma-ray flares from M87, the only active galactic nucleus with a jet resolved almost down to the black hole, provide a unique opportunity to advance our understanding of particle acceleration in black hole driven jets. Despite many ideas, no firm conclusion on the physical origin has been reached, mainly because neither the TeV angular resolution (~size of the entire galaxy), nor the short duration (implying a small emission region of ~10 gravitational radii) constrain the flaring location along the jet. We present here a new strategy of using the multi-wavelength (MWL) data available with much higher angular resolution at other energies to model the jet properties that critically affect the particle acceleration and radiation spectra. The most promising dataset is the 2018 Event Horizon Telescope (EHT) MWL campaign published in 2024, capturing a VHE flare with unprecedented energy coverage including cm/mm-VLBI, EHT, Swift/Chandra/NuSTAR, Fermi-LAT and H.E.S.S./MAGIC/VERITAS. We explore a new two component model: First, a large-scale jet component matching the radio to X-ray spectra and morphology, described by a steady-state multi-zone jet model. Second, a time dependent, single-zone flaring component capable of modeling the excess gamma-rays, with properties linked to the steady-state jet, including background photon fields. This work is also particularly relevant in anticipation of the results from the ongoing 2-month EHT/MWL/VHE movie campaign (Spring 2026), with greater temporal and spectral coverage to better reveal the physics of M87’s super-fast VHE flares.

Speaker: Marc Klinger-Plaisier (University of Amsterdam)

187 Identifying Gamma-ray Sources with Image Processing for Wide-Field Gamma-ray Survey Instruments

The High Altitude Water Cherenkov (HAWC) Observatory continuously surveys the very-high-energy gamma-ray sky from 300 GeV to beyond 100 TeV. With its wide field of view, HAWC is particularly well suited for studying extended emission regions that may contain multiple point-like and diffuse gamma-ray sources. HAWC's current blind search methods for identifying sources, which are adapted from Fermi-LAT algorithms, rely on iterative likelihood procedures to seed and fit new sources. Likelihood methods are known to computationally scale poorly and often require compute resources beyond what is reasonably available. On HAWC, our computational scaling limits the size of fit regions to a few degrees. In this work, we present a faster, source detection method based on an unsupervised image-processing framework. Our method applies a Difference of Gaussian's filtering technique to all sky test-statistic maps then follows with a blob detection algorithm leveraging the Determinant of Gaussian method to generate accurate candidate source locations. Our image analysis technique yields a compute speedup that is 300 times faster than the currently utilized HAWC search pipeline. The source candidates are subsequently seeded into a multi-threaded likelihood fitter to refine the source localization and spectral parameters. We assessed the performance of the improved pipeline and discuss its potential application to other astrophysical datasets, including the public HGPS and Fermi-LAT survey.

Speaker: Mehr Nisa (Michigan State University)

188 Investigating the Hadronic Origin of VHE Emission in HBL Blazars: Spectral Modeling vs. Energetic Constraints

The origin of Very High-Energy (VHE) gamma-ray emission in High-frequency
peaked BL Lac objects (HBLs) remains an open question. While standard leptonic models successfully reproduce the low-energy synchrotron component, they often struggle to account for the observed hard TeV spectra without invoking extreme parameter choices. This has led to increasing interest in lepton-hadronic processes, particularly the unavoidable interactions between relativistic protons and ambient
photon fields within the emission region.
In this work, we investigate a hybrid scenario in which the target photon field is provided by shock-accelerated electrons emitting synchrotron radiation, and high-energy protons undergo proton–photon interactions. Applying this model to the characteristic HBL spectral energy
distributions, we examine whether such interactions can naturally explain the observed VHE spectral hardening.
We find that although proton–photon interactions can, in principle, reproduce the hard TeV emission, the required jet power is extremely large, often reaching highly super-Eddington levels. This poses a significant challenge to leptohadronic interpretations and highlights a fundamental tension between spectral modelling and energetic constraints.
Our results suggest that while proton–photon interactions are inevitable in such environments, their role in producing the observed VHE hardening remains unclear, pointing to the need for alternative or additional mechanisms in explaining blazars emission.

Speaker: Tagtshen Tamang (TIFR Mumbai, Tata Institute Of Fundamental Research)

189 Jet-driven Molecular Cloud Formation in the γ-ray Binary LMC P3: Implications for Cosmic-ray Acceleration by Microquasars

The origin of cosmic rays (CRs) remains one of the most important open questions in astrophysics. Microquasars are considered as efficient CR accelerator due to their ejection of powerful jets. The recent detections of ultra-high-energy γ-ray emission from these systems provides a direct observational evidence of particle acceleration. However, the physical mechanisms underlying CR acceleration in these systems remain elusive. Interaction of jet from microquasars with the surrounding interstellar medium (ISM) is a leading candidate process, capable of accelerating protons to CR energies via hadronic interactions. This makes a detailed study of the gaseous environment surrounding microquasars essential for advancing our understanding of their role as CR accelerators.

Here we present ALMA CO observations of the environment surrounding LMC P3, a γ -ray binary in the Large Magellanic Cloud consisting of a compact object and an O5-type stellar companion, embedded within the supernova remnant DEML241. We report the discovery of a remarkable one-sided, jet-like molecular CO structure extending ~8 pc in length that shows strikingly alignment with LMC P3 (see attached figure). Such jet cloud has sinusoidal velocity structure along the jet axis further supports its origin in the system's relativistic jet. The kinetic temperature of jet-like cloud is found to be significantly higher (33-80 K) compared to non-jet clouds (15-16 K), consistent with shock heating by jet.

Our results provide the first direct observational evidence for jet-driven molecular cloud formation by a microquasar, supporting the accretion-powered jet scenario as the origin of very-high-energy emission in γ-ray binaries. These findings establish LMC P3 as a strong extragalactic candidate for microquasar-driven CR acceleration, offering new empirical support for the role of microquasars as significant contributors to the cosmic-ray budget.

Speaker: Bhuvana Gadikere Rajendra (Gifu University)

ALMA Observations of DEM L241/LMC P3 in the Large Magellanic Cloud: Evidence for the Formation of Cool Molecular Jets Driven by a Microquasar

poster_gamma_conf.pdf

190 Joint spectral and population inference of Galactic pulsars and extragalactic sources in the Fermi-LAT sky with simulation-based inference

Since its mission started more than 17 years ago, the Fermi Large Area Telescope (LAT) has significantly advanced our view of the GeV gamma-ray sky, yet several key questions remain - such as the nature of the isotropic diffuse background, the properties of the Galactic pulsar population, and the origin of the GeV excess towards the Galactic Centre. Addressing these challenges requires sophisticated astrophysical modelling and robust statistical methods capable of handling high-dimensional parameter spaces.
Building on our previous single-population, single-energy-bin analysis of Fermi-LAT data using simulation-based inference (SBI), we present a multi-bin spectral extension of the gamma-ray emission simulator. The updated framework simultaneously models two source populations, Galactic pulsars and extragalactic point sources, with distinct spatial distributions and intrinsic spectral variability informed by the empirical spread observed in detected members of each respective class. This richer forward model enables joint inference of population-level quantities on a per-class basis, including luminosity functions and source-count distributions, alongside individual source detection per class. We present preliminary results that investigate the extent to which SBI can disentangle the two populations and recover their respective spectral and spatial properties from synthetic observations.

Speaker: Christopher Eckner (Instituto de Astrofísica de Canarias (IAC))

poster_Heidelberg_2026_Eckner.pdf

191 Learning to Remove Interstellar Emission via Latent Space Alignment

The Fermi Large Area Telescope (Fermi-LAT), operating since August 2008, has recently released an updated catalog of gamma(γ)-ray sources containing over 7000 γ-ray sources based on 14 years of observations. One of the biggest challenges in analyzing the γ-ray sky is the uncertainty of the interstellar emission model (IEM). This diffuse background significantly complicates the detection and characterization sources.

In this work, we propose a deep learning framework to model the IEM from γ-ray observations. We use simulated Fermi-LAT datasets including source populations from two dominant classes: Active Galactic Nuclei and pulsars. Our approach consists of two stages. First, a U-Net-based network is trained exclusively on background-only simulations to reconstruct the IEM, thereby learning a latent representation of the background emission. In the second stage, a multilayer perceptron is trained to map latent representations of full (signal + background) images to the corresponding representation of the background in the latent space through latent space alignment. The predicted background latent representation is then passed through the decoder to reconstruct the diffuse background.

This approach provides an interpretable representation of latent space that may be useful for identifying anomalous cases that deviate from the learned source-background relations, e.g., due to new components in the diffuse emission, such as dark matter annihilation.

Speaker: Sanja Dumenčić (University of Nova Gorica)

Poster_Gamma_2026.pdf

192 Legacy analysis of the dwarf spheroidal galaxies of the Milky Way: an update

Dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are targets of great interest for searches of Dark Matter (DM) signatures with the Fermi-LAT. In the last decade the number of detected and putative dSphs has been rapidly increasing, allowing for some of the most stringent constraints to be put on models of annihilating DM in the GeV-TeV range. The most recent results even highlight the presence of local significance excesses at the 2-3 sigma level.
With the recent observations of ultra-faint compact stellar systems (UFCSs), that might be the darkest galaxies ever observed, and the predictions on the upcoming results of the Legacy Survey of Space and Time (LSST), which poses to double or more the sample of known dSphs over the next decade, we stand before a fundamental moment for gamma-ray searches of DM signatures.
In this work, we apply key improvements to the analysis of the dSphs. We use stricter cuts on the data, implement a method to adaptively model the background, and assume an updated framework for DM annihilation. We find that our improved background modeling leads to a better agreement between the model and the data. This produces an increase in the local and global significance of the dSphs excess compared to previous studies. Finally, we find that the DM properties obtained in this work are less dependent on the sample of dSphs being considered, while remaining in agreement with the predictions from the Galactic center excess observed by theFermi- LAT and the antiproton excess observed by the Alpha Magnetic Spectriometer (AMS-02).

Speaker: Antonio Circiello (Clemson University)

193 Long-term joint analysis of M87 with MAGIC and VERITAS

The famous radio galaxy M87 is one of the most well studied extragalactic sources in the gamma-ray sky and has been the subject of many multi-wavelength observation campaigns. Despite extensive study, the origin and production mechanism of the gamma rays from M87 remains elusive. We present over 550 hours of data taken during a decade-long (2013-2022) very-high-energy ($E\gtrsim 100$ GeV) campaign using MAGIC and VERITAS. This very-high-energy data is complemented by a decade of high-energy ($100$ MeV $\lesssim E \lesssim 100$ GeV) Fermi-LAT observations taken over the same period. A joint analysis using Gammapy of this multi-instrument dataset was performed, revealing M87 to be mainly in a quiescent state. We present the results of this analysis, including the combined light curve and spectral evolution, and use them to constrain the underlying physical processes.

Speaker: Nicki Bond (University College Dublin)

NickiBondPoster.pdf

194 Long‑term multi‑wavelength SED and variability modeling of the HSP BL Lac PKS 2155-304

PKS 2155-304 is a nearby high‑synchrotron‑peaked (HSP) BL Lac blazar at redshift z ≈ 0.116, and a benchmark source for studying multiwavelength variability in relativistic jets. In this work, we present a time‑dependent modeling study of its spectral energy distributions (SEDs) across the electromagnetic spectrum, using data from H.E.S.S. (very‑high‑energy gamma rays, 2003-2014), Fermi‑LAT (high‑energy gamma rays), Swift‑XRT and UVOT (X‑ray to UV), SMARTS (optical to near‑infrared), Stewarts Observatory, SALT, and Las Cumbres Observatory (LCO) optical/UV facilities (all covering the period from August 5, 2008, MJD 54683, to December 31, 2018, MJD 58483). This long‑baseline coverage enables a detailed view of its quiescent states, multiwavelength flares, and orphan‑flare episodes.
We construct time‑resolved SEDs at multiple epochs and apply a time‑dependent Synchrotron Self‑Compton (SSC) framework in which the relativistic electron population evolves according to injection, acceleration, and radiative cooling. By varying physical parameters such as magnetic field strength, Doppler factor, electron injection rate, and emission‑zone size, we investigate how different activity states - quiescence, gamma‑ray‑dominant, X‑ray‑dominant, and optical‑UV‑dominant flares - emerge from changes in the underlying jet physics. The modeling allows us to derive characteristic acceleration and cooling time‑scales and to assess the degree to which a single emission zone can account for the observed broadband variability.
Our time‑dependent SSC modeling can reproduce the general spectral shapes and flux evolution of PKS 2155-304 across the various wavelength regimes over MJD 54683-58483. The analysis highlights PKS 2155-304 as an excellent benchmark for constraining jet dynamics through time‑resolved SED modeling, and provides a baseline for interpreting future observations with next‑generation facilities such as the Cherenkov Telescope Array and the MAGIC telescope.

Speaker: Shruti Mukherjee (The University of the Free State)

gamma2026_shruti.pdf

195 Markarian 421 with SST-1M: Multi-Year Observations and Analysis

Markarian 421 (Mrk 421) is the closest observed high-frequency-peaked blazars (z = 0.031), and one of the brightest GeV-TeV gamma-ray persistent emitters, also exhibiting frequent flares. Despite extensive multi-wavelength observations, the mechanisms responsible for its emission at the highest gamma-ray energies remain not fully understood, primarily due to the limited sensitivity of current instruments in the multi-TeV regime.

The SST-1M stereo system of two innovative small-sized, single-mirror Imaging Atmospheric Cherenkov telescopes, currently located at the Ondřejov Observatory (Czech Republic), provides enhanced sensitivity in the energy range from ~1 TeV up to several hundred TeV. As part of a dedicated long-term monitoring program, Mrk 421 has been observed with the SST-1M stereoscopic system over multiple campaigns between 2024 and 2026.

These observations have yielded a substantial dataset of stereoscopic events, enabling detailed characterization of the source spectrum and variability, the development of a more accurate physical model of the source, and validation of the SST-1M instrument performance under real observational conditions. In this contribution, we present an overview of the analysis framework developed for SST-1M data, along with a general characterization of the observed emission from Mrk 421. Particular emphasis is placed on the instrument’s capability to probe the multi-TeV regime and its implications for studying blazar emission at the highest energies.

Speaker: Srija Reddy Muthyala (PhD student)

196 Modelling the high-energy emission of the Vela pulsar as synchro-curvature radiation

In an ongoing study we interpret the curved spectrum of the Vela pulsar as seen by H.E.S.S. II (up to 100 GeV) and the Fermi Large Area Telescope to be the result of synchro-curvature radiation due to the acceleration of primary particles in a dissipative magnetosphere, within an extended separatrix region that leads into the current-sheet outside the light cylinder. We investigate the high-energy emission properties via energy-dependent light curve and phase-resolved spectral modelling, using the accelerating (azimuthally dependent) electric field from global magnetospheric simulations. We expect our model to reproduce the observed trends, i.e., decrease of the flux of the first peak relative to the second one, evolution of the bridge emission, near-constant phase positions of peaks, and narrowing of pulses with increasing energy, relatively well. We will compare the predicted energy-dependent light curves and phase-resolved spectra with the observations from the Vela pulsar, expecting an improved phase lag between the radio and gamma-ray light curves upon updating the electric field description compared to our previous work.

Speaker: Dr Monica Barnard (Centre for Space Research, North-West University, Potchefstroom 2520, South Africa)

197 Multi-Instrument Simulations of Gamma-Ray TeV Halo Morphology around Middle-Aged Pulsars with CTA, SWGO, and Fermi-LAT

Middle-aged pulsars and their evolved pulsar wind nebulae are promising sources of extended gamma-ray emission in the form of TeV halos. At this evolutionary stage, relativistic electrons and positrons can escape the nebula and diffuse into the surrounding interstellar medium, producing extended emission whose morphology depends on particle transport, pulsar proper motion, cooling, and the angular response of the observing instrument. Previous studies and recent observational results suggest that such systems may exhibit asymmetric or offset morphologies, and that the apparent halo structure can change significantly from GeV to TeV energies.

Here we present the expected morphology of gamma-ray halos around middle-aged pulsars through dedicated simulations using the software frameworks of the Cherenkov Telescope Array (CTA) and the Southern Wide-field Gamma-ray Observatory (SWGO), together with corresponding simulation and/or real data from Fermi-LAT at GeV energies. The goal is to understand how energy-dependent particle transport and instrument specific observational effects shape the apparent spatial distribution of halo emission, and to assess whether the same underlying source population can produce measurable morphological differences between the GeV and TeV bands. Special attention will be given to extended structures, pulsar-halo offsets, and possible multiwavelength counterparts relevant to middle-aged pulsar systems.

By comparing simulated morphologies across instruments with differing point-spread functions, fields of view, and energy ranges, this work provides a framework for interpreting future detections and identifying promising halo candidates in joint GeV–TeV studies. The results will help clarify the observational signatures expected from evolved pulsar systems and contribute to population studies of gamma-ray halos associated with middle-aged pulsars.

Speaker: Asu Nisa Ünver (LMU)

198 Multi-TeV Observations of VER J2019+368 and CTB 87 with the SST-1M Stereoscopic System

Since 2024, the Single-Mirror Small-Size Telescope (SST-1M) stereoscopic system located at the Ondřejov Observatory (Czech Republic) has been conducting an observational campaign of the Cygnus region in the multi-TeV regime. With a wide 9° field of view, the SST-1M telescopes are well suited for studies of extended sources and complex regions with multiple very-high-energy emitters. In this contribution, we present a detailed analysis of more than 150 hours of high-quality data, focusing on VER J2019+368, one of the brightest and hardest very-high-energy gamma-ray sources in the northern sky, and the evolved pulsar wind nebula CTB 87 (G74.9+1.2), associated with PSR J2016+3711. Our results include a three-dimensional morphological and spectral analysis of both sources. We also discuss these findings in the context of their emission mechanisms and evolutionary stages, aiming to constrain possible scenarios for the origin of the observed gamma-ray emission.

Speaker: Ana Laura Müller (FZU - Institute of Physics of the Czech Academy of Sciences)

199 Optimizing CTAO’s divergent pointing configurations for poorly localised GRBs

Gamma-ray bursts (GRBs) are among the most energetic and enigmatic transient phenomena in the Universe, originating either from the collapse of massive stars or from the merger of compact-object systems, such as neutron star–neutron star binaries. These short-lived, extragalactic, and unpredictable events pose significant challenges for observations at very-high energies (VHE; i.e., E > 20 GeV), particularly due to their rarity and often poor initial localization.
The Cherenkov Telescope Array Observatory (CTAO), with its unprecedented sensitivity and wide energy coverage from approximately 20 GeV to 300 TeV, will provide a unique opportunity to increase the number of GRBs detected at VHE to study their characteristics. While the standard pointing mode of CTAO is optimized for targeted observations, alternative strategies, such as divergent pointing, can substantially increase the instantaneous field of view. This makes divergent pointing particularly well-suited for GRB follow-up observations, where large sky regions must be covered rapidly.
In this contribution, we present simulation-based studies of divergent pointing configurations optimized for poorly localized GRBs and for follow-ups of multi-messenger events, such as the gravitational-wave counterpart of binary neutron-star (BNS) mergers, and we assess the impact of this strategy on CTAO array performance and GRB detection prospects. Our preliminary results indicate that the off-axis sensitivity of the divergent pointing configuration can surpass that of the parallel configuration, and its effective area is enhanced at low energies for large offset angles, while the on-axis performance remains better in the parallel pointing mode, thereby reinforcing the suitability of this approach for transient-driven observations.

Speaker: Jahanvi Jahanvi (University of Udine-INFN Trieste)

200 Particle Acceleration and Emission Signatures in Relativistic large-scale AGN Jets

Relativistic jets from active galactic nuclei (AGNs) are among the most energetic phenomena in the universe, extending over kilo-parsec scales. These jets develop complex structures through different MHD instabilities and turbulence, which strongly influence non-thermal particle acceleration in these systems. In this work, we explore jet-driven turbulence as a site for stochastic (second-order Fermi) acceleration in regimes where strong shocks are absent and magnetization is sufficiently high for stochastic processes to dominate. We develop a semi-analytical framework to model turbulent acceleration, investigating how different prescriptions for momentum diffusion coefficients and particle escape timescales influence energy gain and resulting particle spectra. Our preliminary results from solving the particle transport equation show that stochastic acceleration can efficiently energize particles to X-ray-emitting energies. These high-energy particles can subsequently produce even higher-energy emission via Inverse Compton processes. This framework is highly relevant for interpreting the high-energy emission observed from large-scale jets in nearby radio galaxies such as M87 and Centaurus A. By coupling our acceleration model with numerical simulation and synthetic synchrotron emission, we aim to examine how variations in diffusion physics translate into observable signatures in AGN jets. This approach provides new constraints on the microphysical processes governing particle acceleration in large-scale relativistic outflows.

Speaker: Nikita Nikita (Max Planck Institute for Plasma Physics)

poster_gamma.pdf

201 Particle Acceleration in Collisionless MRI Turbulence in Stratified Accretion Disks

The magnetorotational instability (MRI) is one of the primary mechanisms responsible for driving turbulence and enabling efficient outward transport of angular momentum in astrophysical accretion disks. In low-luminosity systems around black holes—such as Sgr A at the center of the Milky Way and M87—the plasma is hot and tenuous, with infrequent particle–particle interactions, rendering the flow effectively collisionless. Under these conditions, a variety of nonthermal phenomena can take place, including different temperatures between ions and electrons, pressure anisotropy, and particle acceleration. Understanding these processes is essential for interpreting current and future observations with instruments such as the Event Horizon Telescope, GRAVITY, and CTA.

We use particle-in-cell simulations to study the development and evolution of the collisionless MRI in stratified disks that include the vertical component of gravity. We show that this ingredient is essential for the emergence of a large-scale dynamo magnetic field. The resulting field geometry regulates the spatial localization of particle injection, which occurs preferentially in regions where the dynamo field reverses polarity.

The acceleration process develops in two distinct stages. First, particles are injected to suprathermal energies via magnetic reconnection in these localized regions, providing a population of seed particles. Subsequently, the injected particles undergo stochastic energization consistent with a second-order Fermi process mediated by MRI-driven turbulence. In our simulations, the energy distribution of accelerated particles is well described by a power law with an exponential cutoff ($\frac{dn}{d\varepsilon}\propto\varepsilon^{-p}e^{-\varepsilon/\varepsilon_c} $), with power index $p\approx 2$. The cutoff energy scales with the scale separation—defined as the ratio between the cyclotron $\omega_{c,0}$ and orbital frequencies $\Omega_0$— $\varepsilon_c\propto(\omega_{c,0}/\Omega_0)$ within the range of $\omega_{c,0}/\Omega_0$ explored here. In contrast, the acceleration rate appears largely insensitive to scale separation, suggesting that in realistic accretion flows the accretion time may set the maximum attainable particle energy.

Speaker: Astor Sandoval (Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Universidad Adolfo Ibañez)

202 Performance of the Engineering Camera for the Small-Sized Telescope of CTAO

The Cherenkov Telescope Array Observatory (CTAO) will host the next-generation of imaging atmospheric Cherenkov Telescopes, one of which is the Small-Sized Telescope (SST). The SST is a dual-mirrored telescope that will be deployed at CTAO-South in Chile and utilizes a SiPM-based camera. The SST engineering camera will be fully equipped with 32 modules, where each module includes a 64 pixel SiPM tile, preamplifier board, and a TARGET-based electronic readout module. This contribution presents the technical details of the SST engineering camera that will be deployed on-site at the end of 2026. Additionally, the SST engineering camera's intensity resolution, based on laboratory measurements, is presented in comparison to the CTAO requirement.

Speaker: Rhiannon Lake (MPIK)

203 Physical characterisation of MSH 15-52 through a joint analysis of 3D Fermi-LAT and eROSITA data with Gammapy

Several classes of galactic sources, including supernova remnants (SNRs), stellar clusters and pulsar wind nebulae (PWNe), are known to accelerate charged particles to TeV or even PeV energies. While direct observation of these highly energetic particles is not possible, we can reconstruct their properties by probing the non-thermal photon emission they induce in their interactions with local magnetic and radiation fields. This non-thermal emission is emitted across a major part of the electromagnetic spectrum, making multiwavelength (MWL) analyses essential in its study. Joint analyses at event level present a highly desirable approach to this, as they allow the simultaneous fit of e.g. background and absorption models and thus draw a more statistically comprehensive picture of the emission.
We present our work on a thorough physical characterization of the PWN MSH 15-52. With the Gammapy python package for gamma-ray astronomy we perform the first joint analysis of three-dimensional eROSITA X-ray data and Fermi-LAT gamma-ray data and characterize the underlying particle spectrum of the PWN. We use template models to disentangle the thermal and non-thermal X-ray emission. Furthermore we make use of Gammapy's implementation of the ultranest algorithm to map out the complex parameter space of the physical models.

Speaker: Katharina Egg (ECAP, FAU Erlangen-Nürnberg)

204 PIC simulations of particle acceleration at oblique high-Mach number shocks with pre-existing turbulence

Astrophysical collisionless shocks like that of supernova remants are efficient particle accelerators that require some pre-acceleration mechanism in order for electrons to participate in diffusive shock acceleration. The particle-in-cell (PIC) method provides a kinetic description of a system from first principles of collisionless plasma. Using the PIC code, THATMPI, we perform novel simulations of oblique non-relativistic high-Mach-number shocks propagating into an upstream containing pre-existing decaying turbulence. We consider compressive turbulence with density fluctuations with amplitude on the order of 15%, and we find that the turbulence was able to modify the properties of the shock-reflected electrons that drive plasma instabilities ahead of the shock front. When compared to simulations with a homogeneous upstream we find that turbulence results in more efficient electron acceleration, indicated by the non-thermal tail in the energy spectrum. We also perform preliminary shock simulations with an upstream consisting of electromagnetic turbulence of varying amplitude. This turbulence is generated via transverse perturbations in the magnetic field, and a self-consistent current is driven by the ions in order to ensure the properties of the perturbations are similar to that of Alfvenic turbulence. We also compare results from the electromagnetic turbulence with results from the compressive turbulence.

Speaker: Eloise Moore (University of Potsdam)

205 PIC-QED Simulations of Photon-Mediated Particle Acceleration in Relativistic Pair Shocks

Relativistic shocks are a common occurence in high-energy astrophysics and are thought to be efficient sites of non-thermal particle acceleration. However, the loss of efficiency expected in the standard Fermi acceleration model in ultra-relativistic shocks leads to many open problems. In addition to this, previous Particle-In-Cell (PIC) simulations have indicated that acceleration is strongly suppressed in perpendicular magnetized shocks. A proposed way around these limitations is the “converter mechanism”, in which non-local energy and momentum transport across the shock is mediated by the emission and absorption of neutral high-energy photons.
We present 2D PIC simulations of relativistic perpendicular shocks in a magnetized pair plasma embedded in an ambient radiation field. Our simulations include inverse-Compton scattering in the Klein–Nishina regime and Breit–Wheeler pair production, allowing us to model the converter mechanism from first principles. We demonstrate the operation of this mechanism and quantify how the spectrum, energy density, and angular distribution of the background photon field modify the shock structure and the spectrum of accelerated particles.
These results are directly relevant to particle acceleration in relativistic jets propagating through intense radiation environments, such as those associated with active galactic nuclei, gamma-ray bursts, and microquasars.

Speaker: Mr Anindya Guria (MPIK)

206 Probing Dark Matter with the Complementary Capabilities of CTAO and SWGO

Dark matter remains one of the central open questions in astroparticle physics. Indirect searches aim to identify the products of dark matter annihilation or decay through cosmic messengers, with gamma rays providing one of the most promising observational channels. In particular, very-high-energy gamma-ray observations can probe dark matter candidates in the GeV to multi-TeV mass range, where the shape and cutoff of the spectrum may provide important clues about the underlying particle physics. The Cherenkov Telescope Array Observatory (CTAO) and the Southern Wide-field Gamma-ray Observatory (SWGO) will offer highly complementary views of the gamma-ray sky. CTAO will provide excellent angular resolution and energy resolution over selected regions of the sky, while SWGO will continuously monitor a large fraction of the Southern sky with a wide field of view and high duty cycle. These different observational strategies are especially relevant for dark matter searches, where both deep observations of promising targets and broad, long-term sky coverage can play crucial roles. In this work, we study the complementarity between CTAO and SWGO for indirect dark matter searches. We investigate how their combined observations can improve the sensitivity to dark matter signals, help constrain the properties of a potential detection, and strengthen the discrimination between dark matter spectra and alternative astrophysical interpretations. The combination of CTAO and SWGO is expected to provide a more complete view of possible gamma-ray dark matter signatures, improving sensitivity across a broad mass range and enhancing the robustness of future dark matter searches.

Speaker: Micael Andrade

207 Probing TeV Gamma-Ray Emission from Star-Forming Galaxies with Wide-Field-of-View Instruments

With their intense star formation and dense gas, Starburst galaxies are efficient sites for cosmic-ray acceleration, primarily driven by supernovae. TeV emission is expected via cosmic rays' interaction with matter; however, despite numerous GeV detections in galactic star-forming regions, observations of star-forming galaxies at TeV energies remain limited to the two brightest Starburst galaxies detected by VERITAS and H.E.S.S. To overcome HAWC's field of view limitations, we perform population-based studies by analyzing 51 nearby star-forming galaxies using ~3000 days of HAWC data. Its wide field of view and high duty cycle enable stacking analysis to investigate the overall TeV emission. Each galaxy's contribution is weighted by the inverse square of its distance and its infrared luminosity. No significant signals are found, and we set 95% confidence upper limits on the total gamma-ray flux. These limits constrain TeV emission from star-forming galaxies and align with existing measurements of M82 and NGC 253. The proposed Southern Wide-field Gamma-ray Observatory (SWGO) will extend these studies by exploring the TeV sky in the southern hemisphere and complementing HAWC observations. Its wide field of view and expanded sky coverage will allow us to probe a larger population of star-forming galaxies and improve sensitivity to their cumulative TeV emission.

Speaker: Nilanjana Ghosh (Michigan Technological University)

208 Probing the transient gamma-ray Universe with LST-1

Transient astrophysical events produce various phenomena across the electromagnetic spectrum. Several transient event classes are expected to emit also gravitational radiation or produce high-energy neutrinos. Whether arising from gamma-ray emitting binaries, blazar objects, or gamma-ray bursts, these transient systems serve as excellent laboratories for the study of particle acceleration processes, as well as radiative and absorption mechanisms in action on extreme astrophysical systems.

The Large-Sized Telescopes (LSTs) of the Cherenkov Telescope Array Observatory are designed to detect gamma rays with energies ranging from tens of GeV to a few TeV. Their excellent sensitivity at a few hundred GeV, low energy threshold of 20 GeV, and fast-slewing capabilities to point anywhere on the sky within a few tens of seconds make them particularly well-suited for studying transient sources up to redshift z$\sim$1. During its commissioning phase, the first LST (LST-1) is already delivering its first results from various sources, including transient sources. In this contribution, we will present the transient science program of LST-1 and outline the key results on the transient very-high-energy (VHE; $E > 100$ GeV) sky during the commissioning phase.

Speaker: Arnau Aguasca-Cabot (Universitat de Barcelona - ICCUB - IEEC)

209 Prospects of EBL and Hubble constant measurements in the CTAO era

The extragalactic background light (EBL) is the sum of all photons
produced since the epoch of reionization. The EBL is dominated by the
well-constrained emissions of resolved galaxies, but also contains the
contributions from misknown diffuse sources. At very-high energies (VHE;
E > 100 GeV), gamma rays can interact with the EBL to produce pairs of
electrons and positrons, resulting in attenuation features that can be used
to reconstruct the EBL independently from traditional methods. In this
work, we present the measurements derived from gamma-ray observations
over the past decades, spanning particle physics, astrophysics, and
cosmology. We further study the prospects offered by the next generation
of gamma-ray instruments, the Cherenkov Telescope Array Observatory
(CTAO). Using simulations of the gamma-ray sky realistically observable
with CTAO, we assess the impact of its improved sensitivity and energy
resolution, as well as broader energy and redshift coverage, on the
reconstruction of the EBL at z=0. Finally, we highlight the potential of CTAO
to place constraints on diffuse astrophysical emissions and to measure the
local Hubble constant H0, independently from both cosmic microwave
background observations and cosmic distance ladder estimates.

Speaker: Lucas Gréaux (AIRUB)

gamma26_poster_greaux_cosmo.pdf

210 Pseudo-3D simulations of Supernova remnants with RATPaC

Supernova remnants (SNRs) are considered major contributors to the origin of the Galactic cosmic-ray population. Yet, basically all numerical models of particle acceleration rely on simplified geometries that cannot reproduce the complex and asymmetric emission morphologies of SNRs. These asymmetries are either driven by inhomogeneities in the explosions themselves or more so by the complex circumstellar environment shaped by the massive progenitor stars.
In this talk we present a pseudo-3D framework incorporated into the Radiation Acceleration Transport PArallel Code (RATPaC) to model SNR expansion and evolution as well as particle acceleration into complex media shaped by previous supernova (SN) explosions and the combined winds of the progenitor stars. We use pre-computed 3D-(M)HD simulations as initial conditions, which we then decompose into a number of cones, assuming piecewise spherical symmetry. Within each cone, we solve the coupled system of hydrodynamics, cosmic-ray, magnetic fields and magnetic turbulence transport in a fully time-dependent way.
These pseudo-3D models allow us to create projected emission maps in the radio, X-ray and gamma ray bands.

Speaker: Robert Brose (Universität Potsdam)

211 Sapphire++ – MHD: A finite element MHD code for astrophysical plasmas with energetic particles

We present Sapphire++ (https://sapphirepp.org) [1], an open-source community code developed to simulate astrophysical fluids as well as the propagation and acceleration of energetic particles including self-consistent feedback.

The evolution of astrophysical plasmas is often driven by cosmic-ray (CRs) currents. Both around energetic sources, such as supernova remnants (SNR) and on galactic scales, CRs play a vital role in the driving turbulence and winds. Capturing these effects in simulations of large systems is a challenging multi-scale problem, as one needs to capture physical phenomena occurring on vastly different scales. In some scenarios, like the escape of CRs from sources (e.g. SNR), characteristic quantities can vary by many orders of magnitude.

To address these open questions, we developed Sapphire++. It combines and couples the simulation of the plasma with energetic CRs. We use a pseudo-spectral method to solve the Vlasov-Fokker-Planck (VFP) equation for the high energy component. Utilising a decomposition of the distribution function into spherical harmonics, the dimensionality of the problem is reduced allowing for efficient multi-scale simulations. The thermal plasma is modelled using the magnetohydrodynamics (MHD) equations, and likewise solved using a discontinuous Galerkin finite element (DG-FE) method. We highlight, that coupling these two modules, VFP and MHD, enables self-consistent simulations.

This contribution focuses on the MHD module of Sapphire++. We showcase the code's validity using standard test-cases and comparing to existing codes in terms of accuracy and performance.

Finally, we present results for a combined MHD and CR simulation, in the example of CR current driven instabilities.

References:

[1] Nils W. Schween, Florian Schulze and Brian Reville. “Sapphire++: A particle transport code combining a spherical harmonic expansion and the discontinuous Galerkin method”. In: Journal of Computational Physics 523 (Feb. 2025), doi: 10.1016/j.jcp.2024.113690.

Speaker: Florian Schulze (MPIK)

212 SST-1M measurement of the multi-TeV gamma-ray emission of the CTA 1 supernova remnant

CTA 1 is a composite supernova remnant including the radio-quiet pulsar PSR J0007+7303 powering a pulsar wind nebula. While gamma-ray observatories such as VERITAS and LHAASO have detected very-high-energy emission from the region, their derived spectral energy distributions show discrepancies in flux normalization, which cannot be justified by the slight difference in the regions of observations. The Single-Mirror Small-Size Telescope (SST-1M) stereoscopic system of two 4m-diameter gamma-ray telescopes, located at the Ondřejov Observatory (Czech Republic), is sensitive in the ~1-100 TeV energy range. With a wide 9° field of view, the SST-1M telescopes are well suited for studies of extended sources, making them an ideal instrument to bridge the measurement gap between VERITAS and LHAASO.
In this contribution, we present a detailed analysis of 53 hours of high-quality stereoscopic data collected between September 2024 and January 2026. We use a 3D likelihood analysis (two-space celestial coordinates and energy) coupled with a data-driven background acceptance model (BAccMod) developed to handle spatial gradients across the field of view. Assuming power-law spectral behavior and 2D Gaussian morphology, we will discuss the results of comparing frozen spatial components to VERITAS and LHAASO parameters, as well as a morphological fit with free 2D Gaussian parameters, aiming at a comparison and an understanding of the discrepancy of the VERITAS and LHAASO results, mostly in terms of flux normalization. We demonstrate that forward-folding the full spatial template successfully recovers the total intrinsic flux of the extended emission, yielding a spectral energy distribution in strong agreement with recent LHAASO ultra-high-energy measurements.

Speaker: Mr Bastien Lacave (University of Geneva)

213 Supernova remannts as seen by HAWC

We present the results of the analysis of 10 years of HAWC data from the region of the supernova remnants, W51 and gamma Cygni.

Speaker: Suvendu Giri (IFJ PAN)

214 The ASTRI Mini-Array follow-up of transient events in TeV band

The LHAASO and Imaging Air Cherenkov Telescopes (IACTs, such as MAGIC and H.E.S.S.) detections of TeV emission component from gamma-ray bursts (GRBs) proved the importance of ground-based gamma-ray facilities for a better understanding of their emission processes up to the extreme energies. In recent years, significant efforts in improving the strategies for follow-up of transient events have been performed within the IACT community, particularly in the current era of multi-messenger triggers. The association of high-energy neutrinos and gravitational waves with transient astrophysical sources will indeed provide insights into the physics of extreme cosmic accelerators. The ASTRI Mini-Array experiment, composed of nine IACTs in dual-mirror configuration, will play an important role in studying the TeV emission in transient events. The array is being completed at the Teide Observatory site, where the first telescopes are already acquiring data. The ASTRI Mini-Array will be equipped with a dedicated transient handler to perform specific follow-up campaigns on a wide range of astrophysical sources like GRBs, galactic transients, and the possible TeV electromagnetic counterpart of neutrinos and gravitational waves. The presence of multiple observing facilities at the Teide Observatory and the closeness to the Northern site of the Cherenkov Telescope Array Observatory at La Palma island, will guarantee the unique opportunity to perform simultaneous follow-up in a wide energy range and in a multi-band context. We studied the performance of the ASTRI Mini-Array in detecting the possible TeV signatures from nearby on- and off-axis GRBs. The implementation and optimization of a possible ASTRI Mini-Array observational strategy based on specific science cases will also be discussed.

Speaker: Alessandro Carosi (INAF)

ASTRITransients_Gamma26_v0.pdf

215 The impact of Solar magnetic field configurations on the production of gamma rays at the Solar disk

The Sun is one of the closest and brightest gamma-ray source in the sky observed in the GeV by the Fermi LAT and in the TeV by HAWC. Its emission is thought to be caused by the interaction of Galactic Cosmic Rays in the lower layers of the solar atmosphere, i.e., the chromosphere and photosphere. These high-energy particles lead to a steady gamma-ray emission from the solar disk and the solar magnetic field configuration is a key ingredient to understand the origins of the gamma-ray emission as observed.

I will discuss the propagation of GCR protons through the coronal magnetic field and their gamma-ray production. I use a comprehensive modeling framework based on the CRPropa code, which is a well-established open source tool in high-energy astrophysics. The advantage of this approach is that the proton transport and gamma-ray production can be studied with a single, unified code by following individual particle trajectories and testing their interactions in a Monte Carlo approach. I will present the resulting distribution of gamma-ray production on the Solar surface and its connection to the coronal magnetic field configuration. Finally, I will discuss the spectral energy distribution of gamma rays, covering four orders of magnitude in energy, and predict the correlated neutrino signal and its detection probability with current detectors like IceCube.

Speaker: Julien Dörner (Ruhr University Bochum)

216 The Last Breath of Dying Stars: X-ray Flares in Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are the most energetic transients in the Universe, exhibiting a prompt gamma-ray emission followed by long-lasting multi-wavelength afterglows. The early X-ray afterglows of GRBs, observed with the Swift X-ray Telescope (XRT; 0.3–10 keV) onboard the Neil Gehrels Swift Observatory, have revealed distinct temporal features beyond those predicted by the standard forward shock afterglow model. Components in the XRT light curve, such as steep decay, flares, and plateaus, suggest more complex afterglow physics. X-ray flares are commonly attributed to prolonged central engine activity, although their physical origin remains debated. We present a systematic multi-wavelength study of X-ray flares in a sample of 56 GRBs observed by the Swift XRT over 17 years, all located within the field of view of the Fermi Large Area Telescope (LAT; 30 MeV–300 GeV). The flares are classified into prompt, steep-decay, plateau, and afterglow categories based on their temporal behavior in the X-ray light curves. We found that only six events show significant high-energy emission (>3σ). However, detailed modeling indicates that the GeV high-energy emission is consistent with afterglow processes rather than flare-related processes. We explore correlations between spectral properties and energy output at 1 keV, 10 keV, and 1 GeV. The broadband emission is modelled within a forward-shock scenario including synchrotron self-Compton radiation. This framework allows us to constrain the underlying microphysical parameters governing the emission. Our results provide one of the most comprehensive analyses of X-ray flares to date. We also predict very-high-energy emission associated with flares and discuss detections with current and future very high-energy detectors, such as CTAO.

Speaker: Pawan Tiwari (GNOI)

217 The Reality of Gravitational Repulsion, Cosmic Ray Neutrinos and Protons - Their Acceleration and Sources

In the second decade of the twentieth century, Johannes Droste and, independently, David Hilbert showed that particles moving at sufficiently high velocities can exhibit gravitational repulsion in the Schwarzschild field. We argue that this effect corresponds to physical reality.
Applying this framework to cosmic-ray neutrinos, we find that high-energy neutrinos originate from active galactic nuclei, whereas ultra-high-energy neutrinos are produced by stellar-mass sources. The theory further yields a value for the mass of the muon neutrino.
Extending the analysis to cosmic-ray protons, we find that very-high-energy protons are associated with brown dwarfs, while ultra-high-energy cosmic-ray protons are attributed to Jupiter-mass bodies.

Speaker: Charles McGruder (GNOI)

218 Transformer-Based Analysis for Next-Generation Compact Cherenkov Telescopes

The past decade has witnessed a rapid expansion of machine learning applications in very-high-energy (VHE) gamma-ray astronomy. While most recent efforts have focused on Convolutional Neural Networks (CNNs), these approaches remain constrained by existing camera geometries and by the limited ability of Monte Carlo simulations to fully capture real telescope performance. In this work, we employ a simulated telescope optimization framework to investigate how advanced machine learning techniques can fundamentally reshape instrumental design requirements.

We present a transformer-based architecture that appears to achieve an order of magnitude improvement in performance compared to a typical standard analysis pipeline. Our results indicate that, within this framework, a single 5-meter-diameter Imaging Atmospheric Cherenkov Telescope (IACT) operating in monoscopic mode can reach an energy threshold below 150 GeV. Moreover, our study suggests that the sensitivity for point sources of such a compact (virtual) instrument, operating in mono, is expected to be within a factor of a few of the 17-meter-diameter stereoscopic MAGIC telescopes. These findings point toward a viable pathway for the development of cost-effective, potentially autonomous IACTs, capable of substantially increasing the temporal coverage and duty cycle of future gamma-ray observatories.

Speaker: Elli Jobst (Max Planck Institute for Physics, Technical University of Munich)

219 Update from NectarCAM, a camera for the Medium-Sized Telescopes of CTAO

NectarCAM is a Cherenkov camera designed for the Northern site of the medium-sized telescopes (MSTs) of the Cherenkov Telescope Array Observatory (CTAO). It is optimised for the detection of very-high-energy gamma rays in the energy range from approximately 100 GeV to 50 TeV. The camera features a modular architecture comprising 1855 pixels, each sampled at 1 GHz, and provides a field of view of 8 degrees.
The first camera has been fully integrated and is currently undergoing extensive performance testing in a dark room at CEA Paris-Saclay under varying temperature conditions. In parallel, the integration of the next cameras has begun.
In this contribution, we present the current status of the NectarCAM project along with the latest performance results.

Speaker: Vincent Marandon (AIM, CNRS/CEA Paris-Saclay)

220 Young Massive Star Clusters as TeV Emitters: Constraints from H.E.S.S. and LHAASO

Young massive star clusters (YMSCs) can produce gamma rays in the very-high-energy (VHE, E>100 GeV) range and have been proposed as sources that can accelerate cosmic rays up to PeV energies. Observations with current instruments have led to the detection of only a few YMSCs but future instruments should significantly increase this number. However, the details of the production of the VHE emission are not well understood: What is the spectrum of accelerated particles? What is the efficiency of cosmic-ray production? What fraction of the wind luminosity is converted into the turbulent magnetic field? Which diffusion regime takes place in YMSCs?

To address these questions, we simulate the population of YMSCs in the gamma-ray domain, by means of Monte Carlo methods, and apply the constraints based on the subsample of YMSCs currently detected at TeV energies. We confront our simulated populations with the catalogue of the H.E.S.S. Galactic Plane Survey and the First LHAASO Catalogue of Gamma-Ray Sources, allowing us to investigate crucial aspects of particle acceleration at YMSCs. We find that our model can successfully reproduce the YMSC population observed in both the HGPS and the First LHAASO catalogue, with some sets of parameters producing >75% of realisations in agreement with the combined data.

Speaker: Rowan Batzofin (University of Potsdam)

221 Cosmic-ray escape from stellar clusters and posible non-thermal emission from illuminated molecular clouds

Stellar clusters are considered potential sites for efficient particle acceleration, allowing an explanation for the overabundance of Ne$^{22}$ observed in cosmic rays (CRs) and providing a minor but necessary contribution to Galactic CR flux. They could also complement the supernova remnant (SNR) paradigm by accelerating particles up to the knee of the CR spectrum. Meanwhile, the LHAASO observatory is the first to effectively probe the photon detection band above 0.1 PeV, corresponding to multi-PeV hadronic CRs. Combined with complementary instruments such as HESS and Fermi-LAT, this opens a new window to constrain particle acceleration models and to disentangle contributions of different Galactic accelerator classes to the observed CR flux.

In this context, we model the escape and transport of CRs from their acceleration sites to nearby molecular clouds, where proton-proton interactions produce high-energy gamma rays. We focus on scenarios where the source is a young massive stellar cluster, with particles accelerated at shocks (either wind termination shocks or embedded SNRs) before escaping into the surrounding medium. Using a semi-analytical approach, we first explore the conditions required to generate a CR excess at the location of nearby targets, as a function of parameters such as source-cloud distance, source age, or injection slope. We then investigate the conditions for detectable gamma-ray excess, given the sensitivities of current instruments.

This approach allows to identify the regions of parameter space where cluster-cloud systems can produce detectable gamma-ray signals, and thus to select viable candidates for observation. We further compare model predictions with available gamma-ray data in order to constrain key physical parameters, such as the particle acceleration efficiency or the diffusion coefficient. Finally, we investigate whether some of the so-called dark PeVatrons detected by LHAASO could be interpreted within this framework, as the result of CR illumination of nearby molecular material by stellar clusters.

Speaker: Mr Alexandre Inventar (APC)

222 FACT - Overview of Results from a Decade of Blazar Monitoring

The First G-APD Cherenkov Telescope (FACT) has been monitoring blazars at TeV energies between October 2011 and September 2021. Within a decade of operation, more than 15000 hours of physics data have been collected.

Located on the Canary Island of La Palma, FACT focussed on the transient sky of the Northern hemisphere with a regular blazar monitoring program, multi-wavelength campaigns and a target-of-opportunity program following up multi-wavelength and multi-messenger alerts.

Designed for remote and automatic operation and pioniering semiconductor photosensors, FACT's duty cycle is maximized and the gaps in the light curves are minimized. Thanks to an unbiased observation strategy, a unique and unprecedented long-term data sample is available at TeV energies for blazar studies and time series analyses.

The presentation summarizes the results of this legacy data sample.

Speaker: Daniela Dorner (Universität Würzburg)

fact_poster_V6.pdf

223 Gammapy_SyLC 1.0: A simulation and fitting tool for variability in gamma-ray light curves

Variability studies are one of the focal points to study many high-energy sources, such as Active Galactic Nuclei. This class of studies can provide a crucial window into the underlying physical processes occurring within relativistic jets. Accurately characterizing the stochastic behavior greatly benefits from a simultaneous treatment of the Power Spectral Density (PSD) and the Probability Density Function (PDF) of the measured light curves. This contribution presents the 1.0 version of Gammapy_SyLC, a Python-based framework designed for the simulation and statistical modeling of variability in gamma-ray light curves. The package integrates multiple simulation algorithms, including the Timmer-Konig method and the Emmanoulopoulos algorithm, allowing for the generation of light curves with arbitrary PSD shapes and non-Gaussian PDFs. The package also includes the treatment of unevenly sampled data through a modified Timmer-Konig approach and parallelized Monte Carlo fitting routines. Gammpy_SyLC is designed for integration with the Gammapy ecosystem, enabling the direct simulation analysis of FluxPoints objects.

Speaker: Dr Claudio Galelli (INFN Milano)

Poster SyLC Gamma26.pdf

224 IACTrace: A JAX based Raytracer for IACTs

Accurate optical modelling is essential for the calibration and performance characterisation of Imaging Atmospheric Cherenkov Telescopes (IACTs). We present IACTrace, a new open-source Python package for optical ray tracing of IACT, built on JAX.

IACTrace supports single- and dual-mirror geometries, segmented aspheric mirrors, obstruction primitives, hexagonal and square pixel sensors, and error models for mirror roughness, misalignment, and manufacturing tolerances. Telescopes are specified via YAML configurations or can be built directly using the package API, and the choice of JAX as the backbone enables GPU acceleration for fast inference, especially in the context of simulating point sources.

This makes IACTrace well-suited for generating pixel response matrices, studying the impact of optical imperfections on stellar fields in night sky background data, and exploring the optical properties of current and next-generation IACTs. Here, we present the package design and first example applications to IACTs, comparing it to other popular tools in the field.

Speaker: Gerrit Roellinghoff (ECAP, FAU Erlangen-Nürnberg)

225 New VERITAS Investigation of Energy-Dependent Morphology in the CTA 1 Pulsar Wind Nebula

As supernova remnant (SNR) systems evolve, there is a complex interplay between the pulsar wind nebula (PWN) and the surrounding SNR shell that determines how and when particles escape confinement. Observationally, we are just beginning to probe the stage and mechanism by which this escape occurs. CTA 1 is a middle-aged composite SNR system at the onset of PWN-SNR interaction, making it an ideal laboratory for studying these questions. It consists of a radio SNR shell with a central PWN detected across a broad energy range from X-rays to TeV gamma rays, with recent LHAASO measurements of emission above 100 TeV. Since the initial VERITAS detection of the TeV PWN in 2013, VERITAS has more than tripled its exposure on CTA 1, enabling detailed morphological and spectral studies from ~800 GeV up to ~10 TeV. Using this ~120 hour dataset, we explore how the asymmetry and extent of the TeV morphology change as a function of energy. We also probe the source’s multiwavelength spectral properties with a dynamical PWN evolution model. These results are discussed in the context of PWN-SNR interaction shaping particle transport in evolved PWNe, with possible connections to processes behind the formation of TeV halos.

Speaker: Svanik Tandon (Columbia University)

226 Optimizing shower impact positions for Monte Carlo simulations of the Cherenkov Telescope Array Observatory

The Cherenkov Telescope Array Observatory (CTAO) is the next-generation ground-based observatory for very-high-energy gamma-ray astronomy. It will provide unprecedented sensitivity of up to an order of magnitude compared to current imaging atmospheric Cherenkov instruments (IACTs) and cover the energy range from 20 GeV to 300 TeV due to multiple telescope designs deployed across two sites. This heterogeneous system, combined with the broad energy coverage, requires computationally expensive Monte Carlo simulations that model the atmospheric air showers, provide the detector response, and derive the instrument performance. For this purpose, CORSIKA is employed to simulate the particle cascade and Cherenkov emission from extensive air showers, while sim_telarray is employed to derive the detector response.

This project focuses on optimizing the trigger efficiency of CTAO Monte Carlo simulations. The standard uniform placement of detector positions leads to a low trigger efficiency because a large fraction of showers do not trigger the telescopes. As a result, substantial CPU time is wasted when simulating these events. To mitigate this effect, we use of the importance sampling approach of random telescope offsets implemented within the CORSIKA IACT package. This method targets regions with non-uniform higher trigger probability over the full simulated area. It accounts for the dependence of trigger efficiency on both energy and distance through an energy-dependent weighting scheme. We show that the distributions of triggered events can be predicted statistically using three key ingredients: the uniform distribution of array placements, the trigger efficiency, and the sampling weights. The parameter space defining the area weights is searched and optimized for the trigger efficiency. The method was applied to gamma rays and protons simulated for the La Palma and Paranal sites. We find an improved trigger efficiency by a factor of about 30 for gamma rays and more than a factor of 10 for protons.

Speaker: Maria Kherlakian (Ruhr University Bochum)

final_poster_sampling.pdf

227 SWGO Status

The Southern Wide-field Gamma-Ray Observatory (SWGO) will be a next-generation gamma-ray observatory located in the Southern Hemisphere. Building on the experience of past and existing ground particle array observatories, SWGO will employ a large area, high altitude, and southern location to explore the gamma-ray sky, reaching up to PeV energies. Following the recent site selection at Pampa la Bola in Chile, at an altitude of 4770 m a.s.l., the collaboration is now focusing on efforts for an on-site pathfinder. In this contribution, we summarize the current status of SWGO, highlight recent R&D progress, and outline future plans for the observatory.

Speaker: Hazal Göksu (Max Planck Institute for Nuclear Physics)

228 The CMB-Cosmic-ray connection: investigating deformation effects on the CMB distribution

The interaction of ultra-high energy cosmic rays (UHECRs) with the cosmic microwave background (CMB) has been the subject of extensive research in the past 50 years. These studies have concentrated on the impact of such interactions on cosmic ray physics while neglecting the potential influence on the CMB itself due to its presumed minimal amplitude. However, the prospects of ultra-high-precision measurements of the CMB, together with recent advancements in cosmic ray propagation simulations, motivate a reevaluation of this influence, both for local effects and for potential signatures on cosmological scales. This contribution presents an update of our previous studies detailing the large-scale impact of extragalactic protons on the CMB, which was expanded to include other primary UHECR species and a refined selection of possible sources. Moreover, we investigated the impact of possible changes in the emission spectra of sources in the past on this effect.

Speaker: Dr Claudio Galelli (INFN Milano)

Poster CMB Gamma26.pdf

229 The millisecond pulsar contribution to the cosmic-ray positron spectrum

Millisecond pulsars (MSPs) constitute a peculiar subset of the Galactic pulsar population. Having evolved beyond their spin-down phase, they are old pulsars that have been "recycled", or spun-up by accretion of the matter of a companion star. They are characterised by very short rotation periods, ages significantly greater and magnetic fields lower than those of non-recycled pulsars, and they exhibit an approximately constant luminosity over time, typically lower than that of younger pulsars with ages below ~1 Myr. Gamma-ray observations indicate that MSPs are capable of accelerating leptons in their surroundings, which can then escape and propagate through the Galaxy but the maximum energy to which MSPs can accelerate leptons is still unclear. The old MSPs are expected to easily release particles in the ISM whereas younger pulsars are found amid PWNe and SNRs where particles might get trapped.

By assuming a Galactic MSP birth rate, an average constant luminosity for the population and a lepton injection efficiency, one can estimate the total energy density of positrons in the Galaxy contributed by this class of sources. This energy density exceeds that of cosmic-ray positrons measured at Earth above ~10 GeV, suggesting that MSPs may account for at least a fraction of the positron energy density.

We model the cosmic-ray injection at MSPs and solve the transport equation from the pulsars to the Earth with the Green functions formalism. We present an assessment of the contribution of known MSPs from the ATNF catalogue to the local positron flux and evaluate the impact of physical parameters such as the maximum energy of the leptons injected into the ISM by the MSPs.

Speaker: Lioni-Moana Bourguinat (Gran Sasso Science Institute (GSSI, Italy))

230 CTAO SSTCAM Design and Time Resolution

SSTCAM is the Camera for all Small-Size Telescopes (SST) of the Cherenkov Telescope Array Observatory (CTAO). The 37 SSTs will be located in Paranal (Chile) and feature a dual-mirror design (primary mirror diameter 4.2m), providing a wide field of view. The camera uses Silicon Photomultipliers (SiPMs) and the custom TeV Array Readout Electronics with GSa/s sampling and Event Trigger Application-Specific Integrated Circuit (TARGET ASIC) to detect Cherenkov light stemming from primary particles of energies from 2 TeV to 300 TeV.

This Poster provides an overview of the camera design with respect to timing and the time reconstruction method. Afterwards, the time resolution of the latest camera prototype is evaluated with respect to the requirements set by CTAO (time resolution <2 ns above 5 photo electrons, statistical uncertainties <1 ns above 20 photons). Furthermore, a comprehensive overview of the various contributions (different hardware levels and phenomena) to timing uncertainties is provided.

Speaker: Frederik Wohlleben (MPIK)

231 Development and Characterization of a Fast LED Calibration Device for Cherenkov Cameras

Accurate calibration is essential for extracting precise scientific information from modern photon detection systems, particularly in experiments relying on fast and low-intensity light signals such as imaging atmospheric Cherenkov telescopes. Stable, well-characterized, and versatile light sources are therefore a key component for detector calibration, performance monitoring, and cross-system consistency.

We present the development and characterization of the FlashCam Flat-Field Calibration Device based on fast pulsed LEDs, designed to reproduce nanosecond-scale light pulses characteristic of Cherenkov signals. The system provides tunable control of pulse intensity and duration over a wide dynamic range, enabling detailed and systematic studies of the detector response under controlled conditions.

Extensive measurements of pulse shape and intensity are presented, highlighting the dynamic range, pulse-width tuning, rate dependence, pulse-to-pulse stability, and uniformity of the emitted light field, demonstrating the performance and short-term stability of the system. These results establish the device as a robust and flexible calibration tool for FlashCam, with potential applicability to other camera systems. We also outline ongoing and future studies aimed at assessing long-term stability and environmental dependencies.

Speaker: Anne Elise Timmermans (Max-Planck-Institut für Kernphysik)

Thursday 3 September

Plenary: VII

Convener: Andrew Taylor (DESY)

232 Gamma-Ray Searches for Dark Matter: Current Status and Future Prospects

A wealth of astrophysical and cosmological evidence indicates that most of the matter content of the Universe consists of non-baryonic dark matter. Considerable experimental efforts have been devoted to revealing its nature through direct detection, collider experiments, and indirect searches for observable products of dark matter processes in astrophysical environments. Despite remarkable progress over the past decades, no conclusive evidence for a dark matter particle has yet been found.

This presentation reviews the current status of indirect dark matter searches with gamma rays. Recent results from current gamma-ray observatories, including H.E.S.S., MAGIC, VERITAS, HAWC, LHAASO, and the Fermi Large Area Telescope, will be reviewed. The discussion will cover both established targets, such as the Galactic Centre and dwarf spheroidal galaxies, and recent developments involving new astrophysical targets and novel observational strategies to probe non-standard dark matter scenarios. The current status of the Galactic Centre GeV excess and its implications for dark matter searches will also be discussed. Finally, the exciting prospects of the next generation of gamma-ray observatories will be presented, focusing on the Cherenkov Telescope Array Observatory (CTAO) and the Southern Wide-field Gamma-ray Observatory (SWGO), which are expected to substantially extend the discovery potential of indirect searches and probe the thermal relic annihilation cross section over a broad range of dark matter masses and annihilation channels during the coming decade.

Speaker: Aion Viana (IFSC - University of Sao Paulo)

Gamma2026_SearchesForDM_clean.pdf

233 Gamma-ray bursts and electromagnetic counterparts of Gravitational Waves

Speaker: Gor Oganesyan (Gran Sasso Science Institute)

Gor_Oganesyan_GAMMA_2026.pdf

Gor_Oganesyan_GAMMA_2026.pptx

234 TeV gamma-ray halos: a window on cosmic-ray transport

Speaker: Sarah Recchia (Institute of Nuclear Physics Polish Academy of Sciences)

gamma2026_Recchia.pdf

10:30 Coffee break

Plenary: VIII

Convener: Guillem Martì-Devesa (University of Trieste & INFN Trieste)

235 High Energy Emission from High Energy Transients

Speaker: Prof. Daniel Perley (Liverpool John Moores University)

Perley-Gamma26-Transients_v2.pptx

236 Gamma-Ray Astrophysics from Space

Space-based gamma-ray observatories provide continuous all-sky coverage from a few hundred keV to hundreds of GeV, bridging the gap between hard X-ray and very-high-energy gamma-rays observed by ground-based Cherenkov telescopes. Instruments such as Swift-BAT, INTEGRAL, Fermi-GBM, and Fermi-LAT have revolutionized our understanding of the high-energy Universe, enabling precision studies of non-thermal processes in astrophysical sources, cosmological probes through measurements of the extragalactic background light and large-scale structure correlations, and major discoveries in time-domain astrophysics through transient phenomena.

In this talk, I will review key achievements and recent advances in space-based gamma-ray astrophysics, highlighting their impact on astrophysics, cosmology, and multi-messenger science, also in view of future missions designed to close the long-standing MeV sensitivity gap.

Speaker: Giacomo Principe (University of Trieste - INFN Trieste)

Space_based_gamma_ray_astrophysic_Principe_Gamma2026.pdf

237 Future Gamma-ray Space Experiments

Speaker: Prof. Hirokazu Odaka

Gamma2026_Hirokazu_Odaka.pdf

12:30 Lunch

Parallel A: Galactic IV

Convener: Kirsty Feijen (APC, CNRS)

238 Pulsar studies with the first Large Sized Telescope of the Cherenkov Telescope Array Observatory

The recent detection of Very-High-Energy (VHE) emission from the Crab and Vela pulsars opened the doors to new frontiers in the gamma-ray pulsars field. This discovery challenges the capability of classic curvature-radiation-based models to explain the overall gamma-ray emission from MeV to TeV energies. In addition to the young Crab and Vela pulsars, Cherenkov telescopes have also detected Geminga, the only middle-aged pulsar observed at such high energies to date.

The LST-1 is the first Large Sized Telescope (LST) of the upcoming Cherenkov Telescope Array Observatory (CTAO). LSTs are designed to cover the VHE domain down to tens of GeV, making them ideal instruments for pulsar studies, as they bridge the gap between Fermi-LAT and TeV instruments.

We report the results of the analysis of the Crab and Geminga pulsars as observed by the LST-1. Crab’s characteristic phaseogram peaks are both detected with a significance exceeding 10σ, and their power law spectra were reconstructed up to 450 GeV for the first and up to 700 GeV for the second. The second peak of Geminga’s phaseogram was detected with an improved 12σ significance compared to the previous result, and its power law spectrum was reconstructed up to ~100 GeV. For both pulsars, we jointly fitted LST-1 and Fermi-LAT data and compared the results to current theoretical models. Both results prove the excellent performance of LST-1 between tens of GeV and several tens of TeV and that CTAO will be a fundamental player in the future for the discovery and understanding of new VHE gamma-ray pulsars.

Speaker: Giulia Brunelli (INAF-OAS and University of Bologna)

LSTpulsars_gamma26_Brunelli.pdf

239 A Systematic Study of Extended Galactic Star-Forming Regions Using the Fermi-LAT

Galactic star-forming regions (SFRs) are expected to be gamma-ray emitters and sites of cosmic ray (CR) acceleration. However, although a few SFRs have been associated with gamma-ray sources, the majority remain undetected in gamma rays. These systems host known particle acceleration sites as well as the ambient material and radiation fields necessary for CR interactions, making them potentially important sources of gamma rays in the Galaxy. Therefore, we aim to characterize the gamma-ray properties of SFRs in the first systematic study of Galactic SFRs. To achieve this, we use the first 15 years of Fermi Large Area Telescope (LAT) gamma-ray data in the energy range of 1 GeV – 1 TeV, in combination with optical measurements of Galactic star clusters from Gaia, which has angular resolution on the order of 0.1 arcseconds. We present the results of our systematic population study of gamma-ray emission from Galactic SFRs, including the analysis of spectral and physical properties of gamma-ray detected SFRs.

Speaker: Ava Webber (Clemson University)

Gamma 26 Sample Study of SFRs using the Fermi LAT.pdf

240 The second H.E.S.S. Galactic plane survey catalogue

The H.E.S.S. Galactic Plane Survey (HGPS) significantly advanced our understanding of Galactic gamma-ray source populations and remains, even today, the most extensive survey of our Galaxy at very-high energies covering the Southern sky. This contribution presents the 2HGPS catalogue, which provides a new description of the gamma-ray emission in the Galactic plane at very high energies with unmatched precision.

Since the first HGPS catalogue release, the new observations accumulated provide a deeper scan of many Galactic sources, and a number of improvements have been implemented at various stages of the data processing chain. The catalogue workflow was built using Gammapy, and directly inherits from the work carried out in preparation of the CTAO-GPS survey. It features a more sophisticated modelling of the cosmic-ray background, Galactic diffuse emission, and sources.

The 2HGPS catalogue includes 172 objects identified as 130 complexes after considering potential associations, whereas the HGPS catalogue included 98 objects identified as 68 complexes. Half of the new objects in the catalogue are substructures of complex sources as there was no attempt to model these regions in the previous catalogue. The majority of the new objects remains unidentified, weakly associated, or have no associations. However we also report several new detections with strong associations and identifications to pulsar wind nebulae, supernova remnants, and star-forming regions.

Speaker: Quentin Remy (MPIK)

2HGPS_gamma_remy.pdf

241 Reducing the "MeV gap" -- The Second Catalog of Low-Energy Fermi-LAT Sources (2FLE)

The MeV energy band remains one of the least explored windows in high-energy astrophysics, despite its strong potential to reveal key signatures of particle acceleration and radiation processes. Pioneering all-sky studies — such as COMPTEL, the First Fermi-LAT Low-Energy Catalog (1FLE), and AGILE — have already established the richness of this regime and uncovered emission from some of the most powerful Galactic and extragalactic accelerators. Building on these foundations, we present the Second Catalog of Low-Energy Fermi-LAT Sources (2FLE), the most sensitive and extensive survey of the 20–200 MeV sky to date, based on a dedicated all-sky analysis of 14 years of Pass 8 Fermi-LAT data. This new catalog significantly extends previous efforts, reporting 450 sources detected at >5σ for which we provide localization, spectral characterization and multi-wavelength association. Notably, ~14% of the sources lack clear multiwavelength counterparts, pointing to a population of newly detected MeV emitters. The 2FLE provides a critical step forward in bridging the long-standing “MeV gap,” delivering a dataset that complements earlier catalogs and sets the stage for next-generation missions. In this talk, I will present the key results, discuss the scientific impact of the catalog, and outline the implications for future MeV observatories such as COSI.

Speaker: Lea Marcotulli (DESY)

2FLE_Gamma_2026_v3.pdf

242 ASTRI Mini-Array preliminary results on galactic sources

The ASTRI Project is developing an array of small-sized imaging atmospheric Cherenkov telescopes located at the Teide Observatory in Tenerife, Spain, optimized for observations in the 1–200 TeV energy range. ​​Following the commissioning of the first telescopes, the system has recently entered its early science phase.
Here, we present the initial scientific findings obtained with the first two telescopes. We present stereo observations of the Crab Nebula, which highlight the system's improved performance in terms of angular resolution, energy reconstruction, and background rejection.
During the upcoming summer observing campaign, the array of the first 4 telescopes will focus on the Cygnus region, one of the most complex and active areas of the Galactic plane in the very-high-energy gamma-ray sky. Thanks to its large field of view (~10 degrees), broad energy coverage, angular resolution of a few arcminutes, and energy resolution of about 10%, the ASTRI Mini-Array is particularly well suited to investigate extended emission, crowded regions and multiple overlapping sources. We will also present the first results on the Cygnus region in terms of morphological and spectral characterization of the emission.
These results will provide an initial indication of the ASTRI Mini-Array's scientific capabilities, and future observations will validate its performance further and enhance its impact in very-high-energy gamma-ray astronomy.

Speaker: Silvia Crestan (INAF-IASF Milano)

ASTRI-Crestan_Gamma2026.pdf

243 Multiwavelength Study of 1LHAASO J1902+0648 with HAWC, Fermi-LAT, and Archival Radio and Molecular-Cloud Data

We present a multiwavelength study of the unidentified GeV–TeV source 1LHAASO J1902+0648 using HAWC and Fermi-LAT observations together with archival radio, X-ray, and molecular-cloud data. The source is listed in the first LHAASO catalog and is detected in updated HAWC analyses, where its TeV emission is consistent with a point-like morphology. Its location overlaps dense molecular material and lies near the Fermi-LAT source 4FGL J1902.5+0654, while no unique low-energy counterpart has yet been established. This source environment allows multiple interpretations, including a hadronic scenario related to interactions with dense gas and a leptonic scenario associated with an unresolved compact accelerator. We will present the current HAWC analysis status, the GeV–TeV spectral energy distribution, the multiwavelength properties of the surrounding field, and ongoing broadband modeling under hadronic and leptonic assumptions.

Speaker: Tulun Ergin (GNOI)

Gamma26_J1902_TErgin.pdf

Parallel B: Extragalactic IV

Convener: Frank Rieger (IPP)

244 The Puzzling Long-Term Behaviour of the Radio Galaxy PKS 0625-354

PKS 0625-354 is a nearby radio galaxy exhibiting blazar-like properties, which indicates that it might actually be a misaligned blazar. In the present work, we are conducting an analysis of the long-term variability of this source using multiwavelength observations from optical/UV to VHE gamma-rays, including data from H.E.S.S., Fermi-LAT, Swift-XRT, and Swift-UVOT. Over several years, the Fermi flux remained low and relatively steady, but a gradual increase has been observed since 2021, which could be attributed to a possible change in the jet’s viewing angle. However, in 2025, a sudden decline in the flux was simultaneously detected from the optical/UV to high energy gamma-rays. Additional H.E.S.S. observations were conducted in 2026, unfortunately after the flux drop. We will present an analysis of the variability patterns and possible correlations between energy bands lasting for 17 years, including investigation of spectral variability between different flux states, to search for changes in the broadband emission properties of the source. This work aims to provide constraints on the physical processes driving the observed variability and to assess the relative contributions of different emission mechanisms in PKS 0625-354.

Speaker: Julia de Assis Scarpin (LLR, École Polytechnique)

Gamma26_Julia_deAssis.pdf

245 Blazars at the Edge of the Known VHE Gamma-Ray Universe: Broadband picture of quasar OP 313

Flat Spectrum Radio Quasars (FSRQs) are rarely detected at very-high-energy (VHE, E > 100 GeV) gamma rays due to their distance, soft spectra, and strong absorption from the Extragalactic Background Light (EBL). In December 2023, the Large-Sized Telescope prototype (LST-1) of the Cherenkov Telescope Array Observatory (CTAO) and the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) simultaneously detected for the first time VHE emission from the FSRQ OP 313 during an exceptionally bright emission phase. With a redshift of z = 0.997, OP 313 is the most distant blazar ever detected in this energy band, and among the most luminous sources ever detected in the gamma-ray band. The detection marks a milestone for LST-1 and offers a unique opportunity to study both the extreme physics of FSRQs and the opacity of the Universe to gamma rays due to interaction with low energy EBL photons.

OP 313 triggered one of the most extensive and productive multi-wavelength campaigns ever carried out on a flaring FSRQ, resulting in an exceptional dataset spanning from radio to VHE gamma rays. We present the first in a series of multi-wavelength studies of OP 313 extending to the VHE regime, combining data from LST-1, MAGIC, Fermi-LAT, Swift, and complementary optical, IR and radio observations. We model the broadband emission using two leptonic scenarios and investigate the contribution of various external photon fields to the gamma-ray output. The unprecedented brightness of the flare and the high redshift of the source also allow us to place new constraints on the EBL intensity in the optical-to-near-IR range. This detection highlights the scientific potential of CTAO to push the limits for probing the distant Universe and capture the most extreme blazar outbursts.

Speaker: Chitranshi Bakshi (Saha Institute of Nuclear Physics)

Op313_chitranshi_gamma2026.pdf

OP313_chitranshi_Gamma2026.pptx

246 Ultra-long MeV transient from a relativistic jet: a tidal disruption event candidate

Gamma-ray bursts (GRBs) are extragalactic MeV transients lasting from fractions of a second to several minutes, typically associated with mergers of compact objects or the collapse of massive stars. On 2 July 2025, the Gamma-ray Burst Monitor onboard the Fermi Gamma-ray Space Telescope detected three emission episodes with overlapping sky localisations, occurring 1–2 hours apart, collectively designated GRB 250702B. Taken together, the emission lasted more than 3 hours, making it the longest MeV transient ever observed. Follow-up observations with the Neil Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array revealed a rapidly decaying soft X-ray counterpart over the subsequent ten days.

Our time-resolved spectral analysis of the prompt emission phase shows that the gamma-ray spectra are harder than those of long GRBs. The spectra are well described by a single power law extending from 10 keV to 40 MeV, with evidence for a spectral break above 50 MeV.

The extreme duration, broadband spectral properties, and X-ray evolution suggest GRB 250702B likely originates from a relativistic jet launched during the tidal disruption of a star by a compact object. From $\gamma\gamma$ opacity constraints, we derive a lower limit on the jet bulk Lorentz factor of $\Gamma$ > 10. The inferred luminosity and energetics are consistent with those of previously reported relativistic tidal disruption events. The MeV spectra disfavour inverse Compton scenarios and are naturally explained by synchrotron emission from sub-TeV electrons.

Speaker: Annarita Ierardi (Gran Sasso Science Institute)

Annarita_Gamma.pdf

247 GRB 260226A: A rare LAT-triggered burst with broadband coverage from prompt to afterglow

GRB 260226A is a remarkable GeV-bright gamma-ray burst detected on 26 February 2026, notable for being only the second GRB in the 18-year history of the Fermi mission to trigger the Large Area Telescope (LAT) autonomously, after GRB 090510. This independent LAT trigger, combined with contemporaneous Fermi-GBM coverage, provides an exceptionally rich multi-instrument dataset spanning five decades in energy from 8 keV to 1 GeV. We present a comprehensive temporal and spectral analysis of GRB 260226A exploiting this unique broadband coverage. We devised an analysis method for the LAT Low Energy (LLE) data applied to the 30–100 MeV band, probing the spectral transition region bridging the GBM and standard LAT energy ranges, a band rarely utilized in GRB spectral studies. We discuss the temporal transition from the prompt phase to the afterglow, which allows us to directly measure the prompt radiative efficiency. We also estimate the bolometric afterglow flux from T90 out to ~1000 s post-trigger and discuss the spectral evolution during the prompt phase. Finally, we place GRB 260226A in its historical context by comparing its high-energy GeV emission to the broader population of LAT-detected GRBs, highlighting its significance as only the second LAT-triggered burst and what this implies for the energetics of the jet.

Speaker: Biswajit Banerjee (GSSI, Italy)

Banerjee_26A_Gamma26.pdf

248 An extragalactic gamma-ray binary formed in supernova 2022jli

The type Ic supernova (SN) explosion SN 2022jli was discovered to have shown additional optical emissions, which exhibited ~12.5-day periodic undulations and concordant periodic velocity shifts. Also, a faint gamma-ray source was detected at the SN's position and upon checking the gamma-ray photons’ arrival times, it was revealed that the same ~12.5-day periodicity was likely present. These features strongly suggest that in this SN, a compact object in a binary system was formed. In this talk, I will present our detailed analysis results for the gamma-ray source. We have not only determined a strong modulation at period 12.5 day since the discovery time of the SN for approximately one year, but also found a re-brightening of the gamma-ray emission that lasted for four months, during which the periodicity could not be detected anymore. Considering the newly formed compact object to be a neutron star or a stellar-mass black hole, the putative binary, having an orbital period of 12.5 day, is likely the first extragalactic high-energy system detected (beyond the Large and Small Magellanic Clouds). The system may serve as a valuable example for the formation of many such binaries observed in the Milky Way and nearby galaxies.

Speaker: Zhongxiang Wang (GNOI)

g26-wangzx.pdf

249 Particle acceleration at Ultra-fast Outflows

Ultra-fast Outflows (UFOs) are sub-relativistic ($ v \gt 0.1 \rm ~ c $) dense winds launched from Active Galactic Nuclei with wide aperture angle, identified through X-ray spectroscopy lines at which strong shocks are expected to form, typically with Mach number such that $ \mathcal{M} \gg1$.
At these shocks, particle energisation through diffusive shock acceleration (DSA) should lead to the copious production of gamma rays and neutrinos through the interaction of accelerated charged particles and the dense surrounding circumnuclear medium.

We modeled this particle acceleration through DSA at UFO shocks and estimated the associated high-energy gamma-ray and neutrino fluxes given the properties of the sources, and investigated the prospects for detection with current and next generation gamma-ray and neutrino observatories.
We will emphasize that for a selected list of nearby UFOs, we identified the best candidates for detection with next generation gamma-ray observatories such as the CTAO, and discuss the potential for detection with neutrino observatories such as KM3NeT.

Speaker: Baptiste Le Nagat Neher (Observatoire de Paris - LUX)

Gamma_Le_Nagat_Neher_2026_3_09_session.pdf

15:30 Coffee break

Parallel A: Theory III

Convener: Giada Peron (Inaf Osservatorio Astrofisico di Arcetri)

250 Mirror-dominated shock acceleration to the knee

We show that cosmic rays (CR) can be accelerated to the knee in the spectrum at a few PeV by mirror-dominated shock acceleration (MDSA). Standard diffusive shock acceleration (DSA) to the knee by supernova remnants (SNR) requires magnetic field amplification on the scale of the Larmor radius of PeV protons. Unfortunately, the expansion time of a SNR blast wave is insufficient for the amplification of such large fluctuations. Gamma-ray observations of SNR indicate a turnover in the CR spectrum well short of the knee in agreement with theory.
Previously we showed that mirror-transport can mimic Bohm diffusion thus opening up the possibility of CR acceleration to the knee in SNR. Here we show, using particle calculations of CR trajectories around a shock encountering an upstream mirror, that CR can be accelerated to the Hillas energy (in eV) of $uRB_{\rm mirror}$. This outperforms standard DSA because (i) the peak magnetic field $B_{\rm mirror}$ in a mirror may be considerably larger than the characteristic field $B_{\rm char}$ in the upstream medium, (ii) according to diffusion theory DSA accelerates CR to only a fraction of $uRB_{\rm char}$. For example, MDSA acceleration with u=10,000 km s$^{-1}$, $B_{\rm mirror} = 20$mG, R=3pc gives a maximum CR energy of 2PeV. CR could reach higher energies in stronger mirror fields, or if uR is larger.
Acceleration by MDSA terminates with CR escaping into the low density upstream medium in contrast to DSA where the accelerated CR reside in the higher density downstream plasma. This may contribute to the observed turnover in gamma-ray spectrum at the limit of DSA acceleration. MDSA depends on the existence of suitable mirrors in the ambient medium.

Speaker: Prof. Tony Bell (RAL / Univeristy of Oxford)

Bell Thursday 1600.pdf

251 Synthesis of gamma-ray emission from 3D numerical simulations of supernova remnants: a physically motivated model of the observed gamma-ray emission of IC 443

Supernova remnants (SNRs) are widely considered to be among the primary sources of Galactic cosmic rays, providing ideal laboratories to study particle acceleration and high-energy emission processes.

We present a new three-dimensional framework for modeling non-thermal emission from SNRs, based on the post-processing of state-of-the-art HD/MHD simulations. The tool provides spatially resolved and physically consistent predictions of high-energy emission by coupling the dynamical evolution of the remnant with particle acceleration and radiative processes.

In this work, we focus on hadronic gamma-ray emission, modeling the gamma-ray production via pion decay. The framework incorporates time-dependent acceleration of protons at the shock front, their escape during the remnant evolution, and their interaction with shocked ambient material.

We apply this approach to the middle-aged SNR IC 443, interacting with a dense molecular cloud and an atomic cloud. Our model successfully reproduces the observed gamma-ray emission across GeV–TeV energies, as confirmed by a comparison between the synthesized spectral energy distribution and Fermi-LAT and VERITAS observations. On the other hand, we note that the very-high-energy gamma-ray emission recently detected by LHAASO cannot be attributed to the shocked ambient medium and may instead be associated with cosmic rays diffusing away from the shock front into nearby dense material.

These results highlight the capability of physically motivated 3D models to reproduce the gamma-ray emission of complex systems and to constrain cosmic-ray acceleration and transport in supernova remnants.

Speaker: Anna Marretta (INAF-OAPA)

Marretta_Heidelberg_Gamma2026.pdf

252 The impact of wind-blown bubbles on pulsar wind nebulae and supernova remnants

Pulsar wind nebulae (PWNe) seem to be the most common gamma-ray sources in recent surveys of the very-high-energy and ultra-high-energy sky (with HESS, HAWC, or LHAASO). They are formed by the interaction of a relativistic pulsar wind with the stellar ejecta of the parent supernova remnant (SNR). Electron-positron pairs accelerated in the vicinity of the pulsar and diffusing away across the shocked pulsar wind nebula produce spatially and spectrally extended emission through the synchrotron and inverse-Compton scattering radiation processes.

Over the past decades, developments in gamma-ray observations and data analysis techniques have revealed very extended sources and/or multiple emission component in gamma-ray sources coincident with powerful pulsars. This is hard to reconcile with simple model predictions for PWNe, which are commonly performed under the assumption that the parent SNR expands in a uniform interstellar medium (ISM). Moreover, in many cases, the parent SNR is not detected, casting further doubt on such a description of the SNR evolution. Yet, pulsars are born from massive stars, whose powerful winds excavate large cavities around them. Supernovae explode in such stratified wind-blown bubbles (WBBs) that drastically differ from the uniform average ISM.

We investigated how this environment affects the development of an SNR-PWN system. We performed a series of numerical experiments in which we implemented a 1D hydrodynamical analog of a SNR-PWN system and followed its evolution inside a WBB. This revealed many fundamental differences in the SNR and PWN evolutions, in terms of spatial extent, dynamics, energy content, and thermal X-ray signature. In advanced stages, those expected to characterise many gamma-ray PWNe, both the SNR and the PWN are larger by factors of a few, and the former emits very little thermal X-rays while the latter holds much more energy. Our findings therefore provide a convenient explanation for the above-mentioned observational puzzles.

Speaker: Lioni-Moana Bourguinat (Gran Sasso Science Institute (GSSI, Italy))

Bourguinat Heidelberg Gamma2026.pdf

253 From the Fermi Fundamental Plane to TeV Vela: a unified current-sheet framework for gamma-ray pulsars

We present a unified interpretation of phase-averaged high-energy emission from pulsars in which the equatorial current sheet regulates both the observed GeV population and its extension toward TeV energies. Building on the pulsar Fundamental Plane linking gamma-ray luminosity, spectral cutoff energy, spin-down power, and surface magnetic field, we show that the observed population is bounded by two physically motivated limits: a radiation-reaction-limited branch and a potential-drop-limited branch. Their intersection defines a transition in spin-down power that maps onto a practical gamma-ray visibility threshold on the P-Pdot diagram. Above this threshold, detectability is controlled mainly by distance and beaming, whereas below it both the cutoff energy and radiative efficiency decline rapidly.

Placing Fermi-LAT pulsars, together with the broader ATNF pulsar population and magnetars, in this framework reproduces the main occupancy trends in parameter space: millisecond pulsars lie predominantly on the visible side of the threshold, young pulsars cluster near it, and high-field systems lie close to or below it. The framework also predicts a population of MeV-bright, GeV-faint pulsars that is largely inaccessible to current LAT sensitivity but should be relevant for next-generation MeV missions.

At the highest spin-down powers, the observed flattening of spectral cutoffs relative to the maximal radiation-reaction trend suggests additional screening of the accelerating electric field, plausibly linked to enhanced pair creation in or near the current sheet. Finally, using particle distributions from global PIC simulations together with a seed-photon prescription, we show that the same current-sheet particle population can account for both the GeV curvature component and a multi-TeV inverse-Compton component, supporting a unified GeV-TeV origin as suggested by the recent pulsed TeV detection from Vela.

Speaker: Constantinos Kalapotharakos (NASA Goddard Flight Center)

kalapotharakos_heidelberg_2026_final1.pptx

254 The supernova origin of ultra-high energy gamma-rays in the Cygnus region

None of the observed objects related to the Cygnus OB2 association can provide a convincing explanation for the PeV photons detected in the Cygnus region by the LHAASO observatory. Despite hosting hundreds of OB stars and 3 powerful Wolf-Rayet stars, the association itself is too scattered for the collective action of the stellar winds to generate a coherent large-scale wind termination shock, as demonstrated by our recent 3D hydrodynamic simulations. These simulations allow not only to probe the detailed structure of the shocks produced from wind-wind interactions in the depths of Cygnus OB2, but also to study the dynamics of a supernova remnant which likely propagated in the association a few tens kyr ago. Combining the simulation with a 3D model of the molecular cloud complex, I show how the LHAASO data on spectrum and morphology support the relic supernova remnant scenario, while wind-driven models require assumptions that are inconsistent with basic physical constraints.

Speaker: Thibault Vieu (MPIK)

2026_GammaHD_VIEU_Cygnus-1-1.pdf

255 A unified gamma-ray model for Westerlund 1

Westerlund 1 is the most massive young compact star cluster known in the Milky Way. It is surrounded by ring-like TeV gamma-ray emission, which aligns with the low-density bubble excavated by stellar winds, but not with the shell where ambient medium is swept-up. This presents a serious challenge to hadronic models. Instead, a leptonic scenario provides a good description of the data: electrons are accelerated at the termination shock of the star cluster wind and radiate via the inverse Compton mechanism (Härer et al. 2023). Recently, a Fermi-LAT analysis revealed a new source in the region, which is 1.5 deg across and off-set from Westerlund 1 perpendicular to the Galactic plane. This source traces electrons in a nascent chimney flow, which transports cosmic rays towards the Galactic halo (Lemoine-Goumard et al. 2025). In this talk, I present a joint model for all GeV-TeV data, including constraints on particle transport.

Speaker: Lucia Härer

26-09_Gamma_Haerer.pdf

256 Wolf-Rayet stars as tracers of gamma-ray emission: Isolated stars and stellar clusters/associations

Stellar wind termination shocks are considered potential sites for efficient particle acceleration, allowing an explanation for the overabundance of Ne$^{22}$ observed in cosmic rays (CRs) through Wolf-Rayet stars (WRs) and providing a minor but necessary contribution to the observed flux of Galactic CRs. However, only a few powerful stellar clusters/associations such as Westerlund 1 and Cygnus OB2 have been firmly detected in gamma rays, as well as very young star clusters embedded in parent dense molecular cloud. This lack of detection is limiting our ability to test the different scenarios of CR acceleration in stellar clusters. Therefore, a global census enabling to identify the most promising stellar clusters is now crucial to refine our understanding of particle acceleration and transport in these environments.

Previous approaches based on initial mass function extrapolations provide lower-limit estimates of cluster wind power and remain affected by modeling uncertainties. We adopt here a complementary and more direct strategy based on WR stars, which dominate the mechanical wind power of clusters and can individually rival entire systems. We develop a census of WR stars and WR-hosting clusters, and rank them according to a wind-power to distance squared proxy, directly related to the expected gamma-ray flux. We then perform spatial cross-correlations with gamma-ray catalogs, leading to the spatial coincidence of 12 WR-hosting clusters and 4 isolated WR stars with unidentified gamma-ray sources.

We then investigate the surrounding gas content and counterparts, allowing us to interpret the emission in terms of particle interactions with ambient matter or radiation fields. In parallel, we perform dedicated chance-coincidence studies to evaluate the statistical significance of the correlations. Altogether, this work provides a targeted list for detailed follow-up multi-wavelength analyses of promising regions, that could enable to test particle acceleration and gamma-ray production scenarios in stellar environments.

Speaker: Mr Alexandre Inventar (APC)

INVENTAR_GAMMA_2026_WR.pdf

257 Interacting supernovae as cosmic-ray factories: simulations and observations

Supernova remnants (SNRs) are generally considered as the main contributors to the Galactic sea of cosmic rays (CRs). Indeed, they are the only known sources in the Galaxy that are capable of explaining the energy density of CRs at lowest energies. Acceleration of CRs at the shock fronts of SNRs is confirmed by detection of non-thermal emission of radio waves, X-rays, and gamma rays. It is, however, unclear what are the highest energies that can be reached in these objects. There are several reasons to believe that SNRs are the most efficient accelerators during the very initial stages of evolution, right after the explosion, but so far no supernovae (SNe) were detected in gamma-rays despite dedicated observational campaigns. This could be partially due to the observation strategy - the peak of gamma-ray emission for core-collapse (CC) SNe evolving in smooth winds is expected to happen days to weeks after the explosion where most of the emission is attenuated by gamma-gamma interactions with the photosphere. It is known, however, that massive stars such as luminous blue variable (LBV) stars and red supergiants (RSGs) feature circumstellar shells with enhanced density. Interaction of the SN shock with such shells would increase both acceleration efficiency and non-thermal emission. Such episodes of interaction may happen months or years after the explosion where gamma-gamma absorption by photosphere is negligible and gamma-ray emission can be well detectable. In this talk we present numeric simulations of such scenarios and show that current observation strategies for gamma-ray signals from SNe should be re-designed. We report on the status of observation proposals submitted to various instruments in order to finds signs of late-time interactions. At the time of the abstract submission the campaign already resulted in detection of the radio re-brightening of a supernova 18 years after explosion.

Speaker: Robert Brose (Universität Potsdam)

Brose_RISE.pptx

Parallel B: Instrumentation/Analysis II

Convener: Robert Parsons (Humboldt-Universität zu Berlin)

258 Design, Performance and Progress of the LACT Array

The Large-scale Imaging Atmospheric Cherenkov Telescope Array (LACT) is a key upgraded facility of the Large High Altitude Air Shower Observatory (LHAASO). The array is consist of 32 atmospheric Cherenkov telescopes, each with a 6-meter aperture and an 8° wide field of view, featuring comprehensive detection advantages including a large effective detection area, ultra-high spatial resolution, and powerful particle identification capability. Above 10 TeV, LACT achieves an effective detection area of approximately 2 square kilometers with an angular resolution better than 0.05°, delivering a substantial improvement in spatial imaging performance. By combining muon information from LHAASO with Cherenkov imaging parameters, the array enables high-precision separation of gamma rays from cosmic-ray hadrons, effectively suppressing background noise and enhancing the observation sensitivity at ultra-high energies. It strongly complements and optimizes the overall observation capability of the LHAASO facility. The primary scientific objectives of LACT are to precisely measure the energy spectra and spatial radiation morphology of ultra-high-energy gamma-ray sources, investigate extreme astrophysical radiation processes, and unravel the acceleration mechanisms and origin of high-energy cosmic rays. At present, the first LACT telescope has entered trial operation and acquired observational data, while the second telescope is under intensive installation. The full array is scheduled to be fully completed by the end of 2028. This paper presents the overall design and the performance of LACT, reports preliminary observation results of the Crab Nebula obtained by the first telescope, and elaborates on the subsequent construction and observation plans of the array.

Speaker: Jiali Liu (Institute of High Energy Physics)

Gamma2026_LACT_Liujl_1111 - 1.pdf

259 First Light with the Dark100 Array of PANOSETI Telescopes at Palomar Observatory

Dark100 is the first phase of an extensive array of Panoramic SETI (PANOSETI) telescopes at Palomar Observatory, California. PANOSETI telescopes are small, 0.5-m Fresnel lens telescopes instrumented with 1024-pixel silicon photomultiplier (SiPM) cameras. The telescopes were originally designed to search for ultrafast optical transients, but they also function as cost-effective imaging atmospheric Cherenkov telescopes (IACTs). Dark100 will use seven of these widely separated IACTs to map the ultra-high-energy (UHE) gamma-ray sources on the Galactic Plane at high resolution. Dark100 will also leverage the IACT technique to search for evidence of ultraheavy dark matter in a mass range that extends beyond 100 TeV. Deployment of the first four telescopes and enclosures completed in December 2025, and the next three are scheduled to finish commissioning later this year. The array has so far collected more than 100 hours of 3-telescope data on sources including the Crab Nebula and Markarian 421. Here we present the current status of Dark100, and preparations for array roboticization. We also compare early data with Monte Carlo simulations.

Speaker: Nikolas Korzoun (Ruhr-Universität Bochum)

panoseti_gamma26.pdf

260 Prototype to Paranal: The Status of the First Small Sized Telescope of CTAO

The Small-Sized Telescopes (SSTs) of the Cherenkov Telescope Array Observatory (CTAO) are designed to explore the highest-energy gamma-ray sky, covering the energy range from a few TeV to beyond several hundred TeV. As the project transitions from prototyping to construction, the first SST is currently being deployed at CTAO-South in Chile. The Alpha configuration of CTAO will include 37 SSTs, each comprised of a dual-mirror optical design and a compact, SiPM-based camera.

In this contribution, we present the current status of the first SST, focusing on factory-level verification of the telescope structure and optics, together with progress in site construction. In parallel, we report on the status of the SST Camera, including early performance characterisation such as single-photoelectron response, intensity resolution, timing resolution at the sub-nanosecond level, and dynamic range up to thousands of photoelectrons, demonstrating readiness for on-site integration.

Speaker: Richard White (MHKP)

d260830-RW-SST-Gamma26_v1.2.pdf

261 Results from the upgraded Schwarzschild-Couder Telescope

The Schwarzschild-Couder Telescope (SCT) is a dual-mirror, high-resolution design for TeV gamma-ray astronomy using atmospheric Cherenkov imaging. A prototype SCT (pSCT) with a 9.7 m primary mirror diameter and partially instrumented focal plane was inaugurated in 2019 at the Fred Lawrence Whipple Observatory in Arizona. It detected the Crab Nebula in 2020. A similar design is used for both the optics and the camera of the Small-Sized Telescopes for the Cherenkov Telescope Array Observatory. We are now upgrading the pSCT camera, including improved readout and trigger electronics. In April 2026, we achieved first light and first air shower detection with the first 1408 pixels (spanning a 2.7° field of view) of the planned 11,328 pixels (spanning an 8° field of view). The image quality and high-resolution camera of the SCT enable excellent gamma-ray sensitivity and angular resolution across a wide field of view. A future array of SCTs could improve the sensitivity of the Cherenkov Telescope Array Observatory in its core energy range between 0.1 and 10 TeV. I will present initial results from the upgraded pSCT.

Speaker: Justin Vandenbroucke (Univerity of Wisconsin – Madison)

vandenbroucke_260903_gamma_sct_v2.pptx

262 High-Angular-Resolution Sub-GeV/GeV Gamma-Ray Observations of the Galactic Center Region with a Balloon-Borne Emulsion Telescope (GRAINE)

The Gamma-Ray Astro-Imager with Nuclear Emulsion (GRAINE) project aims to perform high-angular-resolution gamma-ray observations in the sub-GeV/GeV energy range using balloon-borne nuclear emulsion telescopes. Owing to the sub-micron spatial resolution of nuclear emulsions, the instrument achieves an angular resolution of about 0.1° at 1 GeV and 1° at 0.1 GeV, representing an order-of-magnitude improvement over the Fermi Large Area Telescope. This capability enables detailed imaging of gamma-ray sources and provides access to observables such as polarization in the sub-GeV region.

In the 4th balloon experiment conducted in 2023, a telescope with an aperture of 2.5 m²—significantly larger than previous flights—was successfully operated, resulting in increased exposure and improved statistical sensitivity. Using this dataset, we report new observations in the sub-GeV band, including results on the Galactic Center region and the Vela pulsar.

For the Galactic Center region, we derive flux upper limits in the sub-GeV range with the highest angular resolution achieved in this energy range. These measurements provide new constraints on models of the Galactic Center GeV excess, particularly on scenarios involving unresolved source populations such as millisecond pulsars. The results demonstrate the unique capability of emulsion-based gamma-ray telescopes to provide constraints on the spatial structure of gamma-ray emission that are difficult to access with current space-based instruments, offering complementary information to existing observations.

We also present updated imaging results of the Vela pulsar and discuss the overall performance of the 2023 flight. Finally, we outline future prospects for long-duration balloon observations and further expansion of the telescope aperture.

Speaker: Hiroki Rokujo (Nagoya University)

2060903GRAINE-Rokujo.pdf

263 Investigating past high-energy fluxes with paleo-detectors

Paleo-detectors provide a unique avenue to reconstruct the multi-million-year history of cosmic-ray (CR) flux, preserving signatures of transient high-energy events such as nearby supernovae. This technique aims to use natural minerals as particle detectors, looking at the persistent damage tracks created by CR-induced nuclear recoils, accumulated over the minerals’ geological lifespan, offering a geological archive of past particle fluxes. Building on our study of Messinian Salinity Crisis evaporites, which demonstrated that minerals with specific geological histories may enable the detection of primary CR flux variations, we have now expanded to diverse terrestrial records. This contribution presents our recent publication proposing olivine xenoliths from Auvergne, France, where eruption chronosequences could allow for the differentiation of CR flux scenarios over the last 50 kyr. Additionally, we discuss ongoing research into applications of the paleo-detector technique on quartz samples from the Cradle of Humankind, South Africa, to disentangle the datation of paleontological samples. This phenomenological work is supported by the INFN-funded PRImuS experiment, which aims to utilize high-throughput optical microscopy and plasma etching to analyze these mineral targets. By validating theoretical track-length spectra and refining background estimates, PRImuS’s goal is to establish paleo-detectors as a powerful tool for very-long-range time-domain astrophysics.

Speaker: Dr Claudio Galelli (INFN Milano)

PRImuS gamma26 (1).pdf

264 The GRAMS Project: A Next-Generation Mission for Gamma-Ray and Antimatter Studies

GRAMS (Gamma-Ray and AntiMatter Survey) is a NASA-funded balloon project designed to carry out both MeV gamma-ray observations and searches for indirect signatures of dark matter. Making use of cost-effective, scalable LArTPC (Liquid Argon Time Projection Chamber) technology, GRAMS will provide unprecedented sensitivity to gamma rays in the under-explored “MeV-gap” region, a crucial window for studying highly energetic, multi-messenger phenomena, including supernova remnants, neutron star mergers, gamma-ray bursts, and active galactic nuclei. Simultaneously, GRAMS will extensively probe the dark matter parameter space via low-energy antideuteron and antihelium measurements.

The GRAMS LArTPC functions as both a Compton telescope and a 3D tracking detector, providing excellent energy resolution and three-dimensional position sensitivity. The detector concept has been successfully demonstrated via a small-scale engineering flight with JAXA in 2023, and through antiproton beam tests at J-PARC in 2025. GRAMS has been funded by NASA for a prototype balloon flight (pGRAMS), slated to launch from Tucson, Arizona in the summer of 2026. In this contribution, I will provide an overview of the GRAMS detection technique, the overall mission progress, and hardware development efforts for next-generation LArTPC technology.

Speaker: Svanik Tandon (Columbia University)

GRAMSGamma2026Final.pdf

265 GECO: the Galactic and Extragalactic Compact-object Observatory for the transient sky

In the last 20 years the study of the transient sky has been boosted thanks several space- and ground-based telescopes. Precise localisation, wide field of view and rapid coordinate disseminations are mandatory requirements to allow for follow-up of transient events such as GRBs and other rapidly flaring sources. The GECO (Galactic and Extragalactic Compact-object Observatory) is a new space mission proposed to the Italian Space Agency. With a launch planned in the 2030's, GECO will fill the observational gap at X-rays thanks to its unprecedented combination of field of view (half-sky), point source localisation accuracy (1 arcmin), broad spectral response (2 keV – 2 MeV) and energy resolution (350 eV at 6 keV). The GECO coded-mask All Sky Monitor (ASM, 2–50 keV) will detect, localise and study steady, variable and transient sources simultaneously appearing anywhere in the accessible 50% of the sky, downlinking real-time arcmin localisations and photon-by-photon data for in depth spectral and timing studies. The GECO Broadband Universal Radiation Spectrometer for Transients (BURST) instrument will extend the energy band up to 2 MeV, enabling the full spectral characterisation of the hardest transients, from gamma ray bursts to magnetar flares and bright black hole transients. GECO will provide crucial and timely triggers to observe sources in their most interesting states, enabling studies of the link between accretion and jet ejection across all classes of transient sources, from stellar mass black holes to active galactic nuclei and gamma ray bursts, through coordinated follow-up observations at the very-high-energies with, e.g., CTAO.

Speaker: Giovanni Della Casa (INAF/IAPS Rome)

GECO.pdf

GECO.pptx

Dinner: Conference Dinner

Friday 4 September

Plenary: Rapporteur I

Convener: Jim Hinton (MPIK)

266 Rapporteur Extra-Galactic Science

Speaker: Elisa Prandini

Gamma26_Prandini_EgalScience_final.pdf

267 Rapporteur Galactic Science

Speaker: Gwenael Giacinti

Rapporteur_Galactic.pdf

268 Rapporteur Instrumentation & Analysis

Speaker: Prof. Jamie Holder (University of Delaware)

Heidelberg_Instrumentation_Rapporteur.pdf

Heidelberg_Instrumentation_Rapporteur.pptx

10:30 Coffee break

Plenary: Rapporteur II + Closing Remarks

Convener: Brian Reville (MPIK)

269 Rapporteur Theory

Speaker: Prof. Roger Blandford (SLAC)

blandford.pdf

blandford.pptx

270 Concluding Session

Gamma2026Conluding.pdf