arXiv subjects

Marie Cassing

Publications and source records attributed to Marie Cassing.

5 recordsLinked to original sources

arXiv2026

GRACE: An Open-Source Framework for GPU-Accelerated Numerical Relativity

We present GRACE, a new GPU-accelerated numerical-relativity framework designed to run efficiently on heterogeneous high-performance computing platforms. Developed from scratch and built exclusively on open-source libraries, GRACE employs Kokkos for performance portability across CPU and GPU architectures and p4est for adaptive mesh refinement. The code evolves the equations of ideal GRMHD -- with divergence-free magnetic fields maintained by constrained transport -- self-consistently coupled to the Einstein equations in the Z4c formulation, on fixed or adaptively refined grids. We validate the implementation against a suite of standard tests, ranging from magnetized shock tubes and the magnetic rotor in flat spacetime, through (magnetized) Bondi accretion onto a Schwarzschild black hole and the ringdown of a perturbed spinning puncture, to neutron-star oscillation spectra in fixed and dynamical spacetimes and the merger of binary black holes. As more demanding applications, we evolve two binary neutron-star mergers -- an equal-mass, unmagnetized system with an ideal-gas equation of state and an unequal-mass, magnetized system with a finite-temperature tabulated equation of state -- finding the inspiral dynamics to agree well with the FIL code. We also report single-device throughput together with strong- and weak-scaling results on multiple GPU and CPU architectures. GRACE is publicly released together with GRACEpy, a basic post-processing and data-analysis environment.

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arXiv2026

A new code for computing differentially rotating neutron stars

We present new initial data codes for constructing stationary, axisymmetric equilibrium models of differentially rotating neutron stars in full general relativity within the Frankfurt University/KADATH (FUKA) suite of initial data codes. FUKA leverages the KADATH spectral library to solve the Einstein equations under the assumption of an isentropic fluid without magnetic fields while incorporating GRHayLEOS to support 3D tabulated equations of state in \textit{stellar collapse} format. The two solvers explored in this work include one using quasi-isotropic coordinates (QIC) in Spherical coordinates while the other solves the eXtended Conformal Thin Sandwich (XCTS) decomposition in Cartesian coordinates, enabling the construction of equilibrium configurations with high accuracy and efficiency. In this work we adopt the Komatsu-Eriguchi-Hachisu differential rotation law, however, the code is designed to be extensible to other rotation laws, allowing for exploration of physically relevant sequences and critical rotation thresholds. Furthermore, we perform convergence tests demonstrating the exponential accuracy of the spectral approach, we validate QIC and XCTS solutions against models well-studied in the literature, and we also compare FUKA solutions against the well-known RNS code. Finally, we explore the impact that initial data resolution has on dynamical simulations and recover the convergence order of the evolution scheme, the dominate source of error in this study. The new FUKA codes and results presented here lay the foundation for future extensions to more general configurations, including magnetic fields, removal of isentropic assumptions, and binary systems, and have been made publicly available to support community efforts in modeling differentially rotating relativistic stars.

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arXiv2024

Realistic models of general-relativistic differentially rotating stars

General-relativistic equilibria of differentially rotating stars are expected in a number of astrophysical scenarios, from core-collapse supernovae to the remnant of binary neutron-star mergers. The latter, in particular, have been the subject of extensive studies where they were modeled with a variety of laws of differential rotation with varying degree of realism. Starting from accurate and fully general-relativistic simulations of binary neutron-star mergers with various equations of state and mass ratios, we establish the time when the merger remnant has reached a quasi-stationary equilibrium and extract in this way realistic profiles of differential rotation. This allows us to explore how well traditional laws reproduce such differential-rotation properties and to derive new laws of differential rotation that better match the numerical data in the low-density Keplerian regions of the remnant. In this way, we have obtained a novel and somewhat surprising result: the dynamical stability line to quasi-radial oscillations computed from the turning-point criterion can have a slope that is not necessarily negative with respect to the central rest-mass density, as previously found with traditional differential-rotation laws. Indeed, for stellar models reproducing well the properties of the merger remnants, the slope is actually positive, thus reflecting remnants with angular momentum at large distances from the rotation axis, and hence with cores having higher central rest-mass densities and slower rotation rates.

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arXiv2023

Equilibrium non-selfgravitating tori around black holes in parameterised spherically symmetric spacetimes

Non-selfgravitating equilibrium tori orbiting around black holes have a long history and have been employed in numerous simulations of accretion flows onto black holes and other compact objects. We have revisited the problem of constructing such equilibria starting from spherically symmetric black-hole spacetimes expressed in terms of a fully generic and rapidly converging parameterisation: the RZ metric. Within this framework, we have extended the definitions of all of the quantities characterising these equilibria, starting from the concept of the von Zeipel cylinders and up to the possible ranges of the specific angular momenta that are employed to construct families of tori. Within the allowed space of parameters we have then encountered both standard ``single-torus'' solutions, but also non-standard ``double-tori'' solutions. While the properties of the first ones in terms of the presence of a single cusp, of a local pressure maximum and of a varying outer radius, are very similar to those encountered in general relativity, the properties of double-tori solutions are far richer and naturally allow for configurations having the same constant specific angular momentum and hence are potentially easier to produce in nature. The existence of these objects is at present very hypothetical, but these equilibrium tori were to be observed, they would provide very valuable information on the properties of the spacetime and on its deviation from general relativity.

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arXiv2022

Exotic Compact Objects with Two Dark Matter Fluids

The generic properties of compact objects made of two different fluids of dark matter are studied in a scale invariant approach. We investigate compact objects with a core-shell structure, where the two fluids are separated, and with mixed dark matter components, where both dark matter fluids are immersed within each other. The constellations considered are combinations of incompressible fluids, free and interacting Fermi gases, and equations of state with a vacuum term, i.e. self-bound dark matter. We find novel features in the mass-radius relations for combined dark matter compact objects which distinguishes them from compact objects with a single dark matter fluid and compact stars made of ordinary baryonic matter, as white dwarfs, neutron stars and quark stars. The maximum compactness of certain combined dark matter stars can reach values up to the causality limit for compact stars but not beyond that limit if causality of the dark matter fluids is ensured.

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