CoCoNuT Meeting 2026

8 Sept 2026, 09:00 → 11 Sept 2026, 18:00 Europe/London

Building 56 (University of Southampton)

The CoCoNuT Meeting is a series of workshops aimed at fostering collaboration among relativistic astrophysics groups, especially within Europe. The series has been taking place yearly since 2009, and this edition will be hosted at the University of Southampton (United Kingdom). 

This edition will focus on Magnetohydrodynamics, particularly in the context of core-collapse supernovae, neutron star mergers, and magnetars. The different topics will be introduced by the invited speakers, followed by contributed talks, which can cover more general topics as well. 

There will also be a day-0 Workshop on the 8th of September about machine learning applications in numerical relativity, jointly organised with members of the CCP-UKNR community. During registration, please mention if you want to attend only the workshop or the CoCoNuT meeting, or both. 

The workshop and the meeting will be fully in-person. It will take place at the Mathematical Sciences Student Centre (Building 56) of the University of Southampton. Please note that the venue has a maximum capacity of 80 participants; early registration is therefore strongly encouraged.

Confirmed Invited Speakers

•    Anna Neuweiler, Universität Potsdam, Germany
•    Martin Obergaulinger, Universitat de València, Spain
•    Daniel Siegel, Universität Greifswald, Germany

Important dates: 

• Registration opening                                                   13 May,  2026
• Abstract submission deadline                                    17 July,  2026
• Communication of Abstract Acceptances                 20 July, 2026
• Registration deadline                                                  31 August, 2026

 

We gratefully acknowledge the support from the Heilbronn Institute for Mathematical Research, the UKRI Engineering and Physical Sciences Research Council, the University of Southampton and the Southampton Theory Astrophysics and Gravity (STAG) Research Centre. We are also grateful to CoSeC and CCP-UKNR for supporting the day-0 meeting. 

                 

 

            

                                           

Tuesday 8 September

10:00 → 10:55 Registration and coffee

10:55 → 11:00 Welcome

11:00 → 12:30 Talks: Morning session 1

Convener: Miquel Miravet Tenés (University of Southampton)

11:00 Subgrid Modelling for Relativistic Magnetohydrodynamics with Machine Learning

Turbulence is a key driver of dynamics in both isolated and binary neutron star (BNS) systems, and can be triggered by magnetic field instabilities. In particular the onset of the Kelvin-Helmholtz and Magnetorotational Instabilities plays a key role in the evolution of the magnetic field in a post-merger remnant from a BNS system. Modelling the impact of turbulence directly in numerical simulations of a BNS is impossible due to the small length scales involved, but the impact of turbulence on large scale physics can be incorporated through a subgrid model for turbulence. In this talk I will present a new such subgrid model, for Newtonian and Special Relativistic Magnetohydrodynamics, developed using machine learning techniques, and demonstrate its ability to capture the impact of turbulence on large scale magnetised fluid evolutions. This demonstrates the capability to deploy such a model in general relativistic simulations of Neutron Star spacetimes, capturing the impact of turbulence on magnetic field evolution and multimessenger observables.

Speaker: William Cook (FSU Jena)

12:00 Action filtering and relativistic turbulence

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Speaker: Prof. Ian Hawke (University of Southampton)

12:30 → 14:00 Lunch

14:00 → 16:00 Talks: Afternoon session 1

Convener: Katy Clough

14:00 Machine Learning applications outside of numerical relativity

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Speaker: Dr Gregory Ashton (University of Southampton)

15:00 An autoencoder-based surrogate waveform model for quasi-circular binary-black-hole mergers

The generation of accurate waveforms from binary black hole (BBH) mergers is a major effort in Gravitational-Wave Astronomy. In recent years, machine-learning–based surrogate models for BBH waveforms have been proposed. Those offer the potential to dramatically accelerate waveform generation while maintaining accuracy competitive with that of traditional waveform approximants. In this work, we investigate the viability of autoencoders as generative models for gravitational-wave signals from quasi-circular BBH mergers. We introduce AESur3dq8, a novel surrogate waveform model based on autoencoders that enables the rapid and accurate construction of large template banks, producing millions of waveforms in under a second using modest computational resources. The model is trained on the numerical-relativity–informed surrogate NRHybSur3dq8 and subsequently fine-tuned using the SXS catalog of BBH simulations. We demonstrate that waveforms generated by AESur3dq8 achieve mismatches of order $10^{-4}$ with respect to Numerical Relativity waveforms, and that parameter estimation performed with these templates yields results fully consistent with those reported by the LIGO–Virgo–KAGRA Collaboration for observed gravitational-wave events.

Speaker: Mr Anastasios Theodoropoulos (University of Valencia)

15:30 DANSurHM: Modularly incorporating higher modes in a deep learning based gravitational wave surrogate

Numerical relativity (NR) simulations are considered to provide the most faithful representation of the gravitational wave (GW) radiation emitted by binary black hole (BBH) systems. However, in the context of GW astronomy, tasks such as parameter estimation (PE) can require thousands upon thousands of waveform evaluations per second across the entire parameter space. Since performing full NR simulations for each evaluation is not computationally feasible, interpolating methods for existing NR waveforms, known as surrogate models, have been developed with marked success. In this paper, we build on previous work to introduce methods to train a fast surrogate model based on neural networks in order to generate BBH merger waveforms, including the fundamental (2,2) mode, as well as the (3,3), (2,1), (4,4), (3,2), (4,3) and (5,5) higher order modes (HM). Applying a pretraining step on approximant data before fine-tuning on NR data allows us to smooth out the parameter space, and making use of the parallelization ability of GPUs to project the NR waveforms in the $(\theta, \phi)$ sphere during training allows the fitting of all the explored modes simultaneously. The developed surrogate model achieves average mismatches of the order of $10^{-4}$, with the worst mismatch at $2.5\times10^{-3}$, and is able to generate a million waveforms in under 100~ms. Parameter estimation tests show that the addition of higher modes allows for better posteriors when compared to the dominant-mode-only model.

Speaker: Osvaldo Gramaxo Freitas (University of Valencia)

16:00 → 16:30 Coffee break

16:30 → 17:30 Talks: Afternoon session 2

Convener: Eugene Lim

16:30 Physics-Informed Machine Learning for Black Hole Initial Data

Constructing accurate initial data for binary black hole systems is a critical step in numerical relativity simulations, traditionally addressed through spectral and finite-difference methods. In this talk, we will present Pinndorama, a physics-informed neural network framework for solving the puncture initial data problem, validated against the established numerical code, NRPyElliptic. Pinndorama achieves relative errors on the order of 1e-4 with trained networks delivering full binary black hole initial data in 3D. We will also introduce the application of the Deep Ritz Method in this setting. Our results show that neural networks are effective in finding solutions to this problem while offering a new computational paradigm for constructing initial data in numerical relativity.

Speakers: Irene Pitsiladi (University of Nottingham), Martin Zinzen (University of Nottingham)

17:00 PINNs methods for the study of the magnetosphere of compact objects

The magnetospheres of compact objects are known to be at the origin of many important astrophysical phenomena, going from pulsar emission to jet formation around black holes through the Blandford-Znajek process. Thanks to variety of numerical tools, these phenomena are increasingly well understood. However in the era of multi-messenger astrophysics comes the challenge of understanding the nature of new transients such as the merger of binary objects, where the interaction of magnetospheres might play an important role. While these have been the subject of recent studies, their parameter space is very big, and still quite unexplored. To tackle these challenges, new numerical schemes have been proposed. In this paper, we use Physics Informed Neural Networks (PINNs) to evolve the magnetospheres of compact objects within the framework of force-free magnetohydrodynamics. We find that these tools can reproduce the expected behaviors for a range of standard situations.

Speaker: Matteo Stockinger (Max Planck Institute for Gravitational Physics (Albert Einstein insstitute))

Wednesday 9 September

08:30 → 08:50 Registration

08:50 → 09:00 Welcome

09:00 → 10:00 Invited talks: 1

Convener: Nils Andersson

09:00 On the role of rotation and magnetic fields in core-collapse supernovae

Speaker: Martin Obergaulinger (Universitat de València)

10:00 → 10:40 Talks: Morning session 1

Convener: Nils Andersson

10:00 3D MHD core-collapse progenitors: Rotation, magnetic-field amplification, and magnetic topology

The most energetic core-collapse supernovae are thought to arise from rapidly rotating, magnetised progenitors. However, the three-dimensional pre-collapse structure of their angular momentum and magnetic fields remains poorly constrained, limiting the realism of magnetorotational core-collapse simulations.

In this talk, I will present two novel progenitor models for magneto-rotational core-collapse supernovae. We evolved two stellar models through their final minutes pre-collapse in full 3D MHD, including both the core and outer layers, allowing us to characterise the stable 3D magnetic field topology and strength throughout the star. We identify strong amplification of the magnetic field in convective regions, alongside a tendency for small-scale fields to develop there. As a result, regions that are magnetically disconnected in the original one-dimensional stellar-evolution description become magnetically linked in the multidimensional models.

We also identify behaviour in the 3D models that diverges from 1D prescriptions, notably in angular momentum transport and convective flows. With this in mind I will further discuss how our models can be used to feed back into 1D modelling to yield more physically consistent pre-supernova models.

Speaker: Adam Griffiths (Keele University)

10:20 Magnetic fields, dissipation, and multimessenger signatures in three-dimensional core-collapse supernova simulations

Rapid rotation and strong magnetic fields can reshape the dynamics, energetics, and multimessenger emission of core-collapse supernovae. We investigate these effects using three-dimensional magnetohydrodynamic simulations together with hydrodynamic control models.

Rotation alone modifies the protoneutron star structure, neutrino emission, post-shock dynamics, and the development of non-axisymmetric instabilities. Magnetic fields can alter this evolution further by redistributing angular momentum, extracting free energy from differential rotation, and reorganizing the flow into magnetically influenced outflows. These effects are reflected in the gravitational wave signal through rotational dynamics, protoneutron star oscillations, and large-scale asymmetries, while the neutrino signal traces changes in accretion geometry and remnant structure.

Interpreting these physical and multimessenger signatures also requires understanding how magnetic energy is processed within the simulations. Energy budget diagnostics suggest that part of the extracted magnetic energy is neither retained in the resolved magnetic field nor accounted for by the resolved channels of magnetic energy conversion and transport through Lorentz work and Poynting flux, but instead appears in a dissipation-like residual. Determining whether this residual reflects unresolved reconnection, numerical diffusion, or both is important for assessing magnetic field saturation, outflow power, energy conversion efficiency, and the connection between rotational energy loss and explosion energetics. We discuss how this uncertainty may affect the physical interpretation of the overall dynamics and the multimessenger predictions of global magnetorotational supernova simulations.

Speaker: Liubov Kovalenko (Stockholm University)

10:40 → 11:10 Coffee break

11:10 → 12:30 Talks: Morning session 2

Convener: Suprovo Ghosh (University of Southampton)

11:10 Simulations of MHD Core-Collapse Supernova Explosions

Core-collapse supernovae from rapidly rotating and strongly magnetised progenitors are promising candidates for producing the highly energetic hypernova explosions we observe in nature. Current theoretical hydrodynamical supernova simulations have yet to reproduce the same high energies beyond $10^{52}$ erg that we associate with the observed hypernova.
I will present four magnetohydrodynamical (MHD) core-collapse supernova simulations of 8, 13, 17, and 20 M$_{\odot}$ progenitors with initial magnetic fields of $10^{10}$ G. For each progenitor, we perform 2D and 3D simulations and follow the evolution for $<1$ s after core bounce, analysing the explosion, proto-neutron star (PNS), and shock-radius evolution. The explosion energies of all four 3D models asymptote to ranges between $5.51\times10^{50}$ and $1.83\times10^{51}$ erg. These exceed those obtained for non-rotating, non-magnetised progenitors of similar mass (Sykes and Müller, 2024), but remain below the $\sim10^{52}$ erg associated with hypernovae. I will discuss outflows and jets in all four 3D simulations, including nucleosynthesis within the outflows and the relative roles of magnetic and gas pressure. Several models exhibit enhanced magnetic pressure along the PNS polar axis, indicating magnetically driven jets.
Although hypernova-like energies are not achieved, PNS spin evolution shows invert advection of angular momentum increasing with time as hydrodynamic transport outweighs magnetic torques, implying an increasing reservoir of rotational energy (aligning with the proposed magnetar models) that could power more energetic explosions if efficiently extracted. Despite our explosions not exceeding $10^{52}$ erg, the additional reservoir of rotational energy enables our models to achieve explosion energies comparable to those found in simulations of progenitors more than twice as massive (Powell et al., 2023). Three of the four simulations also develop pronounced bipolar distributions of iron-group ejecta, consistent with asymmetric, jet-influenced explosion geometries.

Speaker: Jennifer Quinlan (Monash University)

11:30 The role of magnetic fields in neutrino-driven core-collapse supernovae

Core-collapse supernovae (CCSNe) are some of the brightest, most energetic events in the
universe. In order to model these phenomena accurately, we need to have a diverse range of physics such as neutrino transport and neutrino interactions, general-relativistic gravity, detailed equations of state (EoS) of dense matter, magnetohydrodynamic (MHD) and detailed progenitor models. When modelling these events, magnetic fields are usually only invoked along with rapid rotation to explain rare hypernova events. In this talk, I will present recent efforts to understand the role of magnetic fields in non-rotating and slowly rotating neutrino-driven CCSNe. I will describe our current understanding of how magnetic effects can aid the neutrino-driven mechanism and affect the formation of the compact object. Finally, I will present some open problems that need to be addressed.

Speaker: Vishnu Varma (Keele University)

11:50 Deciphering the nature of proto-neutron star oscillation modes

In this talk, I will present a novel scheme to classify the oscillation modes of a newly born proto-neutron star surrounded by a stalled accretion shock in core-collapse supernovae (CCSNe). Our classification is physically motivated, as it is based on the energy of the restoring forces of the mode. We investigate the nature of the modes by considering the different regions of the CCSN that they stem from. In that way, we find that there are different families of p- and g-modes in the system, living in different areas. Our analysis paves the way to systematic studies for parameter inference of the PNS properties from future gravitational wave signals.

Speaker: Dimitra Tseneklidou (University of Valencia)

12:10 Classification of proto-neutron star oscillation modes

Oscillation modes of the proto-neutron star and stalled accretion shock in core-collapse supernovae are a promising gravitational-wave source, potentially detectable at galactic distances. We present progress on a data-driven scheme to sort computed eigenmodes based on their linear similarity, requiring no prior assumptions about the physical origin of each mode. Building on our previous work, we introduce a method that automatically detects groups of related modes, removing the need to manually seed the classification with a representative mode. We discuss its performance and further refinements across simplified and increasingly realistic core-collapse scenarios. This approach is intended to complement existing classification methods and to support the use of gravitational-wave asteroseismology in constraining the properties of the newly formed compact remnant.

Speaker: Guillem Fernández -Rodríguez (Univesitat de Valencia)

12:30 → 12:35 Group picture

12:35 → 14:30 Lunch

14:30 → 15:30 Talks: Afternoon session 1

Convener: Micaela Oertel (University of Strasbourg)

14:30 Inferring the progenitor properties from the gravitational waves and neutrino signals of a CCSN

During the core-collapse of a massive star, the phase of stalled accretion shock following the birth of a proto-neutron star can be directly observed with gravitational waves and neutrinos. Hydrodynamical instabilities responsible for the asymmetric character of this phase, such as the Standing Accretion Shock Instability, produce characteristic oscillation frequencies that can be identified in the multi-messenger analysis of numerical models of the explosion. A perturbative analysis is used to improve the accuracy of the relation between the time evolution of the oscillation frequencies and the stellar parameters, including turbulence and stellar rotation. The results can be tested on numerical simulations and on the shallow water analogue of SASI in the turbulent regime.

Speaker: Thierry Foglizzo (CEA saclay)

14:50 Impact of the equation of state on the evolution of rapidly rotating core collapse supernovae

Core-collapse supernovae (CCSNe) are among the most energetic phenomena in the Universe and are promising multimessenger sources of gravitational waves (GWs) and neutrinos. The nuclear equation of state (EOS) is a key ingredient in CCSN simulations, as it determines the thermodynamic properties of dense matter, the structure and maximum mass of the proto-neutron star (PNS), and consequently the dynamics of the explosion and its multimessenger signatures.
We present three-dimensional neutrino magnetohydrodynamic simulations of the collapse of a rapidly rotating, weakly magnetised 35 M$_\odot$ progenitor performed with five different finite-temperature nuclear EOSs. Despite the different EOS prescriptions, all models develop two similar phases of non-axisymmetric corotation instabilities within the first 1.25 s after core bounce, suggesting that their occurrence is a robust feature largely independent of the EOS. However, the onset time, dominant azimuthal mode, lifetime, and characteristic multimessenger frequencies differ significantly among the models, reflecting EOS-dependent variations in the evolving PNS structure and rotation profile. The resulting large-scale spiral modes produce quasi-periodic GW emission and modulate the neutrino luminosities. The characteristic GW frequencies associated with both instabilities correlate with the PNS compactness and tidal deformability, with stiffer PNSs producing higher-frequency emission. Finally, the EOS strongly influences the overall morphological evolution of otherwise identical models, highlighting its fundamental role in shaping the dynamics and multimessenger signals of rapidly rotating CCSNe.

Speaker: Marco Cusinato (University of Valencia)

15:10 The Power Gap in Two Dimensions

I will present recent results from 60 axisymmetric core-collapse simulations performed with FLASH. I will describe how the power gap, a narrow band of reduced emission observed in predicted gravitational wave signals, correlates with the properties of the numerical simulations. I will give an overview of proposed explanations and how well these explanations work for our set of simulations.

Speaker: Haakon Andresen (Stockholm University)

15:30 → 16:00 Coffee break

16:00 → 17:20 Talks: Afternoon session 2

Convener: Prof. Alejandro Torres Forne (University of Valencia)

16:00 Multimessenger analysis of Core Collapse Standing Accretion Shock Instability

When a massive star dies, the collapse of its core forms a proto-neutron star. The accreting envelope creates a hydrodynamical shock that stalls after a few hundred milliseconds. The growth of the Standing Accretion Shock Instability (SASI) contributes to the asymmetric character of the explosion, the production of gravitational waves and a modulation of the neutrino signal.
In preparation for the next Galactic supernova, we analyse the multi-messenger signal in numerical models of the explosion to better understand SASI and the explosion mechanism. A numerical analysis is used to characterize to radial location of the region of emission of SASI induced gravitational waves.

Speaker: Alphonse MOREAU

16:20 Taylor-Spruit dynamo in protoneutron stars spun-up by fallback

Turbulence and magnetic field amplification in protoneutron stars are important for supernova explosions, their associated multimessenger signal as well as to determine the properties of the neutron stars in particular their rotation and magnetic field. They can be driven by different processes including convection, magnetorotational instability and the Tayler-Spruit dynamo and are sensitive to small scales unresolved by global models. I will present numerical simulations showing the development of the Tayler-Spruit dynamo and discuss its potential to explain the extremely strong magnetic field of magnetars.

Speaker: Jerome Guilet (CEA Saclay)

16:40 Large Pm small-scale kinematic dynamo in protoneutron stars

Magnetars are young, isolated neutron stars that possess exceptionally high magnetic fields. To explain their formation, one plausible scenario relies on a turbulent convective dynamo that develops inside the protoneutron star. However, the short expected duration of the convection phase imposes a stringent constraint on the efficiency of amplification process. We address this question by combining high-performance direct numerical simulations and statistical small-scale dynamo models to investigate kinematic dynamos. We find that the typical growth rate is of the order of 1/ms in the limit of large magnetic Prandtl number. These results thus suggest that a convective dynamo is efficient enough to generate small-scale magnetic fields of magnetar strength in about 10 seconds, but they do not address the generation of the large-scale dipole field which may occur during the saturation phase.

Speaker: Raphaël Raynaud (Université Paris Cité)

17:00 Impact of the Si/O interface on supernovae explosions

Predicting the outcome of core collapse in massive stars (whether the star explodes as a supernova or collapses into a black hole) remains an open problem. The complex physics involved and the uncertainties in the progenitor structure make it difficult to identify which stars are more likely to explode. Recent studies suggest that the density gradient at the interface between the silicon and oxygen shells may play an important role in determining the explosion outcome. To investigate this effect, we perform simulations of synthetic progenitors in which the density contrast at this interface is systematically varied while all other parameters are kept fixed. The simulations are carried out in two dimensions with the CoCoNuT code, including general relativity and M1 neutrino transport. I will present details of the M1 implementation in CoCoNuT, along with results on core collapse outcome.

Speaker: Nicolas Houry (CEA)

Thursday 10 September

09:00 → 10:00 Invited talks: 2

Convener: Miquel Miravet Tenés (University of Southampton)

09:00 Magnetic Fields in Binary Neutron Star Mergers: Challenges in Numerical Simulations

Speaker: Anna Neuweiler (University of Potsdam)

10:00 → 10:40 Talks: Morning session 1

Convener: Miquel Miravet Tenés (University of Southampton)

10:00 Binary neutron star merger simulations with realistic neutrino interaction rates

Binary neutron star (BNS) systems are currently understood as prominent sources of heavy elements and electromagnetic signatures originating from nucleosynthetic processes. These processes are highly sensitive to the thermodynamical state of the ejected material. To model such complex systems, numerical relativity has emerged as a suitable framework, which provides the means to assess information about the matter outflows. Due to the importance of weak interaction processes for setting the thermal and compositional state of the ejecta, BNS simulations should include neutrinos by, for example, the commonly employed two moments (M1) scheme with integrated (grey approximation) neutrino fields. Interaction rates then determine the feedback between neutrinos and matter, which can be computed with varied degrees of sophistication. In this work, we present BNS merger simulations with M1 neutrino transport, aiming to quantify the role of the neutrinos' interaction rates on the predicted properties of the ejecta, disk and remnant. For that, we adopt three prescriptions for beta reactions: the simplest elastic approximation, the more complete rates provided by the NuLib library, and a full kinematics treatment. Finally, we show that the improved microscopical treatment of the neutrino rates produces significant differences in our simulations, possibly impacting observational signatures from such events.

Speaker: Ramon Jaeger (Universität Potsdam)

10:20 Weak Interaction Rates in Mergers: The Nucleon Width Approximation and Applications to Magnetized Neutron Stars

Weak interaction (Urca) processes govern neutrino emission, chemical equilibration, and bulk viscosity in neutron stars and their mergers. Standard treatments inconsistently combine direct and modified Urca processes, but suffer from divergencies near the direct Urca threshold and are difficult to extend to finite temperature and strong magnetic fields. In this talk, I will present the Nucleon Width Approximation (NWA), a systematically improvable framework that provides a unified description of Urca processes across the direct Urca threshold while avoiding any unphysical divergences. I will then discuss the application of this framework to magnetized nuclear matter, demonstrating how enhanced weak interaction rates modify flavor equilibration, reduce relaxation times, and increase bulk viscosity in magnetic fields relevant to neutron star mergers.

Speaker: Alexander Haber (University of Southampton, UK)

10:40 → 11:10 Coffee break

11:10 → 12:30 Talks: Morning session 2

Convener: Alexander Haber (University of Southampton, UK)

11:10 Eccentric low-mass black hole-neutron star mergers

Recently, there have been significant advances in the development of eccentric gravitational-wave models, so that multiple eccentric models are now available. Real-event reanalysis with these models revealed evidence of nonzero eccentricity in the black hole-neutron star (BHNS) merger GW200105. Such evidence for eccentricity points to a dynamical formation channel in dense environments, rather than isolated evolution.

In this talk, I am going to present recent simulations of low-mass black hole-neutron star (BHNS) mergers with moderate eccentricities (about 0.1) in the late inspiral. I will directly compare these systems with those from our previous study of quasicircular BHNS in the equal- and near-equal-mass regimes and with current waveform models, laying a foundation for model calibration, especially the tidal-phase contributions. Moreover, I will show how strong magnetic fields affect the behavior of trapped g-mode oscillations in accretion disks, the modulation of the accretion rate, and their observational implications.

Speaker: Ivan Markin (University of Potsdam)

Ivan Markin - eccentric low-mass BHNS.pdf

11:30 Impact of different physics on black hole-neutron star mergers

We investigate the impact of different physics on numerical-relativity simulations of black hole-neutron star (BHNS) mergers. In particular, we are interested in how magnetic fields and the subsequent inclusion of neutrino transport shape the gravitational radiation and ejecta of certain BHNS mergers. The simulations for this study are carried out with the GPU-accelerated code AthenaK.

Speaker: Oliver Steppohn (Friedrich-Schiller University Jena)

11:50 The stabilizing role of the crust on the interior magnetic field in isolated neutron stars

Neutron stars exhibits the strongest magnetic field known while also being the most compact horizonless objects in the Universe, they are potential sources of coincident gravitational waves and electromagnetic radiation across the entire spectrum. However, the internal configuration of their magnetic field and the mechanism that stabilize them remain open questions. In this talk, via three-dimensional general relativistic magnetohydrodynamics simulations, we study the role of the magnetic field crust in the magnetic field evolution on isolated neutron stars. We find that, the addition of the crust stabilizes some magnetic field configurations, such as purely poloidal magnetic fields, which where believed to not be a stable configuration for the neutron star interior, while also finding mixed poloidal-toroidal stable configurations.

Speaker: Fabrizio Venturi Piñas (Universitat de València)

12:10 Towards Binary Neutron Star Mergers Using High-Order Central WENO Finite Difference Schemes

We present a study on the application of a family of high-order Central WENO (CWENO) Finite Difference (FD) schemes to simulate Binary Neutron Star (BNS) mergers, where the initial data configurations are generated with the Lorene library following [1].
The evolution is performed in the framework of the first-order hyperbolic Generalized Harmonic (GH) formulation of the Einstein equations of general relativity introduced in [2], coupled to the relativistic Euler equations for the matter sector. The proposed numerical methods have previously demonstrated their efficiency and robustness when applied to several numerical relativity benchmark problems within the same formulation as detailed in [3]. Preliminary results show the successful simulation of several orbits of the inspiral phase, and in particular of the merger itself thanks to the strong shock-capturing properties of the numerical schemes adopted, opening the way to their use in more realistic astrophysical applications. We emphasize, nevertheless, that in our work we assume a simple ideal-gas Equation of State (EoS), and consequently an important next step includes the generalization to more involved models of the matter part, adding a thermal component and potentially including also the contribution of microphysics and radiation transport effects.
To conclude, the extraction of gravitational waves is currently under investigation and will provide a comprehensive validation of the developed numerical methodology.

References
[1] E. Gourgoulhon, P. Grandclement, K. Taniguchi, J.-A. Marck, and S. Bonazzola, “Quasiequilibrium sequences of synchronized and irrotational binary neutron stars in general relativity: Method and tests,” Physical Review D, vol. 63, no. 6, p. 064 029, 2001.
[2] L. Lindblom, M. A. Scheel, L. E. Kidder, R. Owen, and O. Rinne, “A new generalized harmonic evolution system,” Classical and Quantum Gravity, vol. 23, no. 16, S447, 2006.
[3] S. Muzzolon, M. Dumbser, O. Zanotti, and E. Gaburro, “High order numerical discretizations of the Einstein- Euler equations in the Generalized Harmonic formulation,” Journal of Computational Physics, p. 115 084, 2026, issn: 0021-9991.

Acknowledgements: S. Muzzolon and E. Gaburro gratefully acknowledge the support received from the European Union with the ERC Starting Grant ALcHyMiA (No. 101114995).

Speaker: Stefano Muzzolon (University of Trento and Verona)

12:30 → 14:30 Lunch

14:30 → 15:30 Talks: Afternoon session 1

Convener: Jerome Novak (CNRS, Observatoire astronommique de Strasbourg)

14:30 Convexity of equations of state in BNSs merger simulations and gravitational wave

This talk examines the impact of nonconvex equations of state (EoS) in binary neutron (BNS) star merger simulations. We extended the work GR et al. 2024 addressing the key distinction between first-order and crossover phase transitions (PT) in the context of the quasi-universal relation $f_{peak}-\Lambda$. We develop a parametrized PT that allows control of its stiffness and convexity. By analyzing the emitted gravitational-wave, we compare the results from different PTs. Outliers in the quasi-universal relation may signal the presence of general nonconvex behavior in the EoS.

Speaker: Giuseppe Rivieccio (Universitat de Valencia)

14:50 Unlocking neutron-star interiors with tidal resonances

Third-generation gravitational-wave observatories will transform our ability to constrain dense nuclear matter by measuring the tide in coalescing neutron-star binaries with unprecedented precision. To date, analyses have focused on the dominant static contribution known as the tidal deformability $\Lambda$. However, the increasing orbital frequency during the inspiral will also excite stellar oscillation modes, which are sensitive seismological probes of the dense interior and offer exciting discovery potential. In this talk, I will present the first fully Bayesian study into whether the imprint of resonantly excited modes on the gravitational-wave signal is detectable by the Einstein Telescope. By simulating one year of observations and analysing the loudest events, I will show that tidal resonances can be identified, and the Einstein Telescope can measure gravitational-wave phase shifts as small as $\Delta \Phi \approx 0.03$. I will also demonstrate that neglecting resonances can bias the inferred tidal deformabilities. These results suggest that tidal resonances may be a measurable route to gravitational-wave asteroseismology with future detectors.

Speaker: Fabian Gittins (Utrecht University)

15:10 Nuclear parameter inference in the context of high precision detections of Neutron Stars

Neutron star astrophysical observables provide a unique insight into the physics of dense matter. The famous binary neutron star merger GW170817 has particularly provided constraints on the equation of state of dense matter, revealing its softening in the core of neutron stars. Such insight is out of the reach of nuclear theory because of the non-perturbative nature of strong-interaction, and also inexplorable in nuclear physics laboratory experiments. I discuss the inference of not only the equation of state, but also of key nuclear empirical parameters that are of particular interest for nuclear physicists, and show that such inference has its limit, even in the context of extremely precise detections of Neutron Star observables.

Speaker: Lami Shetu Suleiman (Deutches Elektronen Synchrotron)

15:30 → 16:00 Coffee break

16:00 → 17:00 Talks: Afternoon session 2

Convener: William Cook (FSU Jena)

16:00 Magnetic field configurations and jet-launching conditions in binary neutron star mergers

Magnetic fields and the associated turbulent dynamics are central to the evolution of binary neutron star merger remnants. In particular, the amplification of magnetic fields from realistic pre-merger seed strengths to the values thought to be required for jet launching, as well as the emergence of large-scale structures sustaining collimated relativistic outflows, remain not well understood. Addressing these questions is challenging because of both the complexity of relativistic MHD turbulence and the computational cost of fully nonlinear direct numerical relativity simulations.
In this talk, I will discuss recent simulations aimed at clarifying the role of the post-merger magnetic field configuration in setting the conditions for jet launching. I will compare simulations initialised with either a strong large-scale magnetic field or with a post-merger magnetic field calibrated to reproduce the magnetic spectrum obtained in previous large-eddy simulations. I will discuss how these different initial configurations affect the subsequent evolution of the remnant and examine their impact on diagnostics associated with the launching of relativistic jets.

Speaker: Thomas Celora (Institute of Space Sciences)

16:20 Small-scale and mean-field dynamics in accretion disks around rapidly rotating neutron stars

The small-scale dynamics in magnetized accretion disks around rapidly rotating neutron stars is inherently coupled to the large-scale evolution of the disk through the action of a mean-field dynamo and turbulent stresses. The mean-field dynamo leads to the formation of coherent magnetic flux bundles that can migrate from the disk and power a collimated polar outflow as well as magnetic buoyancy-driven disk outflows. The small-scale turbulence, driven by the magnetorotational instability, sustains the mean-field dynamo and leads to disk spreading. The vast separation of scales between the turbulent plasma dynamics and the mean field disk evolution makes it challenging to work with resolutions that are high enough to fully resolve the former and an improved understanding of the coupling between the small- and large-scale dynamics is critical to understand how resolution-based effects can skew the simulated large-scale dynamics of post-merger systems. We present new diagnostics and a detailed analysis of the coupling between the small- and large-scale dynamics in the accretion disk around a rapidly rotating matter remnant from a binary neutron star merger. Our reference system is the post-merger remnant from an equal-mass binary, which we evolve with ideal general-relativistic magnetohydrodynamic, M0 radiation transport for neutrinos and a tabulated finite-temperature equation of state and without the use of any symmetries.

Speaker: Michael Müller (University of Greifswald)

16:40 Numerical simulations of neutron star oscillations with thermal effects

The oscillation modes of neutron stars encode valuable information about their internal structure and dense-matter equation of state. These signatures can be studied through gravitational waves, with next-generation detectors offering new opportunities thanks to their improved sensitivity, which will provide better access to the post-merger phase of binary neutron stars.
In this talk, I will present recent developments for ROXAS, a spectral code designed to simulate the dynamical evolution of perturbed rotating neutron stars using a formalism based on primitive variables, allowing the extraction of oscillation frequencies from the resulting dynamics. In particular, I will discuss recent results obtained for differentially rotating configurations and present ongoing efforts to incorporate thermal effects.

Speaker: Jerome Novak (CNRS, Observatoire astronommique de Strasbourg)

19:00 → 23:00 Conference dinner

Friday 11 September

09:00 → 10:00 Invited talks: 3

Convener: Pablo Cerdá-Durán

09:00 TBD

Speaker: Daniel Siegel (University of Greifswald)

10:00 → 10:40 Talks: Morning session 1

Convener: Pablo Cerdá-Durán

10:00 When Force-Free MHD Breaks Down: Dissipation and Emission in Extreme Magnetospheres

Relativistic plasmas in highly magnetized neutron star (NS) magnetospheres, including binary NS mergers and magnetars, are well described by force-free electrodynamics (FFE) on large scales, yet their observable emission is strongly influenced by kinetic physics. Global 3D FFE simulations of interacting binary NS magnetospheres reveal extended current sheets, Kelvin–Helmholtz–driven turbulence, and reconnection flares that launch compressive waves, potentially powering radio and X-ray precursors. I will connect large-scale magnetospheric dynamics to first-principles particle-in-cell (PIC) simulations, like our recent study of wave conversion at relativistic magnetized shocks. Alfvénic perturbations can transform into propagating superluminal O-modes when their frequency exceeds the downstream plasma frequency, providing a mechanism for radio transient generation. Finally, I discuss new GPU-accelerated PIC frameworks incorporating radiation reaction and QED effects, enabling direct modeling of turbulence, pair creation, and radio-wave propagation in extreme magnetospheres, bridging global MHD and plasma microphysics.

Speaker: Jens Mahlmann (Dartmouth College)

10:20 Building and Breaking Magnetar Fields: A 3D GRMHD Study

Magnetars are among the most extreme magnetized environments in the universe, and modeling their magnetohydrodynamic (MHD) evolution is central to understanding how their fields are generated, structured, and sustained. While these objects are observed through energetic transients such as Soft Gamma Repeaters and Anomalous X-ray Pulsars, the configuration and longevity of their internal magnetic architecture remain poorly constrained. In this talk, I will present results from three-dimensional general relativistic MHD simulations of magnetic field evolution in these systems. Our numerical survey isolates how rotation rate and initial magnetic field strength—individually and in combination—drive magnetic instabilities and shape the resulting field topology. By varying these parameters systematically, we disentangle their respective roles in setting the long-term stability and structure of magnetar fields. We identify two distinct evolutionary regimes governed by the competition between rotation and field strength: rapid rotation enables shear-driven growth of the azimuthal field, whereas strongly magnetized configurations develop poloidal instabilities that drive rapid field dissipation. These results bear on the broader question of magnetic field amplification and saturation in compact objects, connecting to MHD processes also at play in core-collapse supernovae and neutron star mergers.

Speaker: Raj Kishor Joshi (CAMK, Warsaw, Poland)

10:40 → 11:10 Coffee break

11:10 → 12:30 Talks: Morning session 2

Convener: Ian Hawke

11:10 Dark Matter in Neutron Stars: Imprints on Stellar Structure and Gravitational Waves

Compact stars due to their enormous gravitational field can accumulate a sizable amount of dark matter in their interior. Depending on its nature, accumulated dark matter may affect the properties of neutron stars in quite different ways. I will give an overview of the impact of dark matter on various observable properties of neutron stars, i.e. the mass-radius relation, tidal deformability, merger dynamics, gravitational waveform, thermal evolution, etc. For two scenarios, asymmetric fermionic and bosonic dark matter, the conditions at which dark matter particles tend to condense in the core of the star or create an extended halo will be presented. I will show how dark matter condensed in a core tends to decrease the total gravitational mass and tidal deformability compared to a pure baryonic star, which appears as an effective softening of the equation of state. On the other hand, the presence of a dark matter halo has the opposite effect, causing an increase in those observable quantities. Thus, observational data on compact stars could be affected by accumulated dark matter and, consequently, constraints we put on the strongly interacting matter at high densities.

In addition, I will review the effect of dark matter on binary neutron star mergers and emitted gravitational wave signals. I will present the numerical-relativity simulations of compact stars admixed with the dark matter component and discuss how the present and next-generation gravitational wave telescopes could shed light on dark matter-admixed compact stars and constrain the dark matter properties.

Speaker: Dr Violetta Sagun (University of Southampton)

11:30 Constraining Dark Matter in Neutron Stars with Next-Generation Gravitational-Wave Detectors

The accumulation of dark matter inside neutron stars may alter their internal structure and tidal deformability, potentially leaving observable imprints on the gravitational-wave signals emitted during binary neutron star mergers. In this work, we investigate the capability of next-generation gravitational-wave detectors, the Einstein Telescope (ET) and Cosmic Explorer (CE), to probe the effects of dark matter on neutron star properties. Considering fermionic, non-interacting, minimally coupled dark matter, we explore how different detector configurations and network combinations influence the sensitivity to dark matter–induced signatures and assess whether the expected detector sensitivities will be sufficient to constrain dark matter properties.

Speaker: Afonso Ávila (University of Coimbra)

11:50 Modelling viscous effects in neutron stars using the BDNK framework

In this talk, we will discuss our efforts in numerically modelling viscous effects in neutron stars (NSs) using the recently proposed first-order dissipative hydrodynamics framework by Bemfica, Disconzi, Noronha and Kovtun (known as the BDNK framework).  We first demonstrate that under a simplified set up, stable evolutions of spherically symmetric NSs can be achieved in 1+1 spacetime, from which we can compare the predictions of the viscous theory with the perfect fluid model.  We then discuss our progress in generalising our setup to 3+1 spacetimes.

Speaker: Lik Hang Harry Shum (University of Nottingham)

12:10 Magnetic Evolution in Superconducting Neutron Star Cores

Neutrons and protons in neutron-star cores are widely expected to exist in superfluid and superconducting states, respectively. These phases profoundly modify the magnetic evolution through the macroscopic manifestation of the interaction between quantized neutron vortices and proton flux tubes. In the simplest limit, neglecting rotation and entrainment, axisymmetric MHD equilibria have been derived for cores composed of superfluid neutrons, superconducting protons, and electrons ($snspe$-matter). In these equilibria, the charged fluid is in hydrostatic balance under the combined action of pressure, gravity, and magnetic buoyancy and tension forces, while the magnetic field satisfies a Grad–Shafranov-like equation. We have developed a numerical framework based on Dedalus to investigate the long-term evolution of $snspe$-matter in both axisymmetry and three dimensions. In this talk, we will present our first results and discuss their implications for neutron-star magnetic evolution and phenomenology.

Speaker: Nicolas Moraga (Newcastle University)

12:30 → 14:30 Lunch

14:30 → 15:50 Talks: Afternoon session 1

Convener: Isabel Cordero-Carrión (University of Valencia)

14:30 A Simple High-Order Method for Accurate Magnetic Field Evolution

The inclusion of magnetic fields in neutron star merger simulations is essential for understanding a wide range of phenomena, from the efficiency of magnetic field amplification during the merger to the formation of a magnetar and/or jet. Accurate magnetic field evolution is also crucial for modelling the electromagnetic counterparts of these mergers. Current GRMHD codes typically rely either on divergence cleaning techniques or on vector potential formulations to enforce the no-monopole constraint. Although the vector potential approach satisfies this constraint naturally, its implementation in existing codes often requires complicated staggered grids, and the resulting magnetic field accuracy can be limited. In this talk, we introduce our new code, which employs a simple and accurate vertex-centred method for evolving magnetic fields and demonstrates seventh-order accuracy.

Speaker: Rahime Matur (University of Southampton)

14:50 Forming extremally charged black holes with a null coordinates code

In 1973, Bardeen, Carter and Hawking announced the celebrated four laws of black hole thermodynamics, by analogy with the laws of thermodynamics. However, recent works suggest that the third law - stating that an extremal black hole cannot be formed from regular conditions in a finite time - could be incorrect.

We investigate the formation of extremal black holes with our null coordinates code for the evolution of a charged scalar field in a regular centre configuration, similarly to the work of Gelles and Pretorius in a null rectangle configuration. Using a one parameter family of initial data, we find a jump in the black hole charge-to-mass ratio leading, on the upper side of it, to black holes arbitrarily close to extremality. We give numerical evidence that the set of initial parameters collapsing into asymptotically extremal black holes is codimension one.

Furthermore, we study the behaviour of evolutions resulting in near extremal black holes and in particular we observe several of the evolution characteristics scaling with the distance to extremality.

Speaker: Laetitia Martel (University of Southampton)

15:10 The influence of magnetic field geometry and stratification on neutron star magnetic stability

While the external magnetic fields of neutron stars are inferred through electromagnetic observations, the structure and strength of their internal fields remain largely unconstrained. The persistence of strong magnetic fields even in old neutron stars requires that their interior magnetic configuration must remain stable over astrophysical timescales. However, a fully consistent theoretical description of such stable equilibria has yet to be established. It is well known that purely poloidal and purely toroidal magnetic field configurations are unstable on dynamical timescales, while mixed-fields can be secularly stable under certain conditions. In this talk, I will revisit a semi-analytical framework originally developed by former members of Prof. Kokkotas’ group and collaborators. The main benefit of the scheme is that it combines the flexibility of analytical models with the ability to include realistic, microphysical ingredients related to stratification, amongst other things. I will show how, given some hydromagnetic equilibrium, (un)stable partitions can be identified as a function of equation-of-state particulars like the stratification set by composition gradients. I will highlight how these results apply to the magnetar population and their high-energy phenomena.

Speaker: Ms Marta Piscitelli (University of Tübingen)

15:30 Investigating field burial by magnetically confined accretion mounds on neutron stars

Neutron stars are known to have two distinct populations in terms of their surface magnetic fields, viz. low magnetic fields of $10^8–10^{10}$ G such as in millisecond pulsars, and those with higher magnetic fields ($10^{12}$ G), comprising of the bulk of known neutron star population. An accretion induced reprocessing scenario is often invoked to explain the higher spin frequencies of millisecond pulsars. However, the reason for the low magnetic field of millisecond pulsars is still an open problem. In a new work Yeole et al. (2025), we explore the mechanism of surface magnetic field burial inside the neutron star due to accreted matter. The accreted matter accumulated near the magnetic poles forms an accretion mountain in which the screening currents are responsible for magnetic field burial. We calculate the magnetic field geometry of the neutron star above the surface or ocean using Grad Shafranov equation. We have developed a self consistent way of accounting for the reduction in the surface magnetic field using a multipolar current free boundary condition. We have explored several types of accretion mountain configurations in this work, including accretion on pre-existing ocean and also multi-polar surface magnetic fields. Furthermore, I will present new results of 2D and 3D MHD simulations of these accretion mountains performed using a newly implemented SemiRMHD solver in PLUTO, along with a force-free magnetosphere. The simulations allow us to explore the onset of possible MHD instabilities in such mountains, their non-linear growth and cross-field spreading of matter, beyond their initial confinement.

Speaker: Saurabh Yeole (PhD student at IUCAA)

15:50 → 16:00 Closure

16:00 → 16:30 Coffee break