arXiv subjects

Joan Fontbuté

Publications and source records attributed to Joan Fontbuté.

7 recordsLinked to original sources

arXiv2026

Exact Mass Conservation in Binary Neutron Star Merger Simulations

A long-standing problem in the simulation of neutron star spacetimes is the treatment of vacuum regions outside the stars. The use of an artificial low-density atmosphere is a common robust approach within Eulerian hydrodynamics that, however, introduces baryon-mass violation even with conservative numerical schemes. We propose a simple numerical algorithm that ensures exact mass conservation by means of an appropriate local rescaling of the atmosphere. The scheme is combined with a low-order flux correction and it can be further augmented by a pseudo-vacuum treatment that enforces strict vacuum in the outer regions far from the central objects. We demonstrate the effectiveness of these vacuum treatments with binary neutron star mergers simulations spanning multiple orbits and the postmerger phase, and including a microphysical equation of state. The rescaling algorithm guarantees mass and electron number conservation to round-off precision. The pseudo-vacuum treatment shows slightly larger but approximately constant violations and can improve the computation of fast tail ejecta as well as provide convergent gravitational waves of quality comparable to the standard atmosphere. Overall, results from different atmosphere treatments and a two-code comparison suggest that current computations of gravitational waves and (dynamical) ejecta in the presence of an artifical atmosphere are robust, provided that conservative adaptive mesh refinement with flux correction is employed.

gr-qc↗

arXiv2026

High-order effective-one-body tidal interactions and gravitational scattering

Using state-of-the-art scattering results in post-Minkowskian (PM) gravity, we improve the tidal sector of four different flavors of the effective-one-body (EOB) formalism. We notably explore both adiabatic and post-adiabatic gravitoelectric and gravitomagnetic quadrupolar tidal effects at the next-to-next-to-leading PM-order. When comparing the predictions of the so-constructed Lagrange-PM-tidal version of EOB to recent numerical-relativity data on the scattering of neutron stars, we find improved agreement with respect to existing EOB models and PM expansions. Our work lays the foundation for the development of an accurate tidal sector of the PM EOB models, and points out the need to explore improved resummation schemes in PN EOB for bound and circularized orbits.

gr-qc↗

arXiv2025

Gravitational scattering of two neutron stars

We present the first numerical relativity simulations of the gravitational scattering of two neutron stars. Constraint-satisfying initial data for two equal-mass nonspinning sequences are constructed at fixed energy and various initial angular momenta (impact parameter) and evolved with Einstein equations through the scattering process. The strong-field scattering dynamics are explored up to scattering angles of $220^\circ$ and the threshold of dynamical captures. The transition to bound orbits is aided by significant mass ejecta up to baryon mass ${\sim}0.1M_\odot$. A quantitative comparison with predictions of the scattering angle from state-of-the-art effective-one-body and post-Minkowskian calculations indicates quantitative agreement for large initial angular momenta although significant discrepancies in the tidal contribution emerge toward the capture threshold. Gravitational waveforms and radiated energy are in qualitative agreement with the analogous black hole problem and state-of-the-art effective-one-body predictions. Toward the capture threshold waveforms from scattering dynamics carry a strong imprint of matter effects, including the stars' $f$-mode excitations during the close encounter. Overall, our simulations open a new avenue to study tidal interactions in the relativistic two-body problem.

gr-qc↗

arXiv2025

Perturbative Hyperboloidal Extraction of Gravitational Waves in 3+1 Numerical Relativity

We present a framework to propagate to null infinity gravitational waves computed at timelike worldtubes in the interior of a 3+1 (Cauchy) numerical relativity simulations. In our method, numerical relativity data are used as the inner inflowing boundary of a perturbative time-domain Regge-Wheeler-Zerilli simulation in hyperboloidal coordinates that reaches null infinity. We showcase waveforms from (3+1)D simulations of gravitational collapse of rotating neutron stars, binary black holes mergers and scattering, and binary neutron star mergers and compare them to other extrapolation methods. Our perturbative hyperboloidal extraction provides a simple yet efficient procedure to compute gravitational waves with data quality comparable to the Cauchy characteristic extraction for several practical applications. Nonlinear effects in the wave propagation are not captured by our method, but the present work is a stepping stone towards more sophisticated hyperboloidal schemes for gravitational-wave extraction.

gr-qc↗

arXiv2025

Covariant and Gauge-invariant Metric-based Gravitational-waves Extraction in Numerical Relativity

We revisit the problem of gravitational-wave extraction in numerical relativity with gauge-invariant metric perturbation theory of spherical spacetimes. Our extraction algorithm allows the computation of even-parity (Zerilli-Moncrief) and odd-parity (Regge-Wheeler) multipoles of the strain from a (3+1) metric without the assumption that the spherical background is in Schwarzschild coordinates. The algorithm is validated with a comprehensive suite of 3D problems including fluid ($f$-modes) and spacetime ($w$-modes) perturbations of neutron stars, gravitational collapse of rotating neutron stars, circular binary black holes mergers and black hole dynamical captures and binary neutron star mergers. We find that metric extraction is robust in all the considered scenarios and delivers waveforms of overall quality similar to curvature (Weyl) extraction. Metric extraction is particularly valuable in identifying waveform systematics for problems in which the reconstruction of the strain from the Weyl multipoles is ambiguous. Direct comparison of different choices for the gauge-invariant master functions show very good agreement in the even-parity sector. Instead, in the odd-parity sector, assuming the background in Schwarzschild coordinates can minimize gauge effects related to the use of the $Γ$-driver shift. Moreover, for optimal choices of the extraction radius, a simple extrapolation to null infinity can deliver waveforms compatible to Cauchy-characteristic extrapolated waveforms.

gr-qc↗

arXiv2025

Effective-one-body modeling for generic compact binaries with arbitrary orbits

We present the first unified model for the general relativistic dynamics and gravitational radiation of generic compact binaries. TEOBResumS-Dalí is a model based on the effective-one-body framework incorporating tidal interactions, generic spins, multipolar radiation reaction/waveform and numerical-relativity information. It allows the computation of gravitational waves and other dynamical gauge invariants from generic binaries (black holes, neutron stars, neutron star-black hole binaries) evolving along arbitrary orbits (quasi-circular, eccentric, non-planar) through merger and including scattering. The performances of TEOBResumS-Dalí in the strong-field regime are showcased by comparisons with a large sample of 1395 high-accuracy numerical-relativity simulations available. TEOBResumS-Dalí allows the computation of faithful waveforms for gravitational wave astronomy, providing at the same time an understanding and a prediction of the strong-field dynamics.

gr-qc↗

arXiv2024

A numerical-relativity surrogate model for hyperbolic encounters of black holes: challenges in parameter estimation

We present a surrogate numerical-relativity model for close hyperbolic black-hole encounters with equal masses and spins aligned with the orbital momentum. Our model, generated in terms of the Newman-Penrose scalar $ψ_4$, spans impact parameters $b/M\in [11, 15]$ and spin components $χ_{i} \in [-0.5,0.5]$, modeling the $(\ell,m)=(2,0)$, $(2, \pm 2)$, $(3,\pm 2)$ and $(4,\pm 4)$ emission multipoles. The model is faithful to numerical relativity simulations, yielding mismatches lower than $10^{-3}$. We test the ability of our model to recover the parameters of numerically simulated signals. We find that, despite the high accuracy of the model, parameter inference struggles to correctly capture the parameters of the source even for SNRs as large as 50 due to the strong degeneracies present in the parameter space. This indicates that correctly identifying these systems will require of extremely large signal loudness, only typical of third generation detectors. Nevertheless, we also find that, if one attempts to infer certain combinations of such degenerated parameters, there might be a chance to prove the existence of this type of events, even with the current ground-based detectors, as long as these combinations make sense astrophysically and cosmologically.

gr-qc↗