Continuous gravitational waves and neutron stars

17 Jun 2024, 08:30 → 20 Jun 2024, 18:00 Europe/Berlin

Hannover, Germany

The continuous gravitational waves and neutron stars workshop will be held from June 17-20 2024, in Hannover (Germany) at the Max Planck Institute for Gravitational Physics. This is the second edition of the previous "Multi-messenger continuous gravitational waves" workshop.

The main purpose of this workshop is for members of the continuous gravitational waves community and of closely related fields (such as electromagnetic detections of neutron stars, neutron star modeling, etc.) to come together and present their work, with plenty of time for discussions.

The main topics of the workshop are defined in scientific program section. The workshop will be divided into sessions for the different topics, and will consist of contributions from some of the participants and review talks from invited speakers. We accept contributions in the format of traditional talks, reviews, or tutorials about different software tools. We especially welcome contributions from early-career scientists.

Important deadlines:

• Abstract submission and registration: 7 March 2024
• Acceptance/rejection of received abstracts and registration: 22 March 2024
• Payment of registration: 22 April 2024

Given the limitation on the number of participants, notification regarding the acceptance of your abstract and/or registration will be provided on March 22th.

Invited speakers:

• Nancy Aggarwal (University of California, Davis)
• Stefano Ascenzi (Gran Sasso Science Institute)
• Ian Jones (University of Southampton)
• Sergey Tsygankov (University of Turku)
• Karl Wette (Australian National University)

More information about possible social events will be published as the starting date approaches.

For any questions, please write to contact.cw2024@aei.mpg.de

LOC: Pep Covas Vidal, Maria Alessandra Papa.

SOC: Colin Clark, Pep Covas Vidal, Fabian Gittins, David Keitel, Andrew Miller, Serena Vinciguerra.

Monday 17 June

08:30 → 08:50 Registration

08:50 → 09:00 Opening remarks

Speaker: Pep Covas Vidal (Max-Planck-Institut für Gravitationsphysik)

Talk.pdf

09:00 → 10:25 Search results CW: 1

09:00 Continuous Gravitational Waves: Methods, Results, and Practical Tools

The next breakthrough in gravitational wave astronomy may be the first discovery of continuous gravitational waves. I will review the data analysis methods used to search for continuous gravitational waves since the early days of LIGO and Virgo. I will present an overview of continuous gravitational wave search results that have been achieved to date, and show how they illustrate the various trade-offs considered in search setup and design. Finally, I will demonstrate a few practical data analysis tools.

Speaker: Karl Wette (Australian National University)

CWMethodsResultsTools.pdf

Link to IPython notebook in Google Colab

10:00 Continuous gravitational wave atlas.

Continuous gravitational wave atlas is a way to distribute data from a wide parameter continuous wave search. We will show the latest atlas produced by Falcon pipeline, with examples of how to use the atlas data in new searches.

The atlas data is provided in a new MVL file format that allows analysis on small computers, such as your notebook. There is also an MVL version of Gaia DR3 data, which can be used to explore the Falcon atlas, or on its own.

The MVL file format has a simple layout, but capable of expressing data structures of arbitrary complexity. The format was designed for memory mapped access so that the data from the same file can be shared between multiple running processes. We will show examples of how you can use MVL files in data analysis.

Speaker: Dr Vladimir Dergachev (AEI/Hannover)

2024-Jun-Hannover-slides2.pdf

10:25 → 10:55 Coffee break

10:55 → 12:20 Search results CW: 2

10:55 20 years of the Hough transform paper

.

Speakers: Prof. Badri Krishnan (Radboud University), Sergio Frasca (GNOI)

hough-anniversary.pdf

Rome_Hough.pdf

Rome_Hough.pptx

11:55 Status of the O4 run and LVK searches

Started in May 2023 and planned to last 20 months, the fourth observing run (O4) of the advanced ground-based interferometric detector network LIGO-Virgo-KAGRA provides the deepest yet reach into the gravitational-wave side of our Universe. I will give a status update on the run's progress and results obtained so far, as well as a brief overview of the science scope of planned O4 searches for continuous waves by the collaboration.

Speaker: David Keitel (Universitat de les Illes Balears)

20240617_CGWNS_David_Keitel.pdf

12:20 → 13:50 Lunch break

13:50 → 15:15 Neutron star modelling: 1

13:50 Modelling continuous gravitational wave emission from neutron stars

I will describe the latest results on the modelling of gravitational wave emission from neutron stars, discussing both mountains and stellar oscillations. In the case of mountains, I review the latest results on both maximum mountains, and the mechanisms by which mountains might form in the first place. I will end by discussing the inverse problem, i.e. what we learn following a detection.

Speaker: Ian Jones (University of Southampton)

hannover_2024.pdf

hannover_2024.pptx

14:50 High priority targets for transient continuous waves from glitching pulsars

The spin of pulsars can be timed with exquisite precision. They are observed to spin-down steadily over long periods but once in a while, they can undergo a rapid and sudden increase in their spin, which is known as a glitch. For some cases, the glitch is followed by a post-glitch recovery. There are several models that predict the emission of continuous gravitational waves from glitches and their recovery and in this talk, I outline a few of them. Using publicly available data, we compare these models and, after accounting for sky location, we are able to provide a list of high priority targets for continuous wave searches.

Speaker: Garvin Yim (Kavli Institute for Astronomy and Astrophysics, Peking University)

High priority targets for transient continuous waves from glitching pulsars.key

High priority targets for transient continuous waves from glitching pulsars.pdf

15:15 → 15:45 Coffee break

15:45 → 17:00 Neutron star modelling: 2

15:45 A (old) new science goal for deci-Hertz gravitational wave detectors

In the next few decades a number of space-based gravitational wave detectors sensitive in the deci-Hertz band should be operational.
Among the main goals of such detectors are a number of "first detections" such as primordial gravitational waves, compact stellar mass inspirals and intermediate mass black hole mergers.
Interestingly enough we find no mention of Continuous Gravitational Waves (CGWs). Such signals, which are yet to be detected, are thought to be emitted by rotating Neutron Stars (NSs) with a reasonably small degree of non-axisymmetry.
In a previous study (arXiv:2303.04714) we show that, not only the frequency band currently covered by ground-based detectors is populated by less than ~ 0.5% of all Galactic NSs, but also how those sources who possess a strong magnetic field and thus are highly deformed in the scenario of a magnetically induced non-axisymmetry, all lie in the deci-Hertz band, outside the currently "visible" one.

Here we investigate on the possible role that deci-Hertz detectors can play for the cause of CGWs detection.
We consider blind surveys as well as directed and targeted searches for CGWs.
In the former case we find that detecting unknown neutron stars will be possible only if these have an almost purely toroidal magnetic field topology. This result is in line with what we find on a previous study of ours.

In the case of directed searches for known Central Compact Objects in Supernova Remnants, we show how, if no detections are made, physically meaningful constraints can be put down to few Hz.

In the case of known pulsars, similarly to the case of directed searches, we cannot make detectability predictions, but we can explore the no-detection scenario.
Already at the DECIGO pathfinder sensitivities we would be able to constrain spin-down ellipticities for hundreds of known pulsars (currently we can do that for ~ 20), and of these, about a tenth would result to have the percentage of CGW spin-down luminosity constrained down to < 1% (currently this is valid only for three pulsars only).
Such estimations will significantly improve (factor of ~ 3-4) when considering the full DECIGO detector.

Speaker: Gianluca Pagliaro (Max Planck Institute for Gravitational Physics / Leibniz University Hannover)

cww_pres_PAGLIARO.pdf

16:10 Improving the understanding of evolution of binary neutron stars with Einstein Telescope

The Einstein Telescope (ET) is a proposed third-generation, wide-band gravitational wave (GW) detector which will have an improved detection sensitivity for low frequencies, leading to a longer observation time in the detection band and higher detection rate for binary neutron stars (BNSs). GWTC-3 detections have already constrained the merger rates of BNS with just two events, assuming that the merger rate is constant in comoving volume out to a redshift of z = 0.15. The evolutionary parameters play a decisive role in determining the merger rate density as a function of redshift. Despite the fact that ET will have a higher detection rate, the bulk of BNSs will remain undetectable. Exploiting the fact that slow frequency-evolution in the early inspiral of a BNS signal can be searched for using continuous wave methods such as those based on the Hough Transform, and the fact that different population models predict different merger rate density over redshift, Miller et al 2024 already showed that different evolutionary scenarios can be constrained with ET. In Singh et al 2024 we presented a population independent method to reconstruct the merger rates of coalescing compact binaries with ET using information from the full inspiral of a coalescing BNS signal to localise and then constrain the intrinsic parameters of the binary. We formulated a scheme to estimate accurate detection efficiency and showed how to estimate the true merger rate density of the underlying population. In this current work, we use the algorithm developed earlier to explore the capability of ET in probing the evolutionary parameters of the evolution of BNSs, by constraining the merger rate density as a function of redshift despite the loss in detection of the bulk of the BNS population. I will discuss the discriminating power of CW methods and also discuss the possibility of providing constraints on the number of solitary neutron stars from the constraints on the binary evolution models using merger rate estimates.

Speaker: Neha Singh (Universidad de las Islas Baleares)

Hannover_CW.pdf

16:35 Detection possibility of continuous gravitational waves from isolated rotating magnetized compact objects

In the past decades, several neutron stars (NSs), particularly pulsars, with mass $M>2M_\odot$ have been observed. On the other hand, the existence of massive white dwarfs (WDs), even violating the Chandrasekhar mass limit, was inferred from the peak luminosities of type Ia supernovae. Hence, there is a generic question of the origin of massive compact objects. Here we explore the existence of massive, magnetized, rotating compact objects using XNS code, which solves stationary stellar equilibria in general relativistic magnetohydrodynamics (GRMHD). We visualise the deformation of the compact objects due to magnetic field (toroidal and/or poloidal) and rotation (uniform or differential), by solving the Einstein equation (describing space-time metric) and Magneto-Hydrostatic Equilibrium (providing distribution of matter/energy) simultaneously. Our aim here is to understand the detection possibility of isolated NSs and WDs, which are difficult to detect in electromagnetic surveys, such as SDSS, Kepler, Gaia. Such isolated rotating objects with magnetic field and rotation axes misaligned, hence (triaxial system) having non-zero obliquity angle, can emit continuous gravitational waves (GWs), which can be detected by upcoming detectors, e.g., Einstein Telescope, DECIGO etc. We discuss the decays of magnetic field, angular velocity, and obliquity angle with time, due to Hall, Ohmic, ambipolar diffusion and angular momentum extraction by GW and dipole radiation, which determine the timescales related to the GW emission. Further, in the Alfvén timescale, a differentially rotating, massive proto-NS and WD rapidly loses angular momentum to settle into a uniformly rotating, less massive compact objects due to magnetic braking and/or viscous drag. These explorations suggest that the detection of massive compact objects is challenging and sets a timescale for detection. We calculate the signal-to-noise ratio of GW emission, which confirms that any detector cannot detect them immediately, but detectable by Einstein Telescope, Cosmic Explorer and BBO, DECIGO over months of integration time, leading to direct detection of NSs and WDs respectively.

Reference: Detection Possibility of Continuous Gravitational Waves from Rotating Magnetized Neutron Stars Mayusree Das and Banibrata Mukhopadhyay, ApJ, 955 (2023) 1, 19, 10.3847/1538-4357/aceb63 arXiv e-Print 2302.03706

Speaker: Mayusree Das (Indian institute of science)

cw2024-1-29.pdf

17:00 → 17:45 Discussion

Tuesday 18 June

09:00 → 10:50 Search results/methods EM: 1

09:00 Polarized emission from accreting neutron stars as seen by the IXPE observatory

Analysis of the polarization of electromagnetic radiation, or polarimetry, is a unique tool that allows us to obtain information about astrophysical objects that cannot be obtained in other ways, for example, regarding their geometry. With the launch of the IXPE (Imaging X-ray Polarimetry Explorer) mission at the end of 2021, this instrument became available in the X-ray range as well. In my talk I will give a brief overview of the results obtained during the first two years of IXPE observations of accreting neutron stars, mainly focusing on the highly magnetised ones (X-ray pulsars, XRPs). It was found that in all observed XRPs, the measured value of the degree of polarization is below 15%, which is much less than the theoretically predicted values. In some pulsars, it was possible to study in detail the variations in the degree of polarization and the polarization angle as a function of the rotation phase of the neutron star, which, in turn, made it possible to determine the geometric parameters of the system. I will discuss the physical mechanisms that could potentially explain the unexpected polarization properties observed in XRPs.

Speaker: Sergey Tsygankov (University of Turku)

tsygankov_IXPE.pdf

10:00 Discovery and timing of unique binary pulsars in globular cluster Terzan 5 and a search for compact pulsar binaries using volunteer distributed computing.

Globular clusters have historically been a prime target for radio pulsar searching owing to their old stellar population and dense environment. The Terzan 5 globular cluster is one such source that houses numerous pulsars and has been extensively studied at multiple wavelengths. In this talk, I will be presenting 9 new binary pulsars discovered in Terzan 5 using the MeerKAT radio telescope. These include a potential double neutron star system with a total mass of ~ 3.2 solar masses and if confirmed, would host the fastest spinning pulsar (P ~ 2.27 ms) for any double neutron star system (DNS) known. Three systems belong to a class of spider pulsars that show radio eclipses due to material from the non-degenerate companion obscuring the pulsed emission. We were also able to constrain the individual masses for a system by measuring the Shapiro delay due to the binary companion. One system possesses the second highest eccentricity known (e = 0.903) for a recycled pulsar. These discoveries pose multiple open questions regarding their stellar evolution and formation. Finally, I will describe Einstein@Home, a volunteer distributed computing project. Through Einstein@Home, we aim to find compact pulsar binary systems (30 min < Pb < 300 min) with neutron star companions in MeerKAT globular cluster data using coherent template-bank methods. This setup has already demonstrated its capability by blindly redetecting several known pulsars in multiple globular clusters.

Speaker: Prajwal Voraganti Padmanabh (Max Planck Institute for Gravitational Physics (AEI))

CW2024_Hannover_PVP.pdf

10:25 Long-term pulsar timing solutions with joint radio and gamma-ray analyses support searches for gravitational waves

Radio searches of unidentified Fermi-LAT gamma-ray sources have discovered more than a hundred new millisecond pulsars. The two types of data, radio and gamma-ray, have very different properties. For example, the radio data provides extremely precise pulse arrival times, but often has gaps and typically only covers the time span between detection and the most recent observation. The gamma-ray data however is sparse, but covers the full 15 years between the launch of the Fermi satellite and today. Previously, these data sets have been treated separately - radio timing solutions are typically only used as a starting point for gamma-ray timing, with some parameters fixed at their radio values. However, a joint analysis of both the radio and gamma-ray data can provide improved precision and sensitivity to effects that cannot be measured from either data set alone. In this talk, I will present four pulsars, discovered with MeerKAT as part of the TRAPUM project, timed jointly in radio and gamma rays. The 15-year span enables continuous gravitational wave follow-up searches in archival LIGO data and speeds up the integration of such pulsars into pulsar timing arrays.

Speaker: Lars Nieder (AEI Hannover)

LN_Hannover_2024_JRAGtiming.pdf

10:50 → 11:20 Coffee break

11:20 → 13:00 Search results/methods EM: 2

11:20 Dusting gravitational waves off spider pulsars' cobwebs

Neutron stars in compact binary systems are potential sources of continuous gravitational waves. While those containing another degenerate object should provide the "cleanest" signals, double neutron stars are rare, and neutron star-white dwarfs tend to be found at wider orbital periods. On the other hand, another type of neutron star binary, colloquially called spiders due to the strongly irradiated and evaporating semi-degenerate companion they host, appears to form an emerging, abundant fraction of the neutron star binary population at orbital periods ranging from a day all the way down to an hour, and potentially shorter. In this talk, I will discuss how recent results from multi-wavelength observations of spiders, which are known to harbour some of the most massive and fastest-spinning pulsars, can provide insights into neutron star and binary physics but could also prove to be key in enabling the detection of continuous gravitational waves.

Speaker: Rene Breton (University of Manchester)

2024-06 - Breton - CW2024.pdf

11:45 Hunting for the Fastest-Spinning and Heaviest Neutron Stars Using Optical Telescopes

Since its launch in 2008, the Fermi Large Area Telescope (Fermi-LAT) has aided to discover more than half of the known millisecond pulsars (MSPs). Among these are Spider pulsars, which are MSP binaries with main sequence or semi-degenerate stars in tight orbit (orbital periods ≲ 1day). These Spider pulsars are among the fastest-spinning and heaviest pulsars due to the recycling scenario. Observing these pulsars in radio wavelengths can be challenging due to eclipses caused by intrabinary material shed away by the companion star which can last up to 70% of the orbit. Optical surveys, on the other hand, offer an alternative channel to search for Spider pulsars. In such tight orbit, the companions are tidally distorted and heavily irradiated by pulsar winds. These effects form unique sinusoidal modulations on optical light curves, distinguishing them from most variable stars. Since 2020, we have been observing unidentified Fermi-LAT sources using the Thai Robotic Telescope, Liverpool Telescope, and New Technology Telescope at La Silla Observatory. A new black widow PSR J1544-2555 was discovered by the survey in 2022 and subsequently confirmed by radio follow-up observations using the radio telescope MeerKAT in 2023.

Speaker: Adipol Phosrisom (Jodrell Bank Centre for Astrophysics, The University of Manchester)

CW2024_Hunting_for_the_Fastest-Spinning_and_Heaviest_Neutron_Stars_Using_Optical_Telescopes.pdf

CW2024_Hunting_for_the_Fastest-Spinning_and_Heaviest_Neutron_Stars_Using_Optical_Telescopes.pptx

12:10 Fast optical photometry as a tool for CW counterpart searches

Millisecond pulsars are ideal targets to probe the strong interaction at supranuclear densities and search for continuous gravitational wave (CW) sources. Either the rotation of their magnetic field or the infall of matter lost by a companion star is assumed to power their electromagnetic emission. Recently, we exploited the fast optical photometer SiFAP2 at 3.6m INAF’s Telescopio Nazionale Galileo to discover optical pulsations from two millisecond pulsars surrounded by an accretion disk. Thanks to the much higher photon counting statistics of an optical telescope compared to high energy instruments, this has opened the intriguing possibility of searching for weak pulsed signals from accreting neutron stars at an unprecedented sensitivity. I will discuss the properties of the optical millisecond pulsars discovered so far and new search efforts carried out in our group to pin down the spin frequency and orbital parameters of strong candidates for CW emission.

Speaker: Riccardo La Placa (INAF - Osservatorio Astronomico di Roma)

Fast_optical_photometry_La_Placa.pdf

Fast_optical_photometry_La_Placa.pptx

12:35 Parameter estimation for a two-component neutron star model with a Kalman filter

Pulsar timing noise is the stochastic deviation of the pulse arrival times of a pulsar away from their long term trend. In the standard two-component crust-superfluid neutron-star model, timing noise can be explained as the perturbation of the two components by irregular torques. Interactions between the crust and superfluid cause these perturbations to decay exponentially with a characteristic relaxation time scale. In this research we assume the two-component model, then use a Kalman filter to track the pulsar frequency over time and produce a Bayesian posterior for the two-component model parameters given the data. Our method is reliable on simulated data, which we show through individual and large-scale Monte Carlo tests. We also show a representative example on publicly available data from a real pulsar, where we test the two-component model and use it to efficiently measure physical properties of the star, including the relaxation time scale. Our measurements of the properties of neutron stars may lead to information about their equation of state and provide evidence for or against the two-component model.

Speaker: Nicholas O'Neill (The University of Melbourne)

hanover_talk.pdf

13:00 → 14:30 Lunch break

14:30 → 15:45 Search results CW: 3

14:30 Deep Einstein@Home search for Continuous Gravitational Waves from the Central Compact Objects in the Supernova Remnants Cas A, Vela Jr. and G347.3-0.5 using LIGO public O3 data

We perform a search for continuous nearly monochromatic gravitational waves from the central compact objects associated with the supernova remnants (SNRs) Cas A, Vela Jr. and G347.3 using LIGO O3 public data.

The searches for all three targets have the board frequency range from 20 and 1500 Hz. The search is deployed on the volunteer computing project Einstein@Home for six monthes, with thousands of participants donating compute cycles to make this endeavour possible. After the search is done, we perform multi-stages sub-threshold follow-up searches for over 10 million most promising waveforms out of 1e19 of the original search.

The follow-up searches are currently on going which are expected to be done before April, 2024. We yet don't know if we can find any signal candidates or not. However, even there is no candidate found in the end, we can set the upper limits on the amplitude of gravitational wave signals from these three targets which are approximately 80% more constraining than the most sensitive seaches up to date.

Speaker: Jing Ming (AEI Hannover)

2024CW_Han_25mins_jing.pdf

14:55 Probing the pulsar explanation of the Galactic-Center GeV excess using continuous gravitational-wave searches

Over ten years ago, Fermi observed an excess of GeV gamma rays from
the Galactic Center whose origin is still under debate. One
explanation for this excess involves annihilating dark matter; another
requires an unresolved population of millisecond pulsars concentrated
at the Galactic Center. We use the results from LIGO/Virgo/KAGRA's most
recent all-sky search for quasi-monochromatic, persistent
gravitational-wave signals from isolated neutron stars to determine
whether unresolved millisecond pulsars could actually explain this
excess. We find that a large set of the parameter space in the pulsar
luminosity function can be excluded.

Speaker: Prof. Yue Zhao (University of Utah)

GeV-CW.pdf

GeV-CW.ppt

15:20 Gravitational wave searches with the Band-Sampled-Data framework

The Band-Sampled-Data (BSD) framework has emerged as a powerful tool in the search for GW signals. With its versatile capabilities and adaptability, BSD has been instrumental in various searches for both standard CW signals and dark matter candidates. Over the past five years, the BSD framework has evolved significantly, offering a comprehensive suite of functions for analysing GW data. Its foundational functions provide a solid basis for implementing search methods tailored to specific research goals. One of the key strengths of the BSD tool lies in its ability to handle data effectively. This allows us to search for a wide range of GW signals with varying characteristics. In this summary talk, I will discuss the framework's capabilities and limitations. I will show some examples of its application in real searches and I will discuss some of the developments anticipated for the BSD framework in the coming years.

Speaker: Ornella Juliana Piccinni (IFAE)

BSD_5years.pdf

15:45 → 16:15 Coffee break

16:15 → 17:30 Neutron star modelling: 3

16:15 Universal relations and equation-of-state inference for rapidly rotating neutron stars

The accurate modeling of rotating neutron stars and their observational signatures is fundamental to exploring the nuclear equation of state at high densities. However, modeling rapidly rotating neutron stars, especially their dynamical properties, becomes increasingly involved with rotation rate. This talk presents a set of novel universal relations that are valid even with high rotation rates. We then demonstrate how one can utilize them in a Bayesian analysis to constrain parametrized equations of state from mock data of hypothetical, high-precision measurements of masses, radii, rotation rate, and f-mode frequencies. By analyzing different sets of such mock data, our method also allows one to quantify systematic biases due to neglecting the modeling of rotational effects.

Speaker: Dr Sebastian Völkel (Max Planck Institute for Gravitational Physics (Albert Einstein Instititute))

talk.pdf

16:40 A microlensing effect in the continuous gravitational wave signal: signal from the spinning neutron star lensed by a point mass

To detect quasi-monochromatic gravitational wave radiation emitted by the non-symmetric rotating neutron stars requires a long observation time to distinguish it from the detector's noise. If, in addition, the signal is microlensed, the mass of the lens magnifies the signal amplitude, which aids in detection of these signals and in probing the physical nature of the lens as well as the source. We study the observational feasibility of microlensing of continuous gravitational wave signals in the point mass lens approximation by performing a proof-of-concept search using simulated data from ground-based detectors with the semi-coherent Time Domain F-statistic method.

Speaker: Sudhagar Suyamprakasam (Nicolaus Copernicus Astronomical Center Polish Academy of Sciences)

sudhagar_CGWNS-2024.pdf

17:05 Computation of spin evolution of millisecond pulsars: a way to probe continuous gravitational waves

The observed spin frequency distribution of millisecond (ms) pulsars is a result of their current or previous spin evolution in the low-mass X-ray binary phase. Such a spin evolution depends on various physical aspects, such as binary evolution, disk-magnetosphere interaction, continuous gravitational wave emission, transient accretion, neutron star equations of state, etc. Thus, the computation of ms pulsar evolution, when performed considering these aspects, provides an excellent way to estimate the ellipticity of some pulsars and to prepare one to utilize the future continuous gravitational wave data from ms pulsars to probe the neutron star physics. Here, we will discuss the crucial effect of transient accretion on the spin evolution of ms pulsars, and how this effect indicates continuous gravitational wave emission from some pulsars. [Reference: (1) Bhattacharyya and Chakrabarty, 2017, ApJ, 835, 4; (2) Bhattacharyya, 2021, MNRAS, 502, L45].

Speaker: Sudip Bhattacharyya (Tata Institute of Fundamental Research)

bhattacharyya_sudip_Hannover_2024.pdf

17:30 → 18:00 Discussion

Wednesday 19 June

09:00 → 10:15 Search methods CW: 1

09:00 A new analytical limit of the Bayes Factor for continuous gravitational waves

The maximum-likelihood F-statistic, a classical tool in continuous-gravitational-wave (CW) analysis, can be represented as a Bayes-factor analytically marginalized over unphysical priors on the four signal amplitude parameters (h0, cosi, psi, phi0).

This makes the F-statistic Neyman-Pearson sub-optimal but computationally attractive, because four search parameters have been eliminated analytically. Despite various attempts to find a more physical Bayes factor with similar computational efficiency, this still remains an open problem.

Here I report on some progress on this topic: by using a better prior on the amplitude h0, one can identify distinctive 'weak-signal' and 'strong-signal' regimes. Remarkably, in the weak-signal regime one can achieve fully analytical marginalization over the four amplitude parameters. I will discuss some of the properties of this 'weak-signal B-statistic', including its relation to the well-known '5-vector' method.

Speaker: Reinhard Prix (Albert-Einstein-Institut Hannover)

Talk_BstatWeak_CWAEI2024-v1.pdf

Talk_BstatWeak_CWAEI2024-v2-fixes.pdf

09:25 Bayesian F-Statistic based parameter estimation of continuous gravitational waves from known pulsars

Searches for continuous gravitational waves targeted at known pulsars use pulsar timing observations to infer the phase evolution parameters of the CW signals they emit. We present a new method and implementation to obtain Bayesian posteriors on the amplitude parameters of the CW signal, combining modern Bayesian parameter estimation techniques with the well-established F-statistic framework. We further explore the benefits of a likelihood function that is analytically marginalized over the initial-phase of the signal. We test the methods on simulated signals, CW hardware injections in Advanced-LIGO detector data, and using percentile-percentile (PP) self-consistency tests of the posteriors and furthermore use it in a first real-world known pulsar CW search.

Speaker: Pep Covas Vidal (Max-Planck-Institut für Gravitationsphysik)

Talk.pdf

09:50 Detecting a long-duration transient signal

Spinning neutron stars are sources of long-duration continuous waves that may be detected by interferometric detectors. We focus on long, but not infinite duration continuous wave signals when there are no electromagnetic observations to inform on the time of occurrence of the signal. We propose a search scheme to identify the signal duration and location of the signal in the data.

Speaker: Liudmila Fesik (Max Planck Institute for Gravitational Physics (Albert Einstein Institute))

Fesik_present.pdf

10:15 → 10:45 Coffee break

10:45 → 11:35 Search methods CW: 2

10:45 A semi-coherent method for detecting continuous gravitational waves from binary inspirals of dark compact objects

Dark compact objects, like primordial black holes, can span a large range of masses depending on their time and mechanism of formation. In particular, they can have subsolar masses and form binary systems with an inspiral phase that can last for long periods of time. Additionally, these signals have a slow increase of frequency, and, therefore, are well suited to be searched with continuous gravitational waves methods. We present a new pipeline called COmpact Binary Inspiral (COBI), based on the Band Sampled Data (BSD) framework, which specifically targets these signals. We describe the method and show an example of its application in a subset of O3 data, discussing the distance reach of the search and its computing cost.

Speaker: Marc Andrés-Carcasona (IFAE)

CW24_Hannover.pdf

11:10 Searching for Mini Extreme Mass Ratio Inspirals with Gravitational-Wave Detectors

I will discuss the mini-EMRI systems that can be detected at LIGO,
which are ideal systems for detecting very light subsolar exotic compact
objects. The detection strategy and method are both similar to those
currently being used in CW searches. This talk is based on the paper
https://arxiv.org/abs/2205.10359.

Speaker: Huaike Guo (University of Chinese Academy of Sciences)

mini-EMRI.pdf

11:35 → 13:05 Lunch break

13:05 → 14:30 Neutron star modelling: 4

13:05 Neutron star populations in the Galaxy and their evolutionary interconnection

In this talk, I will review the main properties of the galactic population of neutron stars, the so-called neutron star zoo. I will focus in particular on the subgroup of isolated stars, such as magnetars, X-ray dim isolated neutron stars (XDINS), and central compact objects in supernova remnants (CCOs).
I will also discuss how different neutron star populations are thought to be interconnected, in particular within a magneto-thermal evolutionary framework.

Speaker: Stefano Ascenzi (Gran Sasso Science Institute (GSSI))

Hannover-CGWmeeting_Ascenzi.pdf

14:05 Mapping the spatial distribution of neutron star binaries within the Milky Way using novel simulations

In the era of multi-messenger astrophysics, we are on the precipice of observing binary systems within our galaxy using gravitational waves in addition to traditional electromagnetism, in the advent of third generation gravitational wave detectors such as LISA. The joint messenger observations can bolster our detection ability of systems throughout our galaxy, providing an unprecedented insight into their populations. By predicting the spatial distribution of binaries within our galaxy, we can test multiple facets of binary evolution, such as stellar, orbital, and kick evolutions.

In this talk, I will present our end-to-end pipeline (work-in-progress) to accomplish track the orbital motion of binaries robustly using population synthesis and hydrodynamical galaxy simulations. By combining our understanding of binary motion throughout the galaxy, we can create simulations to predict the spatial densities of binaries across their host galaxies. Coalescing binaries within our simulations can be used as proxies to map transient rates and offsets. This includes characterising the subset of binaries that are ejected, which is important in characterising merger induced transients that contribute towards a “host-less” demographic (i.e., no known host galaxy).

I will present the current development progress and explore the implications of having such a framework on formulating strategies aiding searches for binaries with at least one neutron star. Whilst this framework can be used to extra-galactic sources (in context of compact binaries – NSNS/BHNS/NSNS), I will further explore the implications of this work on finding galactic pulsar binaries using foreground gravitational wave emission detected by LISA. We apply this work to explore the environments and populations of “Spidery” pulsar systems (millisecond pulsars paired with an ablated low-mass companion).

Speaker: Soheb Mandhai (The University of Manchester)

The Spatial Distribution of Binaries in the Milky Way.pdf

The Spatial Distribution of Binaries in the Milky Way.pptx

14:30 → 15:00 Coffee break

15:00 → 16:15 Neutron star modelling: 5

15:00 The problem with the r-modes of neutron stars

The r-mode instability is among the most promising sources for continuous gravitational-wave emission from rotating neutron stars. However, our present understanding of these purely axial perturbations rely on Newtonian gravity and assume simplistic models of the nuclear matter. Moreover, calculations in general relativity suggest a continuous spectrum, posing challenges for gravitational-wave searches and reliable parameter inference. In this talk, I will introduce the r-modes and their associated gravitational-wave instability. I will then explore how incorporating realistic nuclear composition alters the nature of these oscillations, leading to diverging frequencies that join the generic inertial modes. Finally, I will present reformulations of the r-mode problem that may resolve the singular behaviour in the relativistic calculation.

Speaker: Dr Fabian Gittins (University of Southampton)

presentation.pdf

15:25 R-modes as probe of Dark Matter in Neutron Stars

It is conjectured that dark matter(DM) could also exist within and around neutron star(NSs), accreted into its interior forming a DM core or as DM halo exceeding its radius. Recently there has been a discrepancy in the measurement of neutron decay lifetime, and also a proposition of a dark matter decay channel of neutron to resolve the discrepancy. Although, DM self-interaction strength can be constrained using recent multimessenger astrophysical observations, the rate of such decay is still not well constrained. In this work, we show how we can use the r-mode instability window and possible future detection/non-detection of continuous GW from r-modes to put contraints on the reaction rates of this dark decay process. Assuming chemical equilibrium among DM fermions and neutrons, we estimate the bulk viscosity resulting from the dark decay of neutrons considering different scenarios for the temperature dependence of the reaction rate. The constrained DM interaction strength is used to estimate DM self-interaction cross section and shear viscosity resulting from DM, which is found to be several orders of magnitude smaller than shear viscosity due to hadronic matter. We conclude that DM shear and bulk viscosity may significantly modify the r-mode instability window compared with the minimal hadronic viscosities, depending on the temperature dependence of the process. We also found that for the window to be compatible with the X-ray and pulsar observational data, the rate for the dark decay process must be fast.

Speaker: Suprovo Ghosh (IUCAA)

Suprovo_CW_Slides.pdf

15:50 Neural Simulation-Based Inference of the Neutron Star Equation of State directly from Telescope Spectra

Neutron stars provide a unique opportunity to study strongly interacting matter under extreme density conditions. The intricacies of matter inside neutron stars and their equation of state are not directly visible, but determine bulk properties, such as mass and radius, which affect the star's thermal X-ray emissions. However, the telescope spectra of these emissions are also affected by the stellar distance, hydrogen column, and effective surface temperature, which are not always well-constrained. Uncertainties on these nuisance parameters must be accounted for when making a robust estimation of the equation of state. In this talk, I present a novel methodology that, for the first time, can infer the full posterior distribution of both the equation of state and nuisance parameters directly from telescope observations. This method relies on the use of neural likelihood estimation, in which normalizing flows use samples of simulated telescope data to learn the likelihood of the neutron star spectra as a function of these parameters, coupled with Hamiltonian Monte Carlo methods to efficiently sample from the corresponding posterior distribution. The approach surpasses the accuracy of previous methods, improves the interpretability of the results by providing access to the full posterior distribution, and naturally scales to a growing number of neutron star observations expected in the coming years.

Speaker: Len Brandes (Technical University of Munich)

hannover2024_Brandes.pdf

16:15 → 17:00 Discussion

Thursday 20 June

09:00 → 09:50 Search methods CW: 3

09:00 BinaryWeave: A New Semicoherent Pipeline for Detecting a CW Signal from Scorpius X-1

Second-generation ground-based detectors, Advanced-LIGO and Advanced-Virgo, opened the window of gravitational wave astronomy. Over the past seven years, we witnessed GW astronomy begin with the first detection and rapidly emerge as a well-established field of transient astronomy. However, we have not detected any persistent gravitational wave (GW) signal. Scorpius X-1, the brightest low-mass X-ray binary source, is one of the strongest candidates for detecting a continuous gravitational wave (CW) signal in the near future. In this presentation, I will provide a brief overview of a new detection pipeline suitable to search CW signals over the largely unknown parameter space of Scorpius X-1. In this context, I will briefly mention the relevance and importance of electromagnetic observations to enhance our search efficiency and detection probability.

Speaker: Arunava Mukherjee (DAE-Saha Institute of Nuclear Physics/HBNI, Kolkata)

CW-NS-WS_AEI-Hannover_20Jun2024.pdf

09:25 HierarchMC: A Bayesian Framework for Hierarchical All-Sky Searches for Continuous Gravitational Waves

All-sky searches for continuous gravitational waves (CWs) emitted by yet-unknown neutron stars pose high computational costs and are typically optimized for signal sensitivity through hierarchical pipelines. Initial search stages reduce the required accuracy in estimated signal parameters by accepting high false alarm rates. All recorded alarms are then analyzed for signal consistency in several follow-up searches with increasing scrutiny. Current frameworks identify signal candidates through maximum likelihood estimation on optimized template grids and establish rejection criteria and uncertainty ranges for the likelihoods and signal parameters through Monte Carlo studies of a reference population of simulated signals, i.e., purely frequentist statistics. This process is highly manual and must be repeated for each follow-up search. Here, we present HierarchMC, a new framework designed around Bayesian Parameter Estimation for the rapid, automated follow-up of the many signal candidates produced by the early stages of wide-parameter space searches for CWs. We further examine the viability and performance of HierarchMC relative to the most sensitive all-sky search for CWs emitted by isolated neutron stars published to date.

Speaker: Jasper Martins (AEI Hannover)

CW_NS_WS_Hannover.pdf

CW_NS_WS_Hannover.pptx

09:50 → 12:10 Brunch break

12:10 → 13:25 Search methods CW: 4

12:10 A novel neural-network architecture for continuous-wave all-sky searches

Continuous gravitational waves (CWs) are long-lasting gravitational waves emitted by rapidly spinning neutron stars that can be seen in the LIGO band. The most sensitive classical search method, the coherent matched filter search for continuous waves is not computationally feasible. Instead, a semi-coherent method is used for the search because it has a higher senesitivity than the coherent matched filter search, but at a reasonable computational cost. We present an alternative, new search method based on Deep Learning. In our study, we focus on training a Deep Neural Network (DNN) to perform a blind search for CWs emitted by isolated neutron stars over the whole sky. We have trained multiple DNNs with a convolutional neural network architecture to detect signals with a wide range of signal parameters. We highlight our specific architectural choices that have yielded good results after performing several experiments. We show that such a trained DNN can achieve a very high sensitivity on an all-sky search for continuous waves at a lower computational cost compared to more classical searches.

Speaker: Prasanna Joshi (Max Planck Institute for Gravitational Physics, Hannover)

Prasanna_Joshi-CW_Workshop2024_slides.pdf

12:35 Convolutional neural network for directed searches of continuous gravitational waves

Convolutional neural networks (CNN) have an advantage in computational cost for the search of continuous gravitational waves (CGWs). We are developing a deep learning method for CGW searches. In our previous work, we proposed a method in which the doubly Fourier transformed strain data are used as inputs of CNN and assessed the effects of non-Gaussian artifacts. In this talk, we will talk about the application of our work for the directed searches of CGWs.

Speaker: Takahiro S Yamamoto (Nagoya University)

240620CWNSworkshop.pdf

13:00 Detection of unmodeled continuous gravitational waves using a Transfer Learning approach

The detection of gravitational waves (GWs) has opened new avenues for studying the universe and testing fundamental physics. As LIGO, Virgo and KAGRA undertake another observation run (O4) with an improved sensitivity, non-axisymmetric neutron stars emitting quasi monochromatic, long-standing GWs are expected to be within the detectors' sensitivity band. However, their detection in the presence of noise remains a challenging problem. In recent years, Deep Learning approaches have been proposed as a potential solution to this issue. This study explores the effectiveness of employing a transfer-learning approach with a binary-classifier network, based on ResNext50, for detecting unmodeled CWs embedded in real detector noise from the O3 observation run. This model leverages spectrograms derived from the H1 and L1 detectors, divided into numerous segments each extending over approximately 4-days periods. Through this technique, inference is applied to each segment to accumulate a comprehensive detection statistic across frequency bins. Such method is compared to the SOAP CW pipeline, showing improved efficiency and generalization to lower signal-to-noise ratios. The results not only highlight the benefits of the use of more complex network-architectures but also set a precedent for the further development of the detection of unmodeled CW signals based on artificial neural networks.

Speaker: Diego Dominguez (Tokyo Institute of Technology)

Dominguez_CW_Transfer_Learning.pdf

Dominguez_CW_Transfer_Learning.pptx

13:25 → 13:55 Coffee break

13:55 → 15:20 Search results CW: 4

13:55 Dark matter searches with gravitational wave detectors

I will present various ways in which our group is searching for dark matter using GW detectors.

1. Searching for GW signals from mergers of ultralight primordial black holes using LIGO data.

2. Searching for direct interaction between dilatonic DM and LIGO.

3. Searching for GWs from axion clouds around spinning black holes using a network of radio-frequency GW detectors.

4. Searching for GWs from mergers of ultralight PBHs using a network of rafio-frequency GW detectors.

Speaker: Nancy Aggarwal (UCDavis)

Aggarwal_AEI_CW_workshop_Jun_2024.pdf

Aggarwal_AEI_CW_workshop_Jun_2024.pptx

14:55 Searching for inspiraling planetary-mass primordial black holes in LIGO O3a data

Gravitational waves can probe the existence of primordial black holes (PBHs). If PBHs form in binary systems, they will inspiral and eventually merge, just as stellar-mass black holes do, emitting gravitational waves in the process. Here, we describe multiple methods to probe the existence of planetary-mass PBHs, and focus on one, the generalized frequency-Hough, that we used to run a search for such objects in O3a LIGO data. While we did not find any viable PBH candidates, we set, for the first time, stringent upper limits on planetary-mass PBHs in equal-mass and asymmetric mass ratio binary systems using gravitational-wave data. We also comment on the future of searches for PBHs with continuous gravitational-wave methods.

Speaker: Andrew Miller (Nikhef / Utrecht University)

search_o3a_pbh.pdf

15:20 → 16:15 Discussion