GdR Gravitational Waves WG joint meeting: Source populations and Multi-messenger Astronomy

16 nov. 2026, 13:30 → 17 nov. 2026, 18:00 Europe/Paris

Amphithéatre (IAP)

The second joint meeting of the « Source populations » (Populations des sources) and « Multi-messenger Astronomy » (Prédiction et suivi des signaux multi-messager) Working Groups of the GdR Gravitational Waves (GW) will provide an opportunity to discuss challenges related to gravitational wave and multi-messenger astronomy and how to coordinate our work in the context of upcoming missions.
 
The meeting will take place in-person at IAP from November 16 to 17. Nevertheless, a zoom link will be sent to registered participants who will attend remotely. Presentations will be done in English. 
 
We invite the community to submit contributions on various source populations (e.g. stellar evolution, stellar-mass compact objects, massive black holes, populations constraints from GW observations, stochastic backgrounds) and multi-messenger astronomy (e.g. follow-up strategies for LVK or VRO/LSST events, synergies between LISA, Einstein Telescope, electromagnetic and neutrinos facilities).
 
Please submit your abstract before October 15 and register before October 31.

Students and postdocs are encouraged to participate and present their work.
If students/postdocs need help to fund their mission, please contact SOC members (see below).

 
Invited speakers :

Thomas Jacquot
Roman Le Montagner
Sara Manzini
Alba Romero

SOC members:

Floriane Cangemi (cangemi at apc.in2p3.fr)
Aurélien Chalumeau (aurelien.chalumeau at cnrs-orleans.fr)
Irina Dvorkin (irina.dvorkin at iap.fr)
Olivier Godet (olivier.godet at utoulouse.fr)
Astrid Lamberts (astrid.lamberts at oca.eu)
Marion Pillas (marion.pillas at iap.fr)

lun. 16 novembre

Multimessenger follow-up and alert systems

Présidents de session: Aurélien Chalumeau (LPC2E/OSUC), Dr Marion Pillas (IAP)

1 Invited talk: Multimessenger in the LIGO-Virgo-KAGRA collaboration

MultiMessenger astronomy is some of the most exciting science that we can do with gravitational waves. The LIGO-Virgo-KAGRA (LVK) Collaboration searches for coincident gravitational wave event with an external counterpart and searches for coincident triggers in multiple external data channels (e.g. Neutrinos, Gamma Ray Bursts, Fast Radio Bursts, Soft Gamma Repeaters, pulsar glitches). Searches are done during observing runs and after observing runs to maximize chances of associating multimessenger counterparts. In this presentation, we will have a focused discussion on how GW-GRB searches work, their current results with detections and interesting candidates and finally perspectives for future runs.

Orateur: Thomas JACQUOT (IJCLAB)

2 Contributed talk: Multimessenger astronomy with sub-solar mass neutron stars

The existence of sub-solar mass neutron stars remains uncertain, despite several theoretical scenarios for their formation. Assuming that such objects exist, could they be observed through multimessenger astronomy? The detection of a sub-solar mass neutron star through both gravitational-wave and electromagnetic signals would provide valuable insights into nuclear physics and the formation of compact objects.
In this talk, I will present a simulation-based phenomenological study that I conducted on black hole – sub-solar mass neutron star mergers and their implications for multimessenger astronomy. My results show that the possibility of producing an electromagnetic counterpart depends on the masses of both objects, the black hole spin, and the compactness of the neutron star. I will discuss how these parameters influence the potential for electromagnetic counterpart emission and the prospects for multimessenger observations of such events.

Orateur: Camille Allan (IP2I)

3 Contributed talk: ET-WST synergy for next generation gravitational wave multi-messenger observations

Third generation (3G) gravitational wave (GW) interferometers such as the Einstein Telescope (ET) and Cosmic Explorer (CE) will explore a larger volume of the Universe with respect to now, detecting up to 10$^{5}$ binary neutron star system mergers (BNS) per year and revolutionizing GW multi-messenger (MM) astrophysics. The error volume on the sky position of such BNS will be large and finding the faint optical-NIR electromagnetic (EM) counterparts among a huge number of contaminants will be challenging. Spectroscopy will likely become the bottleneck of GW MM science, being the only secure tool to identify and characterise EM counterparts. New observational strategies will be necessary and they have to be prepared well in advance of ET operations.
I will present the results of the simulations I carried out within the Wide-Field Spectroscopic Telescope science team and the MM Division of the ET Observational Science Board to assess the impact of next generation Integral Field and Multi-Object Spectroscopy (IFS and MOS) on the detection, identification and characterisation of EM counterparts of ET BNS.
I will consider different observational strategies and show the key role of WST-like instruments in answering the observational challenges of MM observations of next-generation GW interferometers.

Orateurs: Dr Sofia Bisero (Irfu, CEA-Saclay), Sofia Bisero (GEPI, Observatoire de Paris, Université PSL, CNRS)

15:10 Coffee Break

4 Invited talk: The Vera C. Rubin Observatory in Operation: First Results and Prospects for Multi-Messenger Astronomy

After first light in 2025, the Vera C. Rubin Observatory formally began the 10-year Legacy Survey of Space and Time (LSST) on 29 June 2026. The public alert stream has been running since February 2026. In this talk I will give an overview of Rubin’s status and early science. I will start with the observatory, the survey strategy and the data products released so far, including the Data Previews. I will then review the first scientific highlights, from thousands of new Solar System objects to early transient discoveries. Finally, I will describe the real-time alert system. Rubin issues alerts within minutes of each exposure, at rates of several hundred thousand per night and growing toward several million. These alerts are distributed through community brokers that classify and filter them for science teams.

The second part of the talk will focus on what Rubin brings to gravitational-wave astronomy. Its depth and wide field make it well suited to searching for optical counterparts inside large GW localization regions. Its cadence also opens the possibility of finding kilonovae serendipitously, independent of a GW trigger. I will discuss the challenges this raises: faint and fast-fading sources, sparse sampling, and a large number of contaminants. I will also present the strategies the community is developing to meet them, such as kilonova filters running inside brokers like the work led by the GRANDMA collaboration, and coordinated follow-up networks. I will conclude with prospects for joint Rubin–LIGO-Virgo-KAGRA observations during the intermediate run IR1 and the future O5 run.

Orateur: Roman Le Montagner

5 Contributed talk: KM3NeT alerts system

KM3NeT is dedicated to detect and study neutrinos from distant astrophysical sources as well as from our own atmosphere. It is composed of the ARCA detector, located offshore Portopalo di Capo Passero, Sicily, Italy, at a depth of about 3,450 m and the ORCA detector, located at a depth of about 2,450 m, offshore Toulon, France. KM3NeT is under construction but already operational and its real-time analysis framework will provide public neutrino candidates alerts in the near future. In this talk we will present the low-latency system used to identify the interesting neutrino candidates in the data, the strategy we will follow to select the neutrino candidates that will be released, the different type of alerts we will provide, their link to multi-messenger observations and multi-messenger alerts, the format and content of alerts.

Orateur: Jean-gregoire Ducoin (CPPM)

6 Contributed talk: Follow-up of gravitational events

Due to the upcoming IR1 GW run, follow-up campaign will restart to identify the optical counterpart of GW events. After briefly presenting the alert system for IR1, I will come back to observing strategies for IR1, especially since LSST and SVOM are both taking data.

Orateurs: Damien Turpin (CEA), Sarah ANTIER (IJCLAB)

7 Contributed talk: Benchmarking the expected performance of Bayesian parameter inference on Kilonovae

In the study of optical transients, parameter inference is a part of the process of extracting physical information, i.e. constraints on the source's characteristics, by comparing the observed lightcurves to the predictions of different astrophysical models and finding the model and parameter combination that make the closest match.
In the developing field of the study of kilonovae (KN), systematic uncertainties in modeling are still very large. Current-day models still struggle to fit adequately the whole multi-wavelength dataset of the AT2017gfo kilonova, associated to the Binary Neutron Star merger GW170817, and thus obtain wide posterior distributions on the inferred source parameters.
After summarizing the many sources of uncertainty embedded in KN analyses and the process of Bayesian parameter inference, I will discuss how to quantify the performance of parameter inference analyses using metrics on information gain from prior to posterior.
Using the example of two KN models from the literature with the Nuclear Multi-Messenger Astronomy (NMMA) Bayesian inference framework, I simulated the expected information gains that dedicated KN follow-ups with telescope networks could reasonably reach. I will highlight the influence of observational cadences and systematic error margins on these information gains, and show that the wind ejecta's mass and electron fraction are typically the easiest KN parameters to constrain.
I will finally show that this method correctly predicts the information gains of analyses of the AT2017gfo dataset, and comment on the complementarity of multi-messenger approaches.

Orateur: Thomas Hussenot

mar. 17 novembre

Massive Black Hole binaries

Présidents de session: Aurélien Chalumeau (LPC2E/OSUC), Irina Dvorkin (Institut d'Astrophysique de Paris), Dr Marion Pillas (IAP)

8 Invited talk: Eccentric supermassive black hole binaries and non-stationarities induced in the PTA gravitational wave background

The nanohertz gravitational wave background (GWB) observed by Pulsar Timing Arrays (PTAs) is usually modelled as a stationary, spatially correlated red noise process. Eccentric supermassive black hole binaries (SMBHBs), however, emit gravitational wave bursts at periastron passage, which can make the background non-stationary. Using a time-dependent model of the common red noise (CRN) power spectrum, we study this effect in two settings.

First, we consider an idealised scenario: a stationary CRN plus a single eccentric binary. At high signal-to-noise ratio (SNR), the binary induces a clearly detectable non-stationary signature. At lower SNR, both the direct search for the binary signal and the non-stationarity search yield informative posteriors, but neither gives a decisive Bayes factor. This suggests that non-stationarity could provide a complementary channel for identifying eccentric sources.

Second, we move to realistic SMBHB populations with different eccentricity distributions. Quasi-circular populations produce a stationary background, as expected. Non-stationarity emerges as the fraction of highly eccentric binaries increases, with the most eccentric population giving the strongest preference for the non-stationary model. This could make non-stationarity a probe of the eccentricity of the SMBHB population.

Orateur: Sara Manzini (APC - Université Paris Cité)

9 Contributed talk: PTA band GWB anisotropies and their cross correlation with galaxies

The compelling evidence for the presence of nHz gravitational waves (GWs) in pulsar timing data calls for a refinement of what can and cannot be expected beyond vanilla scenarios. Under the hypothesis of an astrophysical origin for the GWs, we discuss possible imprints left in the signal by the fact the Universe is inherently clustered, with anisotropic large scale structure.
At the background level, this should imply primary anisotropies in the GW signal to be distinguished from the Poisson anisotropies due to the discreteness of sources. We discuss cross-correlations with galaxy catalogues as a promising approach towards the recovery of this anisotropic astrophysical signal. The expected cross correlation depends on the redshift distribution of the background GW power, and thereby on the astrophysics of supermassive black hole source populations.

Orateur: Martin Pijnenburg

10 Contributed talk: Investigating the sensitivity of the MPTA and extended datasets to alternative gravitational wave polarisations using simulations

We explore the sensitivity to alternative gravitational wave polarisations in current and future pulsar timing array (PTA) datasets. We focus on the Scalar-Transverse (ST) mode, which is not present in a purely quadrupolar (Traceless-Transverse or TT) gravitational wave background as predicted by General Relativity. Our analysis is based on the MeerKAT Pulsar Timing Array dataset, which routinely monitors over 80 millisecond pulsars with a median timing precision∼1 𝜇s and presented its first evidence for nanohertz GWB after only 4.5 yr of observations. We simulate GWB with a multiple strain amplitudes and search for TT-only or ST and monopolar signals combined with the TT modes. We create extended simulations to include (i) an MPTA-based PTA with a 15-year observing baseline and (ii) a PTA with a uniform distribution of 140 pulsars across Southern and Northern hemispheres. I will present the results of these simulations and their implications.

Orateur: Jaikhomba Singha (University of Cape Town)

11 Contributed talk: Searching for Bursts with Linear Gravitational-Wave Memory in the MPTA 4.5-Year Data Release

Close hyperbolic encounters of supermassive black holes produce Gravitational Waves (GWs) having bursts with linear memory (BWM) signature, which appears close to a ramp-like function in pulsar timing residuals. Pulsar timing arrays (PTAs) are therefore well suited to detect such events or to constrain their occurrence out to gigaparsec distances. In this talk, we present initial results from search for BWM events in the MeerKAT Pulsar Timing Array (MPTA) 4.5-year data release, which comprises high-cadence, high-precision observations of 83 millisecond pulsars. We use a computationally efficient prescription based on a post-Newtonian accurate Keplerian-type parametric solution for the hyperbolic orbits. Our Bayesian framework includes pulsar specific intrinsic noises, a GW background that is modeled as a Common Uncorrelated Red Noise (CURN) among the pulsars, along with the BWM signal. We will discuss injection studies that validate the pipeline, reproducing the CURN signal, further plans, and prospects for search using future PTA data releases.

Orateur: Dr Prerna Rana (LPC2E, Orleans, France)

10:30 Coffee Break

12 Contributed talk: Probing zombie binaries: A new window on supermassive black hole binary mergers

By monitoring the times of arrival of radio pulses from millisecond pulsars, pulsar timing arrays (PTAs) serve as unique gravitational wave (GW) laboratories in the nanohertz band. To date, the primary astrophysical sources of GWs targeted in this frequency range have been inspiraling supermassive black hole binaries (SMBHBs) on circular and eccentric orbits. In this work, we demonstrate that, thanks to the so-called pulsar term in the timing residual waveform of GW signals, PTAs can probe individual SMBHBs that merged before timing observations began. Systems that we refer to as zombie binaries. Using SMBHB population models consistent with current PTA constraints, we find that while the probability of detecting such systems in existing PTA datasets remains low, the Square Kilometer Array observatory is expected to achieve sufficient sensitivity to have a few zombie binaries with optimal matched-filter signal-to-noise ratios exceeding 10 in its data. Although their confident identification might be challenging, this new class of PTA sources opens a novel window for studying the most massive SMBHBs in our local universe.

Orateur: Hippolyte Quelquejay-Leclere (Università degli Studi di Milano-Bicocca)

13 Contributed talk: From MUSE Close Pairs of Galaxies to LISA: Observational Constraints on Massive Black Hole Coalescences

In the hierarchical framework of structure formation, galaxy mergers are key drivers of galaxy evolution and can lead to the formation of massive black hole binaries, whose coalescences are among the primary gravitational-wave sources expected for the future space-based detector LISA.
Predictions for massive black hole merger rates generally rely on cosmological simulations, which can struggle to resolve low-mass galaxies and their black hole populations. Deep spectroscopic surveys therefore provide a complementary, observational route to constrain these rates.

This work presents a methodology to infer galaxy merger rates over a broad redshift range extending to z ∼ 6.7, using a deep and homogeneous spectroscopic catalogue of more than 10 000 galaxies compiled from multiple MUSE fields. Applying a Friends-of-Friends grouping algorithm, we identify physically close galaxy pairs and derive galaxy merger fractions fM and merger rates RM across a wide stellar-mass range, 7 ≤ log10 (M/M⊙ ) ≤ 12. Our measurements account for spectroscopic incompleteness, edge effects, and cosmic variance.

To connect these observational constraints to LISA, we model the transition from galaxy mergers to massive black hole coalescences by combining host-galaxy scaling relations, parameterized massive black hole occupation fractions in dwarf galaxies, and dynamical delay timescales from kpc to sub-pc separations calibrated with hydrodynamical simulations. This framework provides an empirical anchor for LISA event-rate predictions and offers new insight into the massive black hole population in the low-mass and high-redshift regimes.

Orateur: Loic BOCHRAWNA (Institut de Recherche en Astrophysique et Planétologie (IRAP))

14 Massive Black Hole binaries: discussion

Orateurs: Astrid Lamberts (Observatoire de la Cote d'Azur), Aurélien Chalumeau (LPC2E/OSUC), Floriane Cangemi (UPC/APC), Irina Dvorkin (Institut d'Astrophysique de Paris), Dr Marion Pillas (IAP), Olivier GODET (IRAP)

Stellar mass objects

15 Invited talk: Isotropic Gravitational-Wave Background Search Results from the 4th LVK observing run

In this talk, I will present latest results from the search for an isotropic gravitational-wave background (GWB) using data from the second and third segments of LVK's fourth observing run[1, 2]. Although no evidence for a stochastic gravitational-wave background has been found, the analysis yields improved constraints on several theoretical models and places new limits on non-standard polarization modes. I will discuss these results in the context of the GWTC-5 compact-binary population models and assess the prospects for detecting the compact-binary background with future A+ detector sensitivity.
References
[1] LIGO Scientific, VIRGO and KAGRA collaborations, GWTC-5.0: Population Properties of Merging Compact Binaries, arXiv:2605.27226 [astro-ph.HE], LIGO-P2600045 (2026).
[2] LIGO Scientific, VIRGO and KAGRA collaborations, GWTC-5.0: Observations from the Second Part of the Fourth LIGO-Virgo-KAGRA Observing Run and Updates to the Gravitational-Wave Transient Catalog, arXiv:2605.27225 [gr-qc], LIGO-P2600152 (2026).

Orateur: Alba ROMERO RODRIGUEZ (Laboratoire D'Annecy de Physique de Particules (LAPP))

16 Contributed talk: Joint multi-messenger inference of double white dwarfs with LISA

Double white dwarfs are persistent multi-messenger sources, emitting millihertz gravitational waves detectable by LISA, while their light curves show eclipses and orbital modulations. Both signals are driven by the same orbital motion and therefore share a common orbital clock, yet they have been analyzed sequentially. We present a joint Bayesian framework that simultaneously models both signals using a single set of physical parameters and a shared, evolving orbital phase. Applying this framework to simulated observations of four binaries with different photometric signatures and gravitational-wave signal-to-noise ratios, we investigate how the two messengers complement each other in constraining binary properties. We show that joint inference can improve constraints on component masses, radii, and orbital inclination, depending on the binary configuration and model assumptions. We find that gravitational-wave distance measurements can break the temperature-distance degeneracy of single-band photometry, even without stellar mass-radius relations. For non-eclipsing binaries, gravitational-wave measurements of the orbital inclination complement optical modulations, helping to break parameter degeneracies and improve constraints on the binary's physical properties. Our results highlight the potential of coherent gravitational-wave and electromagnetic analyses for characterizing compact binaries in the LISA era.

Orateur: Nina Kunert (Observatoire de la Cote d'Azur)

17 Contributed talk: A deep, large-scale survey for LISA white dwarf binary candidates with the Canada-France Hawaii Telescope

Ultra-compact white dwarf binaries ($P_{\rm orb} \lesssim$ 1 hour) are invaluable probes for studying stellar and binary evolution, and will serve as prime multi-messenger targets for LISA, the future space-based gravitational wave observatory operating in the millihertz regime. Preparing for the mission launch in 2035 requires identifying the highest number of electromagnetic counterparts, in order to maximize both mission operations and scientific return. We address this goal through a dedicated program within the 6-year Community Survey of the Canada-France-Hawaii Telescope (CFHT), serving as a first step in a systematic search of LISA binaries. Leveraging the exceptional $u$-band sensitivity of CFHT/MegaCam, we will conduct a multiband photometric survey of the Northern Galactic plane over 172 high-quality nights, down to $u \sim$ 24.5 mag The primary objective is to yield thousands of short-period white dwarf binary candidates via a tailored color-selection method, which will serve as high-priority targets for subsequent follow-up for confirmation and characterization. With calibrated $u, g, r$ photometry spanning about 2500 deg$^2$, this survey will deliver a high-impact legacy dataset also for broader studies of the Milky Way structure and stellar populations.

Orateur: Alice Perego (Observatoire de la Côte d'Azur)

18 Comparing degrees of EOS knowledge in prior choices for gravitational-wave parameter estimation and equation-of-state inference for binary neutron stars (Contribution from NS-SN-Nucleosynthesis WG)

19 Contributed talk: The Parameter Space of BNS Populations: Formation Channels and Merger Rates

Binary neutron stars (BNS) play a central role in various high-energy astrophysical phenomena,from short gamma-ray bursts to kilonovae.

Even though the number of known systems is currently relatively small, upcoming observations are expected to drastically increase the size of both Galactic and extragalactic BNS populations.

Population synthesis codes offer a powerful tool to study large BNS populations. However, the numerous physical processes involved, each governed by uncertain parameters, introduce strong degeneracies making it challenging to interpret the resulting populations.

In this talk, I will present the results of a systematic exploration of key parameters governing BNS formation. I will describe the methodology developed to analyze a large ensemble of synthetic populations and to identify the dominant BNS formation channels. I will then show how variations in these parameters can impact BNS formation and merger rates. Finally, I will discuss the implications for electromagnetic and gravitational-wave observations, linking specific formation channels to the transient phenomena they are expected to produce.

Orateur: Mathieu VENET

20 Neutron stars, EOS, Multi-messenger astronomy, Population: Discussion

Orateurs: Astrid Lamberts (Observatoire de la Cote d'Azur), Floriane Cangemi (UPC/APC), Irina Dvorkin (Institut d'Astrophysique de Paris), Dr Marion Pillas (IAP), Matteo Bugli (UniTo - CEA Saclay), Olivier GODET (IRAP)

15:50 Coffee Break

21 Contributed talk: Natal kicks and r-process enrichment: starting to trace binary neutron star populations in cosmological simulations

Binary neutron star (BNS) mergers are among the leading astrophysical sites of rapid neutron-capture (r-process) nucleosynthesis. However, uncertainties in binary stellar evolution, particularly natal kicks and merger delay times, complicate predictions of their contribution to galactic chemical enrichment. Natal kicks can displace BNS systems from their birth environments, potentially altering the spatial distribution and efficiency of r-process enrichment.

We present preliminary results from high-resolution cosmological zoom-in simulations of a Milky Way-mass galaxy performed with RAMSES. BNS populations are generated using metallicity-dependent distributions derived from the COSMIC binary population-synthesis code. Individual binaries are followed dynamically from formation through the second supernova to merger, when europium is injected into the surrounding interstellar medium. The simulations also follow iron and magnesium enrichment from core-collapse and Type Ia supernovae.

For now, we focus on comparing simulations with and without natal kicks to investigate how binary displacement affects the distribution of merger sites, the transport of r-process material, and the resulting stellar [Eu/Fe] abundances. This controlled comparison provides a means of connecting uncertain binary-evolution prescriptions to observable chemical signatures in Milky Way-like galaxies evolved inside cosmological environments.

Ultimately, this approach aims to complement gravitational-wave constraints on BNS populations with independent information preserved in the chemical abundances of stars.

Orateur: Arturo Nunez-Castineyra (Institut d'Astrophysique de Paris)

22 Inferring dense matter properties from gravitational waves (Contribution from NS-SN-Nucleosynthesis WG)

23 Contributed talk: Modelling astrophysical stochastic signals in LISA

Unresolved Galactic binaries produce a confusion foreground that dominates over LISA’s instrumental noise in part of the millihertz band. Extragalactic binaries add a weaker stochastic background that becomes increasingly important as observations accumulate. Both components affect LISA’s sensitivity, and their spectra depend on uncertain astrophysical processes, including tides, mass transfer and cosmic star formation. I will discuss how these uncertainties shape the two signals, present our work towards modelling them jointly, and describe a first step towards using neural networks to link the astrophysical noise to the binary populations that produce it.

Orateur: Alexandre Toubiana (Université de Milan-Bicocca)