Belgian-Dutch Gravitational Wave Meeting 2025

27 Oct 2025, 10:00 → 28 Oct 2025, 17:00 Europe/Amsterdam

HG00.304 (Huygensgebouw)

Badri Krishnan, Gijs Nelemans (Radboud University)

Welcome to Nijmegen!

The 13^th edition of the Belgian-Dutch Gravitational Wave Meeting will be hosted by the Radboud University in Nijmegen on Oct 27 - 28 2025. This annual meeting aims to bring together physicists and astronomers interested in all aspects of gravitational-wave science, to inform each other of recent developments, discuss (strategic) topics and informally meet each other. 

The registration is open and closes on October 22!

 

 

 

Monday 27 October

10:00 Coffe/tea

Day 1: Welcom

Convener: Badri Krishnan

Instrument updates + synergy discussion

1 LIGO/Virgo/Kagra update

Speaker: Anuradha Samajdar (Utrecht University, Nikhef)

2 Pulsar Timing Arrays update

Speaker: Aditya Parthasarathy (ASTRON)

3 ETPathfinder and Einstein Telescope update

Speaker: Stefan Hild (Maastricht University and Nikhef)

2025_10_27_Belgium_Dutch_GW.pdf

4 LISA update

Speaker: Oliver Jennrich (ESA)

5 Synergies: discussion

12:30 Lunch

Contributed talks

6 Search for gravitational waves associated with high-energy neutrinos detected by IceCube during the LIGO-Virgo run O3

We present an unmodelled, targeted search for generic gravitational wave transients associated with high-energy neutrinos detected by the IceCube Neutrino Observatory during the third observing run of Advanced LIGO, Advanced Virgo, and KAGRA. Our search is sensitive to gravitational wave signals weaker than those reported in real time or in the gravitional wave transient catalog, and can thus uncover coincidence missed by typical neutrino follow-up searches. We find no statistically significant gravitational wave signal and set lower bounds on the distance of possible gravitational wave sources for different emission models.

Speaker: Matthias Vereecken (Ugent)

7 Leveraging the null stream to detect strongly lensed gravitational wave signals

Gravitational lensing of gravitational waves is expected to be observed in
current and future detectors. In view of the growing number of detections, computation-
ally light pipelines are needed. Detection pipelines used in past LIGO-Virgo-KAGRA
searches for strong lensing require parameter estimation to be performed on the gravi-
tational wave signal or are machine learning based. Removing the need for parameter
estimation in classical methods would alleviate the ever growing demand of computa-
tional resources in strong lensing searches and would make real-time analysis possible.
We present a novel way of identifying strongly lensed gravitational wave signals, based on
the null stream of a detector network. We lay out the basis for this detection method and
show preliminary results confirming the validity of the formalism. We also discuss the
next development steps, including how to make it independent of parameter estimation

Speaker: Jef Heynen (UC Louvain)

Belgian-Dutch GW Meeting 2025.pdf

8 Mitigating systematic errors in Parameter Estimation in the era of loud gravitational wave signals

Parameter estimation (PE) for gravitational wave merger events relies on the accuracy of the waveform models and the noise model. The PE result might suffer from systematic errors if any of the above assumptions break down. Waveform systematics may arise due to mismodelling in the waveform models or numerical relativity simulations. The noise model may break down in the presence of glitches or non-stationarity near the signal. As the sensitivity of the detector network improves, it is essential to incorporate systematic error mitigation schemes into the PE data analysis pipelines.

In this work, we present a framework to account for waveform systematic errors by introducing parametrizations that compensate for amplitude and phase errors in the waveform models. The expected error budget in a reference waveform model is used as a prior distribution in PE. However, in the absence of the knowledge of these error budgets, we can still use conservative priors on amplitude and phase errors. We test our framework with simulated signals with and without systematic errors in the waveform model. We demonstrate that our framework can account for waveform systematic errors and also address biases resulting from the omission of physical effects in the waveform model description. We will also show the PE results for the GW events from publicly available data.

We will also discuss the degeneracy of the waveform systematics with those due to detector calibration. Furthermore, we will discuss the feasibility of a comprehensive data analysis pipeline that includes systematic errors mitigation due to waveform models uncertainties and noise artifacts.

Speaker: Sumit Kumar

9 Residual Test to Search for Microlensing Signatures in Strongly Lensed Gravitational Wave Signals

When a gravitational wave signal encounters a massive object, such as a galaxy or galaxy cluster, it undergoes strong gravitational lensing, producing multiple copies of the original signal. These strongly lensed signals exhibit identical waveform morphology in the frequency domain, allowing analysis without the need for complex lens models. However, stellar fields and dark matter substructures within the galactic lens introduce microlensing effects that alter the morphologies of individual signals. Identifying these microlensing signatures is computationally challenging within Bayesian frameworks. In this study, we propose a residual test to efficiently search for microlensing signatures by leveraging the fact that current Bayesian inference pipelines are optimized solely for the strong lensing hypothesis. Using cross-correlation techniques, we investigate the microlensing-induced deviations from the strong lensing hypothesis, which are imprinted in the residuals. Most simulated signals from our realistic microlensing populations exhibit small mismatches (MMs) between the microlensed and unlensed waveforms, but a fraction show significant deviations. We find that 28% (52%) and 34% (66%) of microlensed events with MM ≥0.03 and ≥0.1, respectively, can be discerned with O4 (O5) detector sensitivities, which demonstrates that high-MM events are more likely to be identified as microlensed. Including all events from a realistic population, 11% (21.5%) are identifiable with O4 (O5) sensitivity using our approach.

Speaker: Eungwang Seo (University of Glasgow)

10 Searches for Signatures of Gravitational Wave Lensing in LIGO--Virgo--KAGRA Data

General relativity predicts that a signal passing a massive object will be
deflected by the spacetime curvature induced by that object. This process is
known as gravitational lensing and bears some similarities to e.g. the lensing
of light through glass. Gravitational lensing of light was first observed over
a century ago and was a fundamental test of general relativity. Since then, it
has become commonly observed in images from telescopes. This is also expected
to be observed for gravitaitonal wave data and searches have been conducted in
the LIGO--Virgo--KAGRA data. Thus far, there have been no confident candidates
for detection, however, a recent signal---GW231123---demonstrated some support
for the lensing hypothesis.

This talk will provide a basic introduction to the gravitational lensing of
gravitational waves including the kinds of object that are likely to lens
objects and what signatures these impart on the data that is observed by the
LIGO--Virgo--KAGRA network. It will then detail the various analysis
methodologies that are used to investigate the data for those signatures and
provide some recent results from those analysis efforts.

Speaker: Mick Wright (Utrecht University)

11 Exploring degeneracies between Precession and Microlensing in GW Signals

We have been detecting gravitational wave (GW) signals from coalescing compact binary systems for nearly a decade. The morphology of these signals can be shaped by a variety of intrinsic and extrinsic parameters. With 90 GW events reported up to the end of the O3 run and 128 already uncovered in the ongoing O4 run, the rapidly growing catalog presents new challenges. Morphological similarities between signals produced by different astrophysical effects and/or source parameters can complicate template-based searches. In particular, the modulation in a GW signal caused by the precession of a compact binary system may mimic the beating pattern characteristic of microlensed GW signals. The talk will explore the feasibility of distinguishing these two kinds of signals by utilizing machine learning based classification techniques. We also investigate whether these two kinds of signals could be distinguished from unlensed circular binary GWs.

Speaker: Disha Hegde (UCLouvain)

Contributed talks

12 Signatures of Phase Transition in Neutron Star Mergers

We investigate observable signatures of strong phase transitions in neutron star cores through two complementary approaches. First, we analyze the oscillation spectrum of isolated neutron stars hosting a sharp density discontinuity, comparing full general relativistic simulations with results from the Cowling approximation. This reveals the behavior of interface modes that could become tidally excited in binary systems or emit gravitational waves during merger.
Second, we present preliminary results from magnetized binary neutron star merger simulations where the stars begin in a purely hadronic phase and undergo a phase transition to quark matter during coalescence. We examine how the presence of a sharp interface affects magnetic field amplification in the postmerger remnant and the mass of the surrounding matter torus, both critical factors for kilonova emission. Our findings suggest that phase transitions may leave distinctive imprints on electromagnetic counterparts, particularly in the prompt blue kilonova component, offering a new avenue for constraining the high-density equation of state through multi-messenger observations

Speaker: Pablo Bosch (Utrecht University)

13 Cosmic superstrings in large volume compactifications: PTAs, LISA and time-varying tension

The Stochastic Gravitational Wave Background (SGWB) from cosmic superstrings offers one of the few known possibilities to test String Theory within current experimental reach. However, in order to be compatible with the existing constraints, the tension of a cosmic superstring network is required to lie several orders of magnitude below the Planck scale. This is naturally realized in string compactifications where the volume of the extra dimensions is parametrically large (in string units). We estimate the GW spectrum arising from cosmic superstrings in such scenarios, providing analytical formulae as well as numerical results. Crucially, we do so within a semi-realistic string cosmology scenario, taking into account various modified cosmological epochs (such as kination or early matter domination) induced by the presence of moduli and a time-dependent string tension.We show that part of the spectrum generically lies within reach of LISA and ET, with a large class of models predicting a characteristic drop in the amplitudewhich may be robustly probed by LISA.The corresponding signal would encode information on the dynamics of moduli and reheating. On the other hand, the ultra-high frequency part of the spectrum can be significantly enhanced by a long, early phase of kination with time-varying tension, yielding a spectral index unique to this set-up.

Speaker: Filippo Revello (KU Leuven)

Revello_Nijmegen25.pdf

14 Gravitational Waves from tri-axial rotating stars in Linearized gravity – an analytical boundary value problem approach

Rotating stars with non-uniform deformations are a very important source of continuous gravitational waves, but representing the radiative part of their gravitational field analytically is mostly limited to using the post-Newtonian method (e.g. Blanchet-Damour 1990), which decomposes the zone of the external gravitational field into a ‘near’ (pseudo-stationary) zone and an 'intermediate' (radiative) zone. Such a decomposition is only valid where the rotation rate leads to an emitted wavelength that is longer than the size of the star, and as the rotation rate increases and the emitted wavelength gets shorter, the radiative effects inside the star can no longer be overlooked.

Focusing on the incompressible ellipsoids as this is one of the few examples that can be dealt with analytically, Chandrasekhar (1967) and several others do obtain a representation for the field inside the star, but there is no mention on how to explicitly join it to a suitable vacuum solution as it is not possible to solve for the region outside the tri-axial ellipsoid in closed form if the ellipticity is not small; one typically resorts to ellipsoidal harmonics. To break this impasse we make the assumption that the non-uniform part of the deformation is a small increment on top of the axisymmetric part, which then allows us to demonstrate that all the equations (both interior and exterior) can be explicitly solved in closed form as a boundary value problem. As well as an analytical representation of the radiation over the whole vacuum, this will also allow us to explore radiative behaviour inside the star (e.g. at interfaces).

Speaker: Prakash Sarnobat (East Surrey Gravity Research)

Tri-axial.pdf

15 Equation of State at finite temperature in the era of new nuclear physics and multimessenger constraints

In this contribution, I will start with an overview of different types of equation of state modelling of neutron star matter in the Bayesian formalism, to demonstrate the িmpact of different experimental and observational constraints. Further, I will present equations of state at finite temperature obtained with Brussels-Skyrme-on-a-Grid (BSkG) energy density functionals developed at Brussels, which are unified across the crust and core of the neutron star environment. These models have demonstrated remarkable accuracy over the whole nuclear chart on the masses, and fission barriers of nuclei, but at the same time they also satisfy recent astrophysical constraints. I will also outline the impact of our calculations at finite temperatures on the composition of the crust in the neutron stars. Our next goal is to apply these equations of state in the end-to-end simulation of binary neutron star mergers.

Speaker: Chiranjib Mondal

Nijmegen_GW_meeting_mondal.pdf

16 Thermal Effects on Tidal Deformabilities of Neutron Stars

The observations of the gravitational wave signal GW170817 from a binary neutron star merger have marked the beginning of a new era in astrophysics. Such events offer unique opportunities to probe the properties of matter under conditions so extreme that they cannot be experimentally reproduced in terrestrial laboratories. However, modeling these events remains a major challenge because many nuclear physics inputs are required. The key ingredient is the equation of state of hot dense matter. Although matter effects are predominant during the merger and post-merger phases, we can also probe dense matter during the inspiral stage. In this phase, the neutron star is slightly deformed due to the tidal field of its companion. These deformations are quantified by the so-called tidal Love numbers. These depend entirely on the properties of matter, and therefore on the equation of state.

Love numbers are generally computed using the equation of state of cold catalyzed matter. Although this is a good approximation when the neutron stars are sufficiently far apart, their constituting matter is expected to be heated during the final orbits due to tidal friction. In this talk, I will present how temperature affects tidal interactions in binary neutron star mergers, using a realistic, temperature-dependent equation of state.

Speaker: Ethan Carlier (Université Libre de Bruxelles)

15:30 Coffee/tea + Poster Session

Contributed talks

17 Approach to the separatrix with eccentric orbits

Eccentric binary compact mergers are key targets for current and future gravitational wave observatories. In the small mass ratio expansion, post-adiabatic inspirals have been modeled up to the separatrix, where first-principles modeling currently ends. However, a complete waveform model for eccentric binaries requires an accurate description of the plunge and the transition from the inspiral to the plunge. This latter phase of the motion must be consistently matched to the late inspiral. In this presentation, I will discuss an analytical solution to the post-adiabatic equations governing the inspiral of an eccentric binary in the extreme mass ratio regime. Unlike in the quasi-circular case, these solutions feature Lambert W functions. These asymptotic 0PA equations and their solutions pave the way toward a full description of the transition to the plunge.

Speaker: Guillaume Lhost (UMONS)

Approch_to_separatrix_Lhost.pptx

18 Gravitational waves from the quasi-spherical inspiral of a spinning body in Kerr spacetime

Extreme mass-ratio inspirals (EMRIs), consisting of a stellar-mass compact object spiraling into a massive black hole, are key targets for space observatories like the recently adopted LISA, TianQin, and Taiji. Together with self-force effects, the smaller companion spin is one of the crucial post-adiabatic terms needed for accurate waveform templates. This work presents the adiabatic, quasi-spherical inspiral of a spinning test body in a Kerr spacetime for the first time. We leverage recent solutions for the motion of spinning test particles to efficiently compute the gravitational fluxes. Next, we implement a numerical scheme to derive the quasi-spherical inspiral trajectories and generate frequency-domain waveforms. Finally, we assess the impact of the secondary spin on the gravitational wave signal. Our results indicate that neglecting the secondary spin induces detectable mismatches in the waveform across much of the parameter space.

Based on https://arxiv.org/abs/2506.20726

Speaker: Gabriel Andres Piovano (Université Libre de Bruxelles)

Belgian_Dutch_meeting_2025_Gabriel_Andres_Piovano.pdf

19 Testing modified gravity with the eccentric neutron star--black hole merger GW200105

Direct detections of gravitational waves offer a unique opportunity to test gravity in the highly dynamical and strong field regime. Current tests are typically performed assuming signals from quasi-circular binaries. However, the complex waveform morphology induced by orbital eccentricity can enhance our ability to probe gravity with greater precision. A recent analysis of the neutron star–black hole event GW200105 identified strong evidence for orbital eccentricity. We extend an eccentric-precessing waveform model to test alternative models with this signal by incorporating eccentric corrections induced by Brans-Dicke, Einstein-dilaton-Gauss-Bonnet, and dynamical Chern-Simons gravity at leading post-Newtonian order. We show that analyzing this event with a quasi-circular model leads to a false deviation from general relativity, while the inclusion of eccentricity improves the bounds on the models. Our analysis of GW200105 places tight constraints on Einstein-dilaton-Gauss-Bonnet gravity, $\alpha^{1/2}_{\text{EdGB}} \lesssim 2.38\,\text{km}$, and Brans-Dicke gravity, $\omega_{\text{BD}} \gtrsim 3.5$, while dynamical Chern-Simons gravity remains unconstrained due to the low spin content.

Speaker: Justin Janquart (Université Catholique de Louvain)

20 Non-linear dynamics beyond General Relativity

We present the first numerical simulations of binary black holes and binary neutron star mergers in scalar–Gauss–Bonnet gravity performed in the modified puncture gauge, which is a well-motivated theory from an effective field theory point of view. Implemented within both GRChombo and MHDuet codes, our modified formulation enables stable evolution through merger and post-merger phases. These results open the door to systematic studies of scalar-Gauss-Bonnet effects on the gravitational wave signal and remnant dynamics in this class of modified gravity theories.

Speaker: Llibert Aresté Saló (KU Leuven)

21 Solving Einstein's equations as Bayesian inference

To leverage the full potential of next-generation gravitational wave observatories, such as the Einstein Telescope (in Europe) and LISA (in space), we will need to improve the efficiency of numerical relativity (NR) simulations in terms of computational speed and accuracy. In this talk, we will introduce probabilistic numerics as a promising new approach to gain control over uncertainties in NR. Instead of a single estimate, probabilistic numerical methods model a full probability distribution of the solution, allowing to quantify the uncertainty associated with the model parameters and numerical discretization schemes. This uncertainty quantification can then be used to optimize simulations. This talk will provide a first demonstration of this approach with a probabilistic numerical solver for the Einstein equations.

Speaker: Tom Colemont (KU Leuven)

Contributed talks

22 Combining Modelled and Unmodelled Inference Methods for Compact Binary Coalescences

Gravitational-wave (GW) data-analysis methods for compact binary
coalescences (CBCs) are generally divided into two categories. On the
one hand, there are modelled techniques, in which a detected signal
is compared to a theoretical model of the CBC source. On the other
hand, unmodelled approaches attempt to reconstruct the signal
without relying on a specific model by using a superposition of
wavelets.

In this work, both techniques are combined, allowing unmodelled
features to be incorporated into a modelled CBC signal. This approach
opens up the possibility to recover new physics from CBC signals that
lack precise models. A possible target is the resonant excitation of
neutron stars during inspiral, which could be represented by wavelets
superimposed on a neutron-star CBC model. The joint inference method
can also improve the estimation of astrophysical source parameters
when detector glitches - transient noise spikes - are present, by
separating the glitch from the GW signal during parameter estimation
(PE).
As a proof of concept, this study focuses on adding wavelets to
binary black hole (BBH) merger signals, as these are relatively
inexpensive to analyze. It is possible to jointly estimate both
wavelet and BBH parameters, particularly for high signal-to-noise
ratio (SNR) signals expected in next-generation detectors such as the
Einstein Telescope. The systematics of the joint inference have been
tested on a small set of injections, which indicate a detectability
threshold for wavelets in next-generation detectors. The results also
highlight a systematic issue related to the sampling priors
implemented for this inference method.
The technique also shows promise for mitigating detector glitches,
even in current detectors, without significantly degrading the quality
of the PE.

Speaker: Robin Chan (UGent | Royal Observatory of Belgium)

23 Bayesian Calibration of Gravitational-Wave Detectors Using Null Streams Without Waveform Assumptions

Gravitational-wave (GW) astronomy has revolutionized our understanding of the universe, but the precision of its discoveries hinges on the accurate calibration of GW detectors. In this talk, we present a novel Bayesian null-stream method for self-calibration of closed-geometry GW detector networks, such as the Einstein Telescope (ET) and LISA. Unlike traditional approaches that rely on electromagnetic counterparts or waveform models, our method leverages sky-independent null streams to constrain calibration errors using GW signals alone, independent of general relativity or waveform assumptions. We demonstrate the feasibility of this approach through proof-of-concept studies, showing that calibration constraints improve linearly with increasing signal-to-noise ratio and the presence of multiple overlapping signals. This method has the potential to enable robust parameter estimation, early-warning alerts, and cosmological studies, particularly for next generation detectors.

Speaker: Francesco Cireddu (KU Leuven)

FCireddu_Bayesian_Calibration_of_Gravitational-Wave_Detectors_Using_Null_Streams_Without_Waveform_Assumptions.pdf

24 Simulation-Based Inference for Stochastic Gravitational Wave Transients from Core-Collapse Supernovae

The detection of gravitational waves (GWs) from compact binary mergers has become routine. However, stochastic and non-stationary signals, such as those expected from core-collapse supernovae (CCSNe), remain elusive. These complex, burst-like signals pose a significant challenge for traditional Bayesian inference methods, which can be computationally expensive and struggle with intractable likelihoods. In this talk, we explore simulation-based inference (SBI) as a potential alternative for analyzing such challenging GW transients. We review the application of a traditional sampler to a simulated CCSNe signal, demonstrating its performance on this test case, and discuss preliminary insights into how a modern SBI approach might address key limitations. This work underscores the promise of SBI for enabling efficient inference on stochastic GW sources as we approach the era of next-generation detectors.

Speaker: Chun-Fung Wong (KU Leuven)

25 EMRIs for Mojito Dataset: Simulating and Validating a Relevant Catalog of Extreme Mass-Ratio Inspirals for LISA

Extreme Mass-Ratio Inspirals (EMRIs) are among the targeted gravitational wave sources for LISA, promising valuable insights into fundamental physics and astrophysics. Preparing for the complex task of EMRI data analysis requires realistic, synthetic datasets. This talk presents the EMRIs for the Mojito light dataset, the upcoming Mock Data Challenge by the LISA DDPC designed for LISA data analysis studies. The catalog is constructed based on an astrophysical population model, which predicts the distribution of EMRI sources throughout the universe. Waveforms are generated using the latest waveform model in the FastEMRIWaveforms package, producing relativistic waveforms for eccentric, equatorial inspirals into spinning massive black holes. To validate the dataset and assess the fidelity of the waveform model, we perform a parameter reconstruction analysis. Using Bayesian inference with MCMC sampling, we quantify the accuracy with which source parameters can be recovered from the simulated data. The analysis is performed with and without noise, to test the performance of the new waveform model. Also, an estimate is made on the impact of overlapping signals on parameter reconstruction. Our results demonstrate the robustness of the waveform generator and provide estimates for the accuracy that can be expected for the EMRIs in this dataset. We find that the overlap has negligible impact for on the parameter reconstruction.

Speaker: Bert Depoorter (KU Leuven - Royal Observatory of Belgium)

main.pdf

26 Encoding neutron star information into neural priors for gravitational wave analyses

Bayesian inference, widely used in gravitational-wave parameter estimation, crucially depends on the choice of priors. Yet, for mergers involving neutron stars, priors are often chosen in an agnostic way, leaving valuable information from nuclear physics and independent observations of neutron stars unused. We propose to encode information on neutron star physics into data-driven prior distributions constructed with normalizing flows, referred to as neural priors. These priors use information from population analyses, as well as constraints on the nuclear equation of state. Applying this framework to GW170817, GW190425, and GW230529, we demonstrate its ability to yield more informative and physically consistent constraints on source parameters such as mass ratio, tidal deformability, and spins, compared to agnostic priors. Moreover, neural priors naturally provide a principled Bayesian source classification between binary neutron star and neutron star-black hole hypotheses. Our method paves the way for classifying future ambiguous low-mass mergers and for continuously incorporating advances in our understanding of the equation of state into gravitational-wave data analysis.

Speaker: Thibeau Wouters (Utrecht University)

27 Glitch mitigation in the era of third-generation detectors

Gravitational wave (GW) detectors routinely encounter transient noise bursts, known as glitches, which are caused by either instrumental or environmental factors. Due to their high occurrence rate, glitches can overlap with GW signals, as in the notable case of GW170817, the first detection of a binary neutron star merger. Accurate reconstruction and subtraction of these glitches is a challenging problem that must be addressed to ensure that scientific conclusions drawn from the data are reliable. This problem will exacerbate with third-generation detectors like Einstein Telescope (ET) due to their higher detection rates of GWs and the longer duration of signals within the sensitivity band of the detectors. Robust glitch mitigation algorithms are, therefore, crucial for maximizing the scientific output of next-generation GW detectors. For the first time, we demonstrate how the null stream inherent in ET's unique triangular configuration can be leveraged by state-of-the-art glitch mitigation methodology to essentially undo the effect of glitches for the purpose of estimating the parameters of the source. The null stream based approach enables mitigation and subtraction of glitches that occur arbitrarily close to the peak of the signal without any significant effect on the quality of parameter measurements, and achieves an order of magnitude computational speed-up compared to when the null stream is not available. By contrast, without the null stream, significant biases can occur in the glitch reconstruction, which deteriorate the quality of subsequent measurements of the source parameters. This demonstrates a clear edge which the null stream can offer for precision GW science in the ET era.

Speaker: Harsh Narola (Utrecht University / Nikhef)

Glitch mitigation in the era of third-generation gravitational wave detectors

Dinner

Tuesday 28 October

09:00 Coffee/tea

Keynote

28 Black-Hole Mimickers and Dark-Matter Candidates: Modeling Boson Stars and their Gravitational-Wave Signatures

Boson stars are compact objects composed of fundamental fields
arising in theories beyond the standard model (SM) of particle physics.
From dark-matter halos to ultracompact objects with light rings,
they have acquired an important role in gravitational-wave driven
searches for physics beyond the "GR+SM" paradigm. In this talk
we present an overview of the modeling of boson-star binaries
as sources of gravitational waves, how present detectors would
interpret their signals and what smoking-gun effects may enable
us to distinguish them from other sources.

Speaker: Ulrich Sperhake (University of Cambridge)

10:30 Coffee/tea

Contributed talks

29 Does the Local Universe dominate Astrophysical Gravitational Wave Background Anisotropies?

As gravitational wave detectors become more sensitive, they will detect so many sources that individual events blend together to form a stochastic Gravitational Wave Background (GWB). Detecting anisotropies in this background could provide valuable insights into various cosmological and astrophysical properties. Given the strong expectation that LISA will detect the astrophysical GWB created by the extragalactic white dwarfs, we focus on the anisotropies of this specific background.

Anisotropies in the GWB have been extensively studied under the assumption that the universe is statistically isotropic and homogeneous. However, for binary white dwarfs, these assumptions break down on local scales due to the discrete distribution of galaxies. The properties of this discreteness are analyzed using simulations of isotropic universes. These simulations are also used to find a measure of anisotropy as a function of redshift. These findings are then used to analyze the inhomogeneity of the real stellar mass distribution using the Glade+ galaxy catalogue.

Our simulation results indicate that discreteness effects in the stellar mass distribution become negligible beyond 30 Mpc. Using both simulation data and Glade+, we find that 90% of the total anisotropy originates within 175 Mpc and more than 95% within 450 Mpc. Additionally, the GWB sky map constructed from Glade+ shows potential stochastic point sources from over-massive nearby galaxies. Moreover, incomplete electromagnetic data due to the Milky Way obscuration introduces uncertainties, which need to be addressed using proper masking techniques. Finally, we estimate that LISA can detect the multipoles $\ell=0$ and $\ell=2$, with $\ell=4$ potentially resolvable depending on observation time.

Speaker: Coen Rondeel (Observatoire de la Côte d'Azur)

30 Simulation of Core-Collapse Supernovae with advanced neutrino transport

Neutrinos play a crucial role in the explosion mechanism of core-collapse supernovae. Including them in simulations requires to solve the Boltzmann equation, which is very expensive to solve because of its higher dimensionality. Most simulations therefore rely on approximations to that equation that are more affordable. The effect of these approximations on the evolution of the system and on the observables is however difficult to estimate. Therefore, we implemented a full Boltzmann solver in the code Gmunu that we will use to compare simulations with and without approximations to the equation. In this talk, we present the full Boltzmann solver and present the two parts of the project. The first one consists simulating axisymmetric core-collapse supernovae with the M1 approximate scheme and analyze the gravitational wave emission, for which we discuss preliminary results. The second part consists in using the Boltzmann solver to investigate the impact of the approximate scheme on the neutrino and gravitational wave signals.

Speaker: Arthur Offermans (KU Leuven)

31 Resolving white dwarf binaries within globular clusters with LISA

It is currently uncertain how many white dwarf (WD) binaries LISA will be able to detect in the globular clusters (GCs) around the Milky Way, with predictions ranging from zero to dozens. However, there is another question: if there is a WD binary in a GC, would LISA actually be able to resolve it well enough in terms of sky location and distance that the binary could be distinguished from those in the Milky Way disc? We investigate this question using the GW analysis software package GWToolbox and a Milky Way model made using the stellar evolution code SeBa.

Speaker: Wouter van Zeist (Radboud University)

32 Stable evolutions of ultracompact black hole mimickers in 3+1 dimensions.

Ultracompact black hole mimickers formed through gravitational collapse under reasonable assumptions obtain light rings in pairs, where one is unstable and the other one is not. Stable light rings are believed to be a potential source for dynamical instability due to the trapping of massless perturbations, as the decay of these modes is relatively slow.

We have performed fully nonlinear dynamical evolutions of spherically symmetric, ultracompact solitonic boson stars in 3+1 dimensions. We find no evidence of an instability after evolving the objects for ~1000 light crossing times, suggesting that the proposed mechanism may not be efficient after all to effectively destroy ultracompact black hole mimickers.

Speaker: Seppe Staelens (University of Cambridge)

33 Extreme mass ratio inspirals with eccentricity in a scalar cloud environment

With the upcoming space based gravitational wave detector LISA approaching, many open questions still remain. One of the golden observables for the mission will be Extreme Mass Ratio Inspirals (EMRI); a stellar mass Black Hole (BH) gradually spiralling into its supermassive companion, emitting gravitational waves containing a wealth of information about the BH environment. An exciting possibility is the detection of Dark Matter (DM) signatures from ultralight scalar bosons in these systems , including axions and fuzzy DM. Within my research, I work on extending the framework from Brito and Shah [1] to study the effects of this environment fully relativistically. So far, I have made the first steps to relax the assumption of circular orbits, and study eccentric orbits in this non-vacuum system. Additionally, I am working on a consistent calculation of all the relevant contributions up to lowest order in the expansion, with the goal of making predictions about the evolution of the orbit under reaction with the cloud.

Speaker: Robrecht Keijzer (KU Leuven)

34 3DMHD simulations of stellar mergers & their GW signals

Three-dimensional simulations can provide the most accurate modelling of the merger between stars, this includes the precise computation of their gravitational-wave signals. With the state-of-the-art 3DMHD code AREPO we can study stellar mergers at many different scales, ranging from giant stars to neutron stars, releasing GWs all over the frequency spectrum.
With these simulations we can predict the detectability of such events with present and future GW detectors.

Speaker: Dr Javier Moran Fraile (KU Leuven)

BENL_GW.pdf

Contributed talks

35 Probing gravitational waves using GNSS constellations

The current detection of gravitational waves (GW) in the audible range involves the construction of detectors specifically dedicated, covering a wide spectrum but requiring costly construction. Future detectors are expected to cover the mHz frequency, while the pulsar signal based detection covers the nHz range, leaving the μHz regime largely unexplored. Thanks to their onboard atomic clocks and orbits determined to within a centimeter, constellations of global navigation satellite systems (GNSS), such as GPS and Galileo, offer free access to more than 30 years of clock and orbital data for tests of fundamental physics. In this talk, we present a framework for calculating the deviation in the evolution of GNSS orbits induced by GW signals. We show that when a GW interacts resonantly with a GNSS satellite’s orbit, the effects are amplified, leading to a secular detectable evolution of Keplerian parameters. Therefore, the long time series of GNSS orbital products can be used to bridge the gap in the μHz range of continuous GW emitted by individual binary sources or the stochastic GW background. Finally, we demonstrate that the orbital deviations induced by resonant GW are coherent across the entire constellation, enabling a satellite network to disentangle GW effects from satellite systematic effects.

Speaker: Bruno Bertrand (Royal Observatory of Belgium)

36 Neural likelihood estimators for flexible Gravitational wave data analysis

In this work, we develop a Neural Likelihood Estimator and apply it to analyse real gravitational-wave (GW) data for the first time. We assess the usability of neural likelihood for GW parameter estimation and report the parameter space where neural likelihood performs as a robust estimator to output posterior probability distributions using modest computational resources. In addition, we demonstrate that the trained Neural likelihood can also be used in further analysis, enabling us to obtain the evidence corresponding to a hypothesis, making our method a complete tool for parameter estimation. Particularly, our method requires around 100 times fewer likelihood evaluations than standard Bayesian algorithms to infer properties of a GW signal from a binary black hole system as observed by current generation ground-based detectors. The fairly simple neural network architecture chosen makes for cheap training, which allows our method to be used on-the-fly without the need for special hardware and ensures our method is flexible to use any waveform model, noise model, or prior. We show results from simulations as well as results from \texttt{GW150914} as proof of the effectiveness of our algorithm.

Speaker: Luca Negri (Utrecht University,Nikhef)

FLEX BGNL

FLEX BGNL meeting.pdf

37 The Doppler boosted LISA response to gravitational waves

The future space-based gravitational-wave observatory LISA will detect massive black hole binaries (MBHBs) with signal-to-noise ratios (SNRs) reaching thousands. Such precision demands accurate modeling of the detector response. Current formulations neglect spacecraft motion during light travel time, omitting velocity-dependent terms of order $v/c \sim 10^{-4}$. We derive these corrections and quantify their impact relative to the state-of-the-art simulator lisagwresponse. The corrections yield residual SNRs up to $\sim 2$ for the loudest events, and fractional differences up to $0.04%$ in other regimes. While small, these effects are comparable to waveform modeling uncertainties and leave distinctive imprints on sky localization, making them relevant for parameter estimation and mock data generation.

Speaker: Tom van der Steen (KU Leuven)

38 Exotic Optical Modes for Thermal Noise Reduction in Metrology

My PhD project aims to overcome a fundamental limitation in gravitational wave detectors and optical clocks:
coating Brownian thermal noise (CBTN), which arises from microscopic vibrations in mirror coatings. The
proposed solution is to reshape laser beams into exotic optical modes—specifically Hermite-Gaussian (HG)
modes—which distribute light more broadly across mirror surfaces, reducing thermal noise.
The first objective is to demonstrate HG25,25 modes in optical clocks, improving timekeeping precision without
relying on cryogenics or crystalline coatings. HG25,25 has been generated at high purity but never applied in a
clock, making its implementation novel and feasible.
The second objective is to implement HG33 modes in the Einstein Telescope High Frequency (ET-HF)
interferometers, enabling detection of primordial black holes by reducing thermal noise at 30 Hz. HG33 is the
highest-order mode compatible with ET’s current mirror design, making it optimal for near-term upgrades.
The built cavities will also be used to detect displacements of less than a femtometre, taking advantage of the
precision of the best atomic clocks and a frequency comb for increased sensitivity.
This interdisciplinary project bridges optics, metrology, mechatronics, and GW instrumentation, leveraging
collaborations across UCLouvain, NIKHEF, and ULiege. It is timely, as Belgium strengthens its connection to
international timing networks and advances its candidacy to host the Einstein Telescope. The project promises
advances in ultra-precise timekeeping, GW sensitivity, and optical cavity engineering.

Speaker: Charlotte Bragard (UCLouvain)

12:30 Lunch

LISA community day: LISA - ET synergies in data analysis

39 A roadmap of gravitational wave data analysis

As gravitational wave detectors become increasingly sensitive, the number of observed sources continues to rise. Unlocking the full scientific potential of these detections depends on robust and efficient data analysis, making gravitational wave data analysis a rapidly evolving and essential field. In this talk, we introduce the core principles of gravitational wave data analysis, emphasizing the current limitations in methodology and computational efficiency. We then highlight recent advances in algorithms and computational techniques designed to mitigate the substantial resource demands while preserving statistical integrity. Finally, we discuss the emerging challenges posed by next-generation observatories such as the Einstein Telescope and LISA, and how they are reshaping the landscape of gravitational wave data analysis.

Speaker: Lorenzo Speri (European Space Agency)

GW-NL-Belgium18Oct2025_talk.pdf

40 LISA - ET synergies (in data analysis): intro

41 LISA - ET synergies (in data analysis): discussion/activity

42 LISA - ET synergies (in data analysis): summary

43 LISA community day: report on parallel sessions

LISA community day: Multimessenger science (with LISA)

44 Transient discovery with the GOTO (Gravitational-wave Optical Transient Observer) Network

The impetus for wide-field searches for counterparts of explosive transients including binary neutron-star mergers, supernovae, and tidal disruption events has never been higher. The ongoing operations with the LIGO-Virgo network of gravitational wave detectors in the 4th observing period continues to offer challenges for optical observers, primarily due to the poor (~100 deg^2) localisation regions. The Gravitational-wave Optical Transient Observer (GOTO; http://goto-observatory.org) network, featuring instruments in the Canary Islands and south-east Australia, is designed to overcome these challenges, and also offers high utility for searches and followup of other types of transients including gamma-ray bursts and supernovae. A modular design featuring eight 40-cm telescopes on each mount provides a composite field of view of around 40 square degrees per instrument, capable of promptly and autonomously covering large fields of view in response to observing triggers. I will report on the status and progress of the network and our expectations for the detection of future transients.

Speaker: Duncan Galloway (Monash University, Australia)

45 Multimessenger science (with LISA): intro

46 Multimessenger science (with LISA): discussion/activity

47 Multimessenger science (with LISA): summary

15:30 Coffee/tea