Computational Relativity

GR-Athena++: Binary Neutron Star Merger Simulations with Neutrino Transport

2026-02-23T00:00:00+00:00

In 2602.18290, we present general-relativistic radiation magnetohydrodynamics simulations of binary neutron star mergers performed with GR-Athena++. Neutrino transport is treated using a moment-based, energy-integrated scheme (M1), augmented by neutrino number density evolution (N0). Our implementation is validated through an extensive suite of standard tests and demonstrated to perform robustly under adaptive mesh refinement. As a first application, we simulate the gravitational collapse of a uniformly rotating, magnetized neutron star, demonstrating stable radiation evolution through apparent-horizon formation using a novel excision technique based on the tapering of state vector evolution inside the horizon. To further test robustness in highly dynamic environments, we apply our code to two demanding binary neutron star merger scenarios. We investigate a long-lived remnant with the DD2 equation of state, evolved with full general-relativistic magnetohydrodynamics and M1 neutrino transport. Following this, a gravitational collapse scenario with the SFHo equation of state is explored. We showcase long-term stable evolution on neutrino cooling time-scales, demonstrating robust handling of excision and stable evolution of the post-collapse accretion phase in three-dimensional mergers with magnetic fields and neutrino radiation.

Magnetic Field Configurations in Binary Neutron Star Mergers II: Inspiral, Merger and Ejecta

2025-09-27T00:00:00+00:00

In 2508.19342 we perform a series of simulations of magnetised Binary Neutron Star mergers, with varying magnetic field topologies in the initial data, as well as varying Equations of State, and mass ratios. In this paper, a companion paper to 2506.18995, we analyse the impact of the initial field configuration on the gravitational wave signal, the amplification of the magnetic field, and the ejected material. We investigate the dependence of the phase evolution of the gravitational wave in the post-merger on the initial magnetic field, finding that dephasing between the (l=2,m=2)(l=2,m=2) mode of the gravitational wave, and the (2,1)(2,1) and (3,3)(3,3) modes may be strongly impacted by the numerical reconstruction scheme. The magnetic field amplification during the Kelvin-Helmholtz dominated phase may be considerably enhanced by anti-aligned fields, or suppressed by toroidal fields. The post-merger amplification of the field due to winding may be suppressed by toroidal fields, and enhanced by asymmetries or mixtures of poloidal and toroidal fields. The field strength in the ejecta may be impacted by the initial magnetic field, with configurations which lead to large amplifications and those with mixtures of poloidal and toroidal fields preferentially emitting highly magnetised material in the polar regions, showing a weaker dependence of the magnetic field on the density of the ejecta than in cases that amplify the magnetic field less. We find that the magnetic field is largely randomly oriented in the ejected material, supporting such models used to estimate thermalisation timescales of ejected material. We find that configurations which begin with an initial bitant symmetry break this symmetry uniformly, independent of the initial configuration, when evolved without an enforced symmetry. This behaviour suggests the presence of a spontaneous symmetry breaking bifurcation in the solution.

Magnetic Field Configurations in Binary Neutron Star Mergers I: Post-merger Remnant and Disk

2025-09-26T00:00:00+00:00

In 2506.17885, we present the first of a set of two companion papers where we analyze a set of general relativistic magnetohydrodynamic (GRMHD) simulations of binary neutron star (BNS) mergers performed with the code GR-Athena++. Our study explores how variations in the pre-merger magnetic field configuration, the nuclear equation of state, and the binary mass ratio influence the magnetic and thermodynamic evolution of the post-merger remnant and its surrounding disk. In addition, we assess the consequences of imposing the commonly adopted equatorial reflection (bitant) symmetry. Magnetic field amplification begins promptly after the stars merge, driven by the Kelvin-Helmholtz instability. At later times, the field continues to evolve under the combined action of differential winding and turbulence, ultimately settling into a predominantly toroidal configuration. Although the field is strongly reprocessed through the merger, the initial magnetic field topology still leaves a measurable imprint on the post-merger configuration and can affect the overall amplification achieved, particularly for the magnetic field strengths typically assumed in the literature and at the finite resolutions feasible for GRMHD simulations. Moreover, we find that enforcing equatorial reflection symmetry partially suppresses turbulence in the vicinity of the midplane, altering the subsequent magnetic field evolution by enhancing its rate of amplification.

Covariant and Gauge-invariant metric-based gravitational-waves extraction in numerical relativity

2025-08-05T00:00:00+00:00

In 2508.03799 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.

Black-hole - neutron-star mergers: new numerical-relativity simulations and multipolar effective-one-body model with spin precession and eccentricity

2025-06-30T00:00:00+00:00

In 2507.00113 we present 52 new numerical-relativity (NR) simulations of black-hole-neutron-star merger (BHNS) mergers and employ the data to inform TEOBResumS-Dalí: a multipolar effective-one-body model also including precession and eccentricity. Our simulations target quasicircular mergers and the parameter space region characterized by significant tidal disruption of the star. Convergent gravitational waveforms are produced with a detailed error budget after extensive numerical tests. We study in detail the multipolar amplitude hierarchy and identify a characteristic tidal signature in the (ℓ,m)=(2,0), and (3,0) modes. We also develop new NR-informed models for the remnant black hole and for the recoil velocity. The numerical data is then used to inform next-to-quasicircular corrections and the ringdown of TEOBResumS-Dalí for BHNS. We show an overall order of magnitude improvement in the waveform’s amplitude at merger and more consistent multipoles over our older TEOBResumS-GIOTTO for BHNS. TEOBResumS-Dalí is further validated with a new 12 orbit precessing simulation, showing phase and relative amplitude differences below ∼0.5 (rad) throughout the inspiral. The computed mismatches including all the modes lie at the one percent level for low inclinations. Finally, we demonstrate for the first time that TEOBResumS-Dalí can produce robust waveforms with both eccentricity and precession, and use the model to identify the most urgent BHNS to simulate for waveform development. Our new numerical data are publicly released as part of the CoRe database.

Gravitational scattering of two neutron stars

2025-06-12T00:00:00+00:00

In 2506.11204 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 [deg] and the threshold of dynamical captures. The transition to bound orbits is aided by significant mass ejecta up to baryon mass ~0.1Mo. 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.

Turbulence in Magnetised Neutron Stars

2025-06-04T00:00:00+00:00

The magnetic field configuration in the interior of neutron stars and its stability are open problems and may be impacted by the influence of a turbulent cascade within the star. Assessing the impact of turbulent flow with numerical simulations requires incredibly high resolution as well as long lived simulations covering multiple Alfven times. In 2506.08037 we present a series of simulations of magnetised neutron stars with resolution up to 29m and lasting at their longest 1.2s to assess this issue, the longest lasting and highest resolution such simulations to date. At the highest resolution we find evidence for a turbulent cascade absent in an unmagnetised star which cannot be captured with lower resolution simulations, consistent with Kolmogorov power law scaling. The presence of turbulence triggers an inverse cascade of helicity, while at late times the net helicity appears to vanish, suggesting that a twisted-torus is not formed in the magnetic field. We find that the presence of the magnetic field excites a characteristic quadrupolar oscillation of the density profile at 145 Hz, consistent with Alfvenic modes proposed as the source of quasi-periodic oscillations observed in magnetars.

56Ni production in long-lived binary neutron star merger remnants

2025-03-25T00:00:00+00:00

In 2503.17445 we investigate the nucleosynthesis and kilonova emission based on numerical-relativity binary neutron star merger simulations that incorporate a two-moment neutrino-transport scheme. Unlike in previous works with simpler neutrino treatments, a massive, fast (up to v=0.3c), proton-rich neutrino-driven wind develops in the post-merger phase of the simulations as long as the merger remnant does not collapse to a black hole. We evolve the ejecta for 100 days after the merger using 2D ray-by-ray radiation-hydrodynamics simulations coupled in-situ to a complete nuclear network. The most abundant nucleosynthesis products are He, 56Ni, and 56Co. We find a total yield of ∼1e−3Mo of 56Ni for all mergers that produce massive neutron star remnants, independently of the mass ratio and equation of state. After a few days, the decay of 56Ni and later 56Co becomes the primary source of heating in the matter expanding above the remnant. As a result, the kilonova light curve flattens on timescales of days for polar observation angles. γ rays emitted by the decay of 56Ni and 56Co peak around 700 – 800 keV and will be detectable by future instruments like GammaTPC for an event up to a distance of 40 Mpc. The observation of these effects could serve as smoking gun for the presence of a long-lived neutron star remnant in future kilonova observations.

Cocoon shock breakout emission from binary neutron star mergers

2025-03-11T00:00:00+00:00

In 2408.15973, we investigated the shock breakout emission produced by the jet/cocoon system launched in a binary neutron star (BNS) mergers. After a coalescence, an engine remnant which may be a black hole or a fast-rotating neutron star is formed. Such an engine may launch a collimated jet along the polar axis. For this jet to efficiently produce observable radiation, it must first propagate through the surrounding dense ejecta produced in the merger. The earliest escaping photons from this interaction correspond to the so-called shock breakout emission. We employed NR simulations spanning multiple binary configurations and equations of state to obtain realistic ejecta density and velocity profiles. Building on these profiles, we developed a semi-analytic model to track jet and cocoon propagation and to compute the resulting shock breakout signal across a range of central engine parameters. Our results show that the shock breakout emission is highly sensitive to the properties of the outermost ejecta layers. In particular, ejecta distributions featuring an extended, fast-moving tail yield predictions consistent with the low-luminosity short gamma-ray burst observed in coincidence with GW170817.

Binary Black Hole Waveforms from High-Resolution GR-Athena++ Simulations

2024-11-18T00:00:00+00:00

The detection and subsequent inference of binary black hole signals rely heavily on the accuracy of the waveform model employed. In the highly non-linear, dynamic, and strong-field regime near merger, these waveforms can only be accurately modeled through numerical relativity simulations. Considering the precision requirements of next-generation gravitational wave observatories, we present in this paper high-resolution simulations of four non-spinning quasi-circular binary black hole systems with mass ratios of 1, 2, 3, and 4, conducted using the GR-Athena++ code. We extract waveforms from these simulations using both finite radius and Cauchy characteristic extraction (CCE) methods. Additionally, we provide a comprehensive error analysis to evaluate the accuracy and convergence of the waveforms. Our self-mismatch study shows that the (2, 2) mode of the CCE strains, for the world tube extraction radius of R=50, reaches the level of ~1e-12 mismatch for mass ratios of 1, 2, 3, and ~1e-11 mismatch for the mass ratio of 4. However, when larger extraction radii are considered or when more modes are included the mismatches increase. These results highlight both the promise and limitations of current simulations in achieving the precision required for upcoming detectors such as LISA, Cosmic Explorer, and Einstein Telescope.

The waveforms are publicly available on ScholarSphere, and represent the first set of waveforms of the new GR-Athena++ catalog.