Belgian-Dutch Gravitational Wave Meeting

7 Oct 2026, 13:00 → 9 Oct 2026, 23:00 Europe/Brussels

Aula Magna

This year, the 14^th edition of the Belgian-Dutch Gravitational Wave Meeting will take place at UCLouvain, from the 7^th to the 9^th of October 2026. The aim of the meeting is to bring together people in our communities involved in gravitational wave science at large, ranging from instrumentation, to astronomy, to theory, and data analysis. This year, we propose a slightly longer duration to allow for more time to discuss various aspects in informal settings, in addition to offering room for presentation across all topics. 

 

Both plenary and parallel sessions will take place at the Aula Magna. 

Registration 

Please register via the registration form on the Registration page.

Registration will proceed in two steps, with an early-bird registration until the 6th of September at 120€. Then, until 30^th of September, registration will still be possible, at a 170€ rate. The conference fee includes coffee breaks, and the conference dinner on the Thursday. You are also welcome to join the outreach event organised on Wednesday evening. 

Payment should be done via bank transfer by the 25th of September, or can be done cash upon arrival. Payments via credit card are not possible. Bank transfer details are

IBAN: BE58370125488179

BIC: BBRUBEBB   

Beneficiary: UCLouvain

Comment/Justificative: it must include "BeNLGW" and your NAME and SURNAME (otherwise we will not be able to match your payment to your indico registration)

Beneficiary address (if needed): 1, Place de l'Université, B-1348 Louvain-la-Neuve, Belgium

Bank address (if needed): ING Belgique 2, rue des Wallons - B-1348 Louvain-la-Neuve

In case you need to cancel your registration, please get in touch with the LOC via email (benlmeeting@listes.uclouvain.be). After the 25th of September, a cancellation fee of 80€ is requested.

Additional activities close to the event (in space and time)

KU Leuven: LGI & LSI Academic Year Opening Reception

 

The Leuven Gravity Institute (LGI) and the Leuven Space Initiative (LSI) invite you to join them for a reception to open the 2026–27 academic year, on Tuesday 6 October from 17:00 at the STUK café.

The evening includes a short performance at 17:30 by artist-researcher Klaas Verpoest (LUCA School of Arts/LGI) and cellist Benjamin Glorieux: "The Solitary One - No Outside", a new work that takes a fall into a black hole as its point of departure.

We would like to propose this occasion to bring together different member audiences. As such, we warmly welcome LGI and LSI members, participants in theBelgian-Dutch Gravitational Wave Workshop, and attendees of the preceding LGI Art-Science "Event Horizon" workshop. (Note that you are also warmly invited to attend the LGI Art-Science workshop, which starts at 14:00 at STUK.)

 

Please register here to confirm your attendance to the opening reception by 29/09.

 

UCLouvain outreach event (07/10 after reception)

Scienceinfuse and the IRMP invite the general public to step into this fascinating universe for one evening.
 
How do we detect a gravitational wave? What does it tell us about the universe? How do we distinguish its signal amid a sea of data? And what role might the future Einstein Telescope play, which is planned for the cross-border region between Belgium, the Netherlands, and Germany?
 
You don’t need to be a physicist to participate: games, challenges, experiments, hands-on activities, and mini-talks will help you discover gravitational waves in all their facets, alongside the researchers who study them.
An evening to explore at your own pace, to play with space-time, ask all your questions… and meet those who are currently striving to listen to the Universe. 

 

Info: https://uclouvain.be/fr/scinfuse/space-day-0  

Directions

By car …

Louvain-la-Neuve is accessible via the E411, N4, and N25 motorways. Its town center is, for the most part, a pedestrian zone.

Note that the center of the city is fully pedestrian, with paid parking almost everywhere. Three car parks are accessible to all motorists, free of charge, on the outskirts of Louvain-la-Neuve.

• The Baudouin 1st car park (210 spaces)
• The Lauzelle car park (180 spaces)
• The Rédimé car park (74 spaces)

No access permit or sticker is necessary to park your vehicle there.

By train…

Many stations are directly accessible from Louvain-la-Neuve, including along the line to Brussels. At Ottignies station, trains run to Brussels Airport, Namur, Charleroi, and Leuven.

Attention: Do not confuse Louvain-la-Neuve with Louvain (the French name for Leuven), which is another city on a different line.

From Brussels

Allow approximately 30 minutes. A change is required at Ottignies, except for certain direct trains (which take slightly longer).

From another country

Arrive at Bruxelles-Nord (Brussels North) or Bruxelles-Midi (Brussels South) train station, then take a train to Ottignies (which may also stop at Bruxelles-Central, Bruxelles-Schuman, and Bruxelles-Luxembourg).

Check real-time timetables and plan your trip at www.belgiantrain.be or download the SNCB mobile app. 

Accomodation

Some preferential rates have been guaranteed for people attending the conference. 

• Hotel Martin's Louvain-La-Neuve (website): use code BeNlGW2026 when reserving your room to the following address aline.debuyser@martinshotels.com (reservation via website may not be able to handle the code). Offer is Cosy Single room 160.46€/night, Cosy Double Room 171.92/night.
• Ibis hotel Louvain La Neuve (website): reserve via email with reference "UCL Cosmology 2026" via email to marc.brassart@accor.com. The rate is 110.30/night and the offer is valid only until 21/08.

Other accomodation options are possible in and around Louvain-La-Neuve, but would not have preferential rates. 

Conference Dinner and Reception 

On Thursday, we will have a conference dinner at the Cercle du Lac

 

Local Organizing Committee

Charlotte Bragard, Giacomo Bruno, Ricardo Cabrita, Swapnil Dhage, André Fuzfa, Aaron Goodwin-Jones, Disha Hegde, Justin Janquart

 

Wednesday 7 October

Welcome and introduction: Registration

1 Registration

2 Welcome words & information about the conference

invited speaker: Artem Basalaev (AEI Hannover/Leibniz University Hannover): Challenges for next-generation ground-based detectors at low frequencies

3 Challenges for next-generation ground-based detectors at low frequencies

Future ground-based gravitational wave detectors, such as the Einstein Telescope and the Cosmic Explorer, will reveal the universe in gravitational waves throughout cosmic history. However, their realisation presents a set of unprecedented challenges. In this talk, I will focus on low-frequency gravitational waves, from a few Hz to a couple of dozen Hz. I will discuss the unique discovery opportunities that arise from detecting gravitational waves at these frequencies, particularly when in synergy with space-based detectors. I will then elaborate on the specific challenges and solutions for overcoming noise in this regime, focusing on gravity gradient noise, seismic noise and associated control noise, as well as quantum noise. Significant progress has been made in all these areas; however, achieving the incredible sensitivity of a detector like the Einstein Telescope still relies on optimistic assumptions about technological development in the near future. I will discuss some of the specific risks and mitigation strategies. Finally, I will conclude by sharing my personal perspective on how we can design such future detectors by working together as an interconnected but diverse community.

Speakers: Artem Basalaev (AEI), Artem Basalaev (AEI Hannover)

2026-10-07_InvitedTalk_BelgianDutch_GW_Meeting_A.Basalaev.pdf

Coffee break

News from major scientific Collaborations

4 Matthias Vereecken: New from LIGO-Virgo-KAGRA

General overview of news in Ligo-Virgo-KAGRA Collaboration

5 Henri Inchauspé: News from LISA

General overview of nieuws in LISA.

6 Archisman Ghosh: New from Einstein Telescope

General news from the Einstein Telescope collaboration

Talks data analysis and theory

7 Scalable and sequential inference of the neutron star equation of state with the Einstein Telescope

Future gravitational wave observatories, such as the Einstein Telescope, will detect thousands of mergers between compact objects annually. By detecting the effects of tidal deformation in binary neutron star mergers, these detectors will be the dominant way to study the equation of state of ultradense nuclear matter in the coming decades. However, this requires a scalable Bayesian hierarchical inference framework that can be easily updated as soon as new detections are made. We show that sequential Monte Carlo is a natural framework for analyzing such growing datasets. Paired with GPU accelerators, this allows us to infer the equation of state of a mock catalog of one month of binary neutron star mergers detected by the Einstein Telescope within a few hours. We project that the extension to a full year of detectable sources would take our sequential Monte Carlo sampler just over a couple of days on modern hardware.

Speaker: Thibeau Wouters (Utrecht University/Nikhef)

main.pdf

8 Scalable early warning for BNS mergers in next-generation ground-based detectors using SBI

Third-Generation (3G) observatories will observe Binary Neutron Star coalescences for hours before merger. Over these extended windows, Earth's motion induces signal modulations that render standard approaches computationally prohibitive for low-latency searches. To overcome this, we propose a 2D Convolutional Neural Network (CNN) processing multi-detector, complex Short Fourier Transform (SFT) tensors to implicitly marginalize over extrinsic parameters, effectively reducing the pipeline to a mass-only search. By optimizing a Binary Cross-Entropy loss, the network learns the signal's non-linear time-frequency morphology, approximating the optimal Bayesian detection statistic. We conduct a large-scale campaign of software-simulated signals to compare the CNN's performance with a GPU-optimized SFT-based Matched Filtering baseline for both $\triangle$ and 2L configurations for the Einstein Telescope, across three signal duration regimes: $\approx 25 $ minutes ($f_\text{min} = 5$ Hz), $\approx 2$ hours ($f_\text{min} = 3$ Hz), and $\approx 6$ hours ($f_\text{min}=2$ Hz). We found that the CNN achieves comparable detection sensitivity while reducing computational costs by over four orders of magnitude, slashing inference latency from minutes to milliseconds. This efficiency unlocks crucial low-latency early-warning triggers for 3G multi-messenger astronomy.

Speaker: Martin Gerini

MARTIN_GERINI_EARLY_WARNING.pdf

MARTIN_GERINI_EARLY_WARNING.pptx

9 Probing Binary Black Hole Mergers in AGN Accretion Disks with Gravitational Waves and Neutrinos

We present a method to search for coincident gravitational-wave and high-energy neutrino emissions from binary black holes (BBHs) embedded in the accretion disks of active galactic nuclei (AGNs). Such environments are promising sites for multimessenger emission from BBHs, as the surrounding matter and radiation fields can enable electromagnetic and neutrino emission in addition to GWs. Moreover, interactions between BBHs and AGN disks may facilitate binary formation and hierarchical mergers, potentially contributing to the population of black holes in the mass gap detected by the LIGO-Virgo-KAGRA (LVK) collaborations.
Our search method combines GW and neutrino data with AGN optical catalogs to enhance sensitivity to potential sources. We assess the sensitivity of the search using subthreshold GW candidates detected by LVK, neutrino candidates from public IceCube data and AGN candidates from the Quaia catalog.
A confident detection would constitute the first evidence for an association between GW and high-energy neutrinos and provide crucial insights into BBH formation channels.

Speaker: Leonardo Ricca (CP3)

Be-NL GW Presentation.pdf

Public Event

Public event on gravitational waves

Thursday 8 October

Parallel Sessions: Instrumentation: Session 1

10 First results from a borehole network for the seismic characterization of the Einstein Telescope Euregio Meuse-Rhine candidate site

The low-frequency sensitivity of the Einstein Telescope depends on the seismic environment of its site, both through the mechanical coupling of ground motion to the test masses and through Newtonian noise.
We present the first results of the seismic characterization of the Euregio Meuse-Rhine candidate site based on a network of recently instrumented boreholes covering the first seven months of 2026 at four underground stations and their co-located surface sensors. Most of these boreholes were deliberately sited next to railway infrastructure, so that the levels reported here describe how anthropogenic transients propagate to depth at different distances from the source. The analysis was carried out with SiteChar, a Python framework developed for this work to provide a common processing pipeline across different sensors, networks and observation periods. It processes the data in sequential chunks and separates the noise floor from the contribution of transients.
We present a characterization of the catalog of observed transients and a tentative classification.
These measurements form part of an integrated site characterization approach that combines the observed seismic field with three-dimensional geological and wave-propagation models to predict Newtonian noise and ultimately guide its mitigation at the Euregio Meuse-Rhine candidate site.

Speaker: Luca D'Onofrio

Belgian-Dutch conference 2026.pdf

11 Addressing coupling limitation in low-frequency active isolation systems with Fiber-Optic Gyroscopes

Seismic noise remains one of the principal challenges to extending the detection bandwidth of gravitational-wave detectors toward lower frequencies. As the Einstein Telescope aims to achieve unprecedented low-frequency sensitivity, the seismic isolation of its test masses must be significantly improved.
A major difficulty in active isolation at very low frequencies arises from the strong coupling between the different degrees of freedom of the suspension chain. The sensors currently employed for low-frequency isolation are sensitive to both translational and rotational motion, making it difficult to distinguish between them. This limits the performance of isolation systems in the frequency range where both effects are significant, typically below 1 Hz. Several approaches exist to mitigate these couplings, ranging from direct mechanical decoupling solutions to digital compensation techniques.
An alternative approach is the direct measurement and control of the suspension's rotational motion. This requires a sensor capable of detecting extremely small rotations, on the order of nrad/√Hz, while completely rejecting translational motion. Fiber-optic gyroscopes (FOGs), based on the Sagnac effect, are attractive candidates due to their intrinsic insensitivity to translation. Their main limitation is sensitivity, with the best commercial devices typically reaching noise floors of about 10 nrad/s/√Hz.
In this work, we investigate the use of FOGs for low-frequency isolation and evaluate their added value on the ET-CRISTAL platform, a full-scale hybrid isolation system prototype for ET test masses.

Speaker: Brieux Thibaut (Uliege)

BDGWmeeting2026_BrieuxThibaut.pdf

12 Cryogenic Interferometric Inertial Sensor for next-generation gravitational-wave detectors

Technology validation for the ET-LF must be conducted in an extremely low-vibration, cryogenic environment to simulate adequate operational conditions. This harsh environment calls for a highly sensitive, cryogenic-compatible inertial sensor for vibration monitoring and reduction in the test facilities. To that end, cryogenic, vacuum-compatible, horizontal and vertical inertial sensors with a sensitivity of 10 fm/√Hz from 1 Hz at 20K are developed. This presentation collects the first validations conducted on the cryogenic sensors. Their proper operation under cryogenic conditions was first assessed by testing critical sub-components, such as the interferometric readout and suspension mechanisms, and by validating the sensor design's robustness to thermal variations. The vertical version of the sensors was integrated into a large-scale cryostat at the Institute for Gravitational Research (IGR) at the University of Glasgow to monitor vibration levels in this future ET technology validation facility. The vertical cryogenic interferometric inertial sensor operated perfectly down to 5.12K. Its measurements were validated over the range of 0.2 to 100Hz using witness sensors placed near the cryostat, proving its reliability in this harsh environment. Similar results were observed for the horizontal cryogenic sensor at 19K. Once the sensors proper operation under target conditions is demonstrated, their resolution is measured in a dedicated low-vibration cryogenic environment.

Speaker: Morgane Zeoli (Université de Liège)

261008 - Zeoli DBGW UCLouvain.pptx

13 Continuous tracking of the eigenmodes of a locked cavity: inferring mirror aberrations during a thermal transient

Detector upgrades typically include increasing circulating power in the interferometer. In particular, next-generation detectors plan to have circulating powers in the MW range. Meanwhile, current detectors have achieved a few hundreds of kWs of circulating power, well below the intended design values. High power operation comes with a particular set of challenges, due to optical absorption at the test masses, which causes thermal deformation of the mirror and a shift of the cavity eigenmodes. In turn, these lead to increased optical losses and a plethora of control issues such as parametric instabilities.

Here, we present an experimental scheme using a phase camera to monitor the eigenmodes of a cavity while it is locked. In combination with the experimental scheme, we introduce a machine learning pipeline capable of deriving mirror aberrations in a table-top linear cavity from the phase camera data. This is achieved by mapping the cavity eigenmode amplitudes into Zernike mode coefficients which describe the various mirror aberrations. We will present experimentally measured modal amplitudes of an optical cavity in a tabletop set-up and discuss the next steps. Such an ability to monitor cavity mirror deformations in the presence of high circulating power will be crucial when dealing with the long thermal transients that are expected in next-generation detectors.

Speaker: Ricardo Cabrita

BelNL_XIV_RicardoCabrita.pdf

Coffee break

Parallel Sessions: Instrumentation: Session 2

14 Output Mode Cleaner for ETpathfinder

Gravitational-wave detectors require a purely Gaussian beam in output, which ensures a smooth distribution of the optical power across the transversal plane, and the lowest angular divergence. For this reason, an Output Mode Cleaner (OMC) optical cavity is placed before the detection photodiode, to filter out the control sidebands created with phase modulation, and spurious higher-order modes (HOM) that can arise from optical mismatch and misalignment in the interferometer. The performance of an OMC is evaluated in terms of HOM suppression factor, Gaussian mode transmittivity and noise from back-reflected stray light.
The present contribution illustrates the current status and plans for optical and mechanical design of the OMC for the ETpathfinder experiment, at a wavelength of 1550 nm. The chosen cavity layout is the so-called “bow-tie”, with two highly reflecting mirrors and two input and output couplers. All the mirrors will be glued to a base plate of the same material (fused silica), to reduce cavity length noise due to thermal expansion. At the moment, the initial assessments on the input beam specifications, available space on the optical bench, and target performance requirements are ongoing. Simulations in FINESSE and OSCAR are used to build the best optical configuration. Once the optical layout will be fixed, the mechanical design of the base plate and mirrors will begin. Future steps include the whole process of characterization of the single optical elements, mounting, installation and commissioning of the OMC at the ETpathfinder site.

Speaker: Francesco De Marco (Ghent University)

2026-10-08_DeMarco_OMC_BeNl_Meeting.pdf

15 Calibrating GW detectors using scattered light

The Virgo gravitational-wave detector is calibrated using two independent methods: the Photon Calibrator (PCal) and Newtonian Calibrator (NCal), that can validate h(t) with an accuracy of ~1% up to 2 kHz. A new independent technique that uses scattered light as a signal for the detector calibration was recently proposed and it is planned to be installed in Virgo for its fifth observing run.
Through this technique, scattered light is injected in the interferometer from the back of the end mirrors, using a scattering element that can be modulated in amplitude and frequency. This method will give an independent validation of h(t) in the range 50 Hz to few kHz with an accuracy of a few percent. The main advantage of this method is the possibility to validate h(t) also in the band 1.5 to 2 kHz, which is within the range of the standard GW data analysis. Furthermore, we will be able to inject a calibration signal in the detector without moving any ITF mirror, which will allow us to highlight systematic errors in the suspension modelisation.
In this talk we will explain the working principles of this method and we will give an update on the status of the project.

Speaker: Daniela Pascucci (Ghent University)

Pascucci_B-NL_meeting_2026.pdf

16 Demonstrating Transverse Optical Mode Control for Gravitational Wave Detection

The proposed Einstein Telescope (ET), a third generational European gravitational-wave (GW) detector, plans to extend the frequency band below 10 Hz and achieve a broadband sensitivity improvement by at least an order of magnitude over current ground-based detectors such LIGO, Virgo and KAGRA. ET will have 10 km underground interferometers in a triangular configuration with each arm consisting cryogenic low-frequency (ET-LF) and high-frequency (ET-HF).
To achieve such prominent sensitivity, ET-HF is expected to operate at 3 MW intra-cavity power, to raise GW sidebands signal above vacuum fluctuations and reduce quantum shot noise impact. Such high-power is more susceptible to optical loss mechanisms, in particular, thermo-elastic and thermo-optics deformations, resulting due to distortion in eigenmode of the interferometer cavities from fundamental Gaussian mode to higher-order modes (HOM). Such mode mismatch reduces the coherent power and increases losses.
Current detectors use open-loop thermal compensation systems (TCS) to ease such distortions using CO2 projections, ring heaters and Hartmann sensors. But these systems lack the robustness and adaptability as HOM sensing and control will be critical in managing optical losses in the megawatt regime.
I will talk about how I plan to address this research issue by demonstrating first-ever closed loop transverse mode sensing and control. I will also talk about ET-OPT laboratory of UCLouvain, which will consist of 10 m long Fabry-Perot suspended optical cavity operating at same ET-HF power densities. I will also discuss implementation of thermally actuated suspended active mode-matching stages (TSAMS) for mode actuation and phase camera as mode diagnostics. Upon successful demonstration of this technique, it will be deployed in close collaboration with ULiege at Virgo for O5 full-scale detector operations and ETpathfinder to integrate with cryogenic operations and squeezed light injections.

Speaker: Swapnil Dhage (UCLouvain)

2026_10_08_Transverse_mode_control.pdf

17 Development of a displacement sensor based on a maser reference

Inertial sensors are essential tools for gravitational-wave detectors.
These detectors typically rely on Fabry-Pérot cavities that are actively stabilized to minimize mechanical noises in the frequency range of interest.
This stabilization demands precise monitoring of the mirrors motion using displacement sensors.
Various techniques have been developed for such inertial sensing, achieving impressive sensitivities using Fabry-Pérot cavities [1] or Michelson interferometers [2][3].

Heterodyne cavity tracking relies on measuring the beating between two optical cavities.
The stability of each cavity impacts the precision of this measurement since it involves a relative motion between the two cavities.
In this talk, we present proof of concept for a new method for heterodyne inertial sensing that enables absolute displacement measurement.
Using an optical frequency comb beating separately with two optical cavities, we can eliminate common-mode noise from the reference oscillator, effectively forming a balanced inertial sensor.
With this method, we reach a sensitivity of 700 fm/$\sqrt{\text{Hz}}$ at 1 Hz, close to the state-of-the-art of 260 fm/$\sqrt{\text{Hz}}$, and 10 fm/$\sqrt{\text{Hz}}$ at 1 kHz [1].

A projected two-orders of magnitude improvement in sensitivity will be discussed along with its potential applications in gravitational waves detectors.

[1] S. Chalathadka Subrahmanya, C. Darsow-Fromm, and O. Gerberding, Optics Express 33, 4044 (2025).
[2] M. B. Gray, D. E. McCLELLAND, M. Barton, and S. Kawamura, Optical and Quantum Electronics 31, 571–582 (1999)
[3] G. Zhao, B. Ding, J. Watchi, A. Deraemaeker, and C. Collette, Mechanical Systems and Signal Processing 145, 106959 (2020)

Speaker: Charlotte Bragard (UCLouvain)

BelDutch_2026_Charlotte_Bragard.pdf

BelDutch_2026_Charlotte_Bragard.pptx

18 Investigating Astigmatism in the Stable Folded Recycling Cavities of Virgo

The current performance of the Virgo Gravitational Wave Detector is limited by an excess noise source, caused by marginally resonant higher order modes in the recycling cavity. Virgo is proposing a substantial update, to shift to a “stable cavity” geometry. This would suppress higher order modes in the recycling cavity. However, the design introduces new challenges with Astigmatism associated with the angles of incidence (AoI) in the folded recycling cavities represents a potentially critical limitation, which affects the interferometer’s sensing and control signals.

Astigmatism introduces higher-order Hermite–Gauss modes that can contaminate longitudinal and alignment error signals, producing offsets, cross-couplings and changes in the sensing response, with potential consequences for the longitudinal controllability and accuracy of the interferometer. The current design considers astigmatism primarily in terms of power losses within the recycling cavities, which are estimated by design to be of the order of a few tens of ppm; while this requirement is adequate in terms of recycling gain, its impact on longitudinal controllability remains to be studied.

In this talk, we present the simulation results for the longitudinal controllability of Virgo in the stable cavities scenario. In close collaboration with the Virgo ISC group, this work investigates how the AoI and resulting astigmatism of the folded recycling cavities affect the higher-order mode content and the locking signals of the interferometer. We compare the resulting offset with the designed AoI values with the accuracy requirements. These results provide a first assessment of the impact of astigmatism on the longitudinal controllability of the upgraded Virgo interferometer and establish a framework for extending the study to other geometrical defects.

Speaker: Jue Zhang

Astigmatism.pdf

Lunch break

Parallel Sessions: Theory & Data Analysis

19 Self-force inspiral-merger-ringdown waveforms from flux-balance laws

With the upcoming third-generation gravitational-wave detectors comes the need to build complete, faithful, and fast waveform models for asymmetric-mass-ratio compact binaries. Most efforts within the self-force community have focused on modeling these binaries’ inspiral regime, but for ground-based detectors the final merger can represent the dominant part of the signal. In the multiscale self-force expansion, binaries undergo three distinct dynamical phases: inspiral, transition-to-plunge, and plunge. Previous work on non-spinning quasicircular binaries has established waveform models at first post-adiabatic order (1PA) for the inspiral, second post-leading-transition order (2PLT) for the transition, and leading geodesic order (0PG) for the plunge. In this work, I present the first multiscale self-force inspiral-merger-ringdown (IMR) waveform model built from first-principles, as well as its implementation in the FastEMRIWaveforms (FEW) Python package. This self-force IMR waveform model consistently assembles the inspiral, transition and plunge trajectory and amplitudes into a single composite solution built from matched asymptotic expansions. The matching is performed directly at the level of the fluxes, the binding energy and the waveform amplitudes, allowing for a resummation of the entire IMR trajectory in terms of a flux-balance law. This work is a crucial milestone towards building fast and accurate waveform models for asymmetric-mass-ratio compact binaries.

Speaker: Loïc Honet (Université libre de Bruxelles)

imr_ode_solution.gif

LoicHonet.key

LoicHonet.pdf

20 A merger-ringdown model for eccentric EMRIs.

In the small mass ratio limit, eccentric binary systems can be modelled up to the separatrix crossing within the self-force framework by using first principles. At this stage, the timescale of the dynamics changes, and the prescription of post-adiabatic expansion ceases to be applicable. In the quasi-circular case, the development of a transition-to-plunge model, together with a post-geodesic expansion makes it possible to calculate full trajectories and waveforms from the inspiral to the plunge.
In this presentation, we introduce a model that addresses the transition to plunge for eccentric binary systems. Starting at the separatrix crossing, the secondary is expected to evolve along a perturbed homoclinic orbit before it transits to the merger phase. We present different phases of the motion with different expansions in the mass ratio, and compute waveforms on these trajectories.

Speaker: Guillaume Lhost (University of Mons)

Lhost.pdf

21 Spatial projection of Kerr quasinormal modes from numerical data

We present a practical algorithm for the numerical spatial projection of Kerr quasi-normal mode excitations from numerical data. We study criteria for quasinormal mode completeness. We conclude by discussing prospective applications to linear and generically nonlinear evolutions of perturbed Kerr black holes.

Speaker: Stef Husken

BeNLGW_Stef_Husken.pdf

22 Tidal deformations of neutron stars: role of superfluidity and temperature

The observation of the gravitational-wave signal GW170817 from a binary neutron-star merger 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 reproduced in terrestrial laboratories. During the inspiral phase, neutron stars undergo slight deformations induced by the tidal field of their companion. These deformations are quantified by the so‑called tidal deformabilities. With the advent of third‑generation detectors, tidal deformabilities are expected to be measured with far greater precision, motivating the development of more accurate tidal models that incorporate richer microphysics.

Most existing calculations of tidal deformabilities rely on a cold, barotropic, perfect‑fluid description of neutron‑star matter. In this talk, I will discuss how to go beyond this approximation by including both superfluidity and finite‑temperature effects. Neutrons in the core and in the inner crust are indeed expected to be superfluid, while protons are superconducting in the core. Additionally, neutron‑star matter is likely to heat up during the late inspiral because of tidal friction. Although these two aspects may appear physically distinct, they can be naturally incorporated within a unified multifluid hydrodynamic formalism. Using this framework, I will show how superfluidity and finite temperature impacts the tidal deformabilities of neutron stars.

Speaker: Ethan Carlier (Université Libre de Bruxelles)

Talk Ethan Carlier BeNL.pdf

23 Memory effect from neutron star merger counterparts

The gravitational-wave (GW) memory effect is a prediction of general relativity, characterized by a permanent change in spacetime geometry. Although it has not yet been observed, next-generation detectors such as the Einstein Telescope (ET) and the Laser Interferometer Space Antenna (LISA) are expected to provide the sensitivity required for its first detection. GW memory comprises two distinct contributions: the nonlinear memory, generated by the self-interaction of GWs, and the linear memory, originating from the anisotropic emission of matter and radiation associated with astrophysical transients. In this talk, I will show how we quantify both nonlinear and linear memory contributions produced during binary neutron star mergers, including those from GWs, gamma-ray bursts (GRB), afterglows, neutrinos, dynamical and disk-wind ejecta, and kilonova emission. I will then discuss their detectability with ET and LISA through two specific applications. First, I will focus on the multi-messenger event GW170817-GRB 170817A-AT2017gfo, for which we find that the combined memory signal would have been detectable by ET with a signal-to-noise ratio of 10.7, dominated by the nonlinear memory. Second, I will present our assessment of GRB memory across a synthetic population, showing that only extreme-energy and favorably oriented configurations could produce a detectable signal.

Speaker: Thomas Brabant (Université de Liège)

Memory_Effect_talk.pdf

Workshop on ET Modelling: Session 1

main.pdf

Coffee break

Parallel Sessions: Theory & Data Analysis

24 A track search for extreme mass-ratio inspirals in LISA data with template banks

Extreme mass-ratio inspirals (EMRIs), the inspiral of a stellar-mass compact object into a supermassive black hole, are one of the primary targets for LISA. They are long-duration, multi-harmonic signals that capture the complicated two-body dynamics of the system, which is exactly what makes them such precise probes of the source parameters. Detecting them in realistic LISA data is a different matter: the prior-to-posterior volume ratio is of order $10^{40}$, which rules out a fully coherent matched-filtering template bank search. It was recently shown that near-circular systems can still be detected using a semi-coherent detection statistic based on a single-harmonic template model. In this talk we continue on that approach and search directly in the physical parameter space, using a template bank built on the semi-coherent local metric. For this we introduce a JAX-based module that rapidly solves the EMRI phase evolution in batches, and we use its auto-differentiation to obtain the metric that informs the template placement. We show initial estimates of the resulting bank size and compare them to existing work. We also comment on what still limits the method and where it needs to go next.

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

BertDepoorter_EMRI_Track_Search.pdf

25 Disentangling the Galactic binary zoo with LISA

The Laser Interferometer Space Antenna (LISA) will open the millihertz gravitational-wave window and provide a new way to survey the fossil stellar population of our Galaxy through compact binaries composed of white dwarfs, neutron stars, and black holes. This offers enormous potential for population studies, Galactic astrophysics, and multi-messenger astronomy with LISA. However, once a source is detected, can we tell what kind of system we are observing from the LISA data alone? I will present recent work addressing this question for different classes of stellar-remnant binaries. Using simulated LISA catalogues and machine-learning classification, we find that even rare populations such as neutron-star–white-dwarf binaries can be identified with high efficiency, despite substantial overlap with the dominant double-white-dwarf population.

Speaker: Valeriya Korol (SRON Space Research Organisation Netherlands)

Disentangling the Galactic binary zoo with LISA_Korol.pdf

26 Spin-up of white dwarfs through non-adiabatic dynamical tides and GW-driven resonance locking

Short-period white dwarf (WD) binaries are prime targets for LISA, where both gravitational-wave (GW) emission and dynamical tides drive the spin-orbital evolution. Previous treatments of resonance locking — in which the tidal torque and GW-driven decay evolve in concert to hold the forcing frequency constant — rely on a quasi-adiabatic description in which the tidal torque is dissipative by construction. We couple the non-adiabatic oscillation codes MAD and MAD_tides with Peters' equations to study this evolution in DA white dwarfs before and into the ZZ Ceti instability strip.
We find that the sign of the tidal torque at a given resonance varies strongly with forcing frequency and stellar spin, a behaviour inaccessible to quasi-adiabatic formalisms. Systems undergo successive locking episodes rather than remaining locked on a single mode, each episode ending when the torque reverses sign as the primary spins up. Tidal effects leave the merging timescale essentially unchanged, while spinning the primary up significantly, reaching 0.3–0.4 of critical rotation at Roche-lobe filling.

Speaker: Vincent Ernst

27 On the evidence of lensing in GW231123

Just like light, gravitational waves may be gravitationally lensed by massive objects along their travel path, however, this has not been observed so far. GW231123 is the most massive binary black-hole merger observed to-date, and standard analyses infer uncommon source properties that are unusually sensitive to the waveform model used. Here we investigate whether gravitational lensing can account for these features. We model the signal as a superposition of multiple overlapping images of the same merger and find support for lensing across four waveform models. The lensing interpretation reduces the support for unusually high spins, bringing both the inferred spins and masses into closer agreement across waveform models. A comparison with a background of unlensed signals gives the lensing interpretation an estimated significance of $3.96^{+0.68}_{-0.30} \sigma$. This makes lensing the most compelling interpretation of this event to-date.

Speaker: Mick Wright (Utrecht University)

wright-gw231123.pdf

28 On the interplay between waveform systematics and lensing signatures in gravitational-wave signals

The exceptional gravitational-wave event GW231123 exhibited unusually high total mass and near-extremal spins, while revealing strong waveform-dependent variations in its inferred source properties. Several follow-up studies favored a lensed interpretation, with reduced waveform systematics,raising whether apparent lensing evidence could arise from waveform modeling systematics. Using GW231123-like numerical-relativity injections, we find that lensing degrees of freedom can reduce waveform discrepancies and yield non-negligible lensing support. However, this behavior is not systematic across injections, and cases where all waveform and lens models favor lensing while reducing discrepancies are not found.

Speaker: Disha Hegde (UCLouvain)

BE-NL Meeting 08_10 slides-1.pdf

29 GPU-accelerated Bayesian inference for diffracted gravitational wave signals

As the rate of gravitational wave detections continues to grow, the probability of observing a gravitationally lensed gravitational wave also increases. When the Schwarzschild radius of the lens is comparable in size to the gravitational wave wavelength, wave-optics effects induce a frequency-dependent modulation on the signal. CPU-based parameter estimation analyses for wave-optics lensing are computationally intensive, limiting the scale of studies to determine the significance of any potential lensing candidate. In this work, we implement a point-mass lens model in jim, a high-performance GPU-based parameter estimation pipeline. This enables, for the first time, Bayesian analysis in full wave-optics to be accelerated without requiring training data. We call this framework Jim-lensing. Using this framework, we are able to speed up a full parameter estimation run under the point-mass lens hypothesis from the order of days to the order of 10 minutes compared to the current state-of-the-art CPU-based pipeline. We show that our implementation is stable and obtains comparable results to this state-of-the-art pipeline. The substantial speed gain over existing CPU pipelines paves the way for analyzing gravitational wave data under a broader range of lensing models with greater capacity to produce robust frequentist significance estimates.

Speaker: Jef Heynen (UC Louvain)

BeNL_GW_meeting_2026.pdf

Workshop on ET Modelling: Session 2

main.pdf

Visit ET-OPT

Meet point: E.161. Marc de Hemptine building. https://maps.app.goo.gl/tkmE7NYucL6SzkvA7

Meet time: 18h10

Conference dinner

Friday 9 October

Miguel Zumalacarregui: Discovering distant black-hole mergers and low-mass dark-matter halos with wave-optics lensing

Like light, gravitational waves (GWs) are gravitationally lensed by massive objects in the Universe. Their long wavelengths, phase coherence, and lack of absorption make GW lensing highly complementary to its electromagnetic counterpart. Beyond magnification and the formation of multiple images, lensed GWs can exhibit genuine wave-optics effects such as diffraction and interference. These imprint frequency-dependent modulations on the signal that encode information about the lens mass and its distribution.

I will describe how wave-optics lensing can be used to discover high-redshift, magnified binary mergers and probe small-scale structure, including dark matter. After reviewing the basic theory, I will focus on two applications: 1) the interpretation of GW231123 as a diffracted and magnified compact binary coalescence, and 2) the potential of LISA to probe dark-matter halos from 10 to 10,000 solar masses. These examples will highlight the need to go beyond the isolated-lens picture, as external gravitational fields or the collective effect of many structures produce qualitatively new diffraction signatures.

Coffee break

News from Belgian and Dutch GW facilities

30 ET-CRISTAL: A full scale prototype of ET-LF seismic isolation

In order to reach the targeted low-frequency sensitivity of the Einstein Telescope, large cryogenic mirrors will have to be mounted on ultralow-frequency seismic isolation stages. In this context, ET-CRISTAL is developing a full-scale prototype suspending a 100 kg silicon test mass cooled to cryogenic temperatures by radiative cooling and isolated at low frequency from seismic noise. The proposed isolation strategy is hybrid, combining a large inverted pendulum mounted on a soft active platform. This pairing of passive and active isolation enables a compact suspension design that achieves seismic isolation below 10 Hz and has been shown to fulfil the ET-LF DARM requirements. In addition, the two stages reinforce one another: the inverted pendulum provides an excellent reference to keep the active platform aligned, and the active platform in turn helps align the inverted pendulum and control its resonances. This talk will present the latest achievements in active seismic isolation on the ET-CRISTAL prototype. First, an overview of the objectives will be presented, focusing on the isolation performance targets and the control-oriented roadmap. It will then address the general strategies adopted for seismic isolation. This includes techniques for decoupling in all six directions, and virtual sensor fusion to boost low-frequency performances while mitigating the drift of inertial sensors. To conclude, the latest performances at the active platform level will be presented and correlated with theoretical predictions.

Speaker: Thomas Giordano (University of Liège)

BDGW_Oct26_GIORDANO.pdf

31 ETpathfinder: prototyping low-frequency technologies for the Einstein Telescope

The European third generation gravitational-wave detector Einstein Telescope will rely on cryogenics and silicon as mirror and suspension material to achieve unprecedented low-frequency sensitivity. Prototyping and characterisation of these new technologies on a system-level is needed to inform the design and future upgrades of the large-scale observatory. This is the mission of ETpathfinder, a 10m prototype facility hosted by Maastricht University, the Netherlands, and involving more than 20 research institutions from 8 different countries.

The inner mechanics of the ETpathfinder vacuum system are currently under construction with two suspended optical tables already commissioned to host the injection system of the 1550nm pre-stabilised laser, the beamsplitter and steering optics. At the moment, we are installing the first cryogenic mirror tower with a cryostat hosting two 3kg-silicon test masses at 15K. In this talk, we give an overview of the current status, recent results from ongoing activities and future plans with a focus on the technologies being tested and deliverables to current and future detectors.

Speaker: Elise Van den Bossche (Vrije Universiteit Brussel)

ETpathfinder@BE-NL_GW26.pdf

32 ET-OPT a Next Generation Prototype for ET-HF

The proposed Einstein Telescope high-frequency interferometers (ET-HF) in their nominal configuration are limited by coating Brownian thermal noise (CBTN) and quantum shot noise. Achieving the target sensitivity requires up to 3 MW of laser power with advanced optical coatings. At these power levels, thermo-elastic and thermo-optic effects induce wavefront distortions, causing transverse mode mismatch, increased optical loss, and degraded squeezing performance. Parametric instabilities (PI) and CBTN impose additional critical limits.

We present ET-OPT, a 10 m coupled-cavity at UCLouvain, in collaboration with ULiege and UNamur, currently being developed to study advanced optical techniques and optical mode control at power densities exceeding that of ET-HF.

In this talk, we will introduce the designs for ET-OPT. This will include the optical mode control strategy, first results analyzing parametric instability suppression, first results on the vacuum envelope and latest results on the generation of the high order optical modes. Our work will facilitate current detectors to reach the high-power levels demanded in O5. Additionally, this will unlock new techniques to facilitate the use of high order optical modes, reducing CBTN impact, and increasing detectable BNS events in ET at redshift ~10 by a factor of three.

Speaker: Aaron Goodwin-Jones (UCLouvain)

2026_10_09-BeNL.pdf

Talks data analysis and theory

33 Rapid Classification of Eccentric Binaries with the Wavelet Scattering Transform

The gravitational-wave detections reported by the LVK collaboration have so far been consistent with quasi-circular compact binary coalescences. Nevertheless, a small fraction of binaries may retain measurable orbital eccentricity in the LVK frequency band. Confident measurement of eccentricity would provide strong evidence for dynamically driven mergers; however, eccentric gravitational-waveform models are computationally expensive, and performing production-level inference on all detected signals is not an efficient use of resources. An intermediate step between detection and analysis, in which the signal is assessed for the potential presence of eccentricity, could provide quick recommendations for which signals should undergo full eccentric inference. We apply the wavelet scattering transform (WST) to a large set of synthetic waveforms in realistic noise and assess its discriminatory power using a shallow neural-network classifier. We find that the WST representation enables effective discrimination between eccentric and quasi-circular binaries and provides a compact multi-scale representation of GW signals. We also examine the ability of our classifier to distinguish eccentricity from spin-induced precession and find robust performance across a range of spin-precession magnitudes.

Speaker: Seppe Staelens (University of Cambridge)

Ecc_WST_BeDutch26.pdf

34 New hardware-accelerated and accurate gravitational waveform approximants for fast parameter estimation

Gravitational wave parameter estimation demands millions of waveform evaluations per event, a bottleneck set to worsen as next-generation detectors like the Einstein Telescope and Cosmic Explorer increase detection rates and signal duration. We present a major expansion of ripple, a JAX-based waveform package offering GPU acceleration and exact autodifferentiation, which now covers higher-order-mode, precessing and tidal waveform families. We validate these implementations against LALSuite via mismatch analysis and against numerical relativity simulations from the SXS and CoRe catalogues, finding strong agreement overall. We also benchmark the speedup gained from GPU-based waveform generation with respect to CPU-based waveforms.

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

GW-JAX BelgianDutch26.pdf

Lunch break

Highlights in Einstein Telescope landscape

35 When can Fisher forecasts be compared? Pre-sampling validity checks for gravitational-wave detector networks

Fisher-matrix forecasts are the standard tool for comparing proposed gravitational-wave detector networks, such as the triangular and double-L designs of the Einstein Telescope. When the likelihood is nearly flat in some direction and only the prior constrains it, the Fisher interval overstates the uncertainty — and because the size of this bias depends on detector geometry, it need not cancel when two networks are compared. I will present the prior-whitened Fisher spectrum: the generalized eigenvalues of the Fisher matrix measured against the prior precision. Each eigenvalue fixes exactly how much the likelihood-only width is inflated relative to the prior-informed one, identifying prior-dominated modes before any posterior sampling. Combined with checks on location, prior–likelihood conflict and likelihood curvature, it tests whether the Gaussian picture behind a Fisher comparison holds. Applied to eight reference networks, the triangular Einstein Telescope is more prior-dominated than the double-L design for 99.5% of 2770 jointly detected binary neutron stars, with a median worst-mode inflation factor of 87 against 7.6 (a comparison of Gaussian-equivalent diagnostics, not of true posterior widths). For binary black holes relocated to a signal-to-noise ratio of 200, at most 7% of sources violate the spectral conditions, yet the curvature condition still fails for 96–99% of sources: a high signal-to-noise ratio alone does not make a Fisher forecast valid. None of these checks requires posterior sampling, so they can screen a forecast before it is trusted.

Speaker: Chun-Fung Wong (KU Leuven)

36 A fish out of water: How the Fisher Information Matrix can bias comparisons between gravitational waves detectors

The Fisher Information Matrix is the most widely used tool to forecast and compare the parameter estimation capabilities of future gravitational waves detectors. This method relies on a linear approximation of the detectors' response which speeds up computation of confidence intervals by millions of times with respect to stochastic sampling methods. However, neglecting higher orders terms, could lead to inaccurate and biased comparisons, especially if the detector networks have very different response morphologies. We estimate the confidence intervals for 130 signals injected in simulated future generation networks, both by Fisher Matrix and by more accurate nested sampling methods. We find that the Fisher Matrix introduces biases by overestimating the errors for networks with shorter baselines and lower SNRs, artificially exaggerating the difference between the performances of the instruments, for several parameters: the fisher matrix is biased against the triangular ET design and overall biased against ET only networks when compared to ET+CE ones. We test the assumption that the Fisher matrix should become a good and unbiased approximation for louder signals, and find that the assumption doesn't fully hold for all but 1 signal in the CoBA dataset.

Speaker: Luca Negri (Utrecht University & Nikhef)

A Fish out of Water - BE-NL meeting.pdf

37 Einstein Telescope: the triangle in a network and the impact of the relative orientation of detectors

The geometrical configuration of the Einstein Telescope (ET) gravitational-wave observatory is one the most important design choices to be made for this future scientific facility. Two baseline configurations are currently being considered, a single triangle or 2 distant L-shaped detectors tilted by about 45◦. In this document we present a study of the scientific performance of the ET baseline configurations in both stand-alone mode and in the presence of either an L-shaped Cosmic Explorer detector with 40-km long arms located in the United States or the existing LIGO-Virgo-KAGRA network. We also include in the study an alternative ET configuration consisting of a triangle plus a distant L-shaped detector. We find that in stand-alone mode this alternative configuration performs better than the other two. The differences in performance between the ET configurations are reduced in the presence of the considered, additional non-ET detectors. We also study the impact on the ET scientific performance of the relative orientation of the future detectors in the network. We find that, while the options that include a triangular ET detector are basically insensitive to this orientation, a balanced compromise for the ET configuration consisting of two L-shaped detectors could be represented by a tilt angle between the two detectors of about 30◦.

Speaker: Antonin Oswald (UCLouvain)

ET_triangle_in_a_network_GW_AntoninOSwald.pdf

38 Updates on Einstein Telescope in the EMR Region

Speaker: Jo van den Brand

39 Updates on EMR site for ET

Speaker: Frederic Nguyen (ULiège)

40 ET computing and data analysis initiatives in EMR

Speaker: Tjonnie Li (KU Leuven)

Concluding words