News

In an article recently published on Monthly Notices of the Royal Astronomical Society, we considered the fate of the neutron star merger GW190425, which is widely believed to have formed a black hole promptly upon merger. Motivated by the potential association with the fast radio burst FRB 20190425A, which took place 2.5 h after the merger, we revisited the question of the outcome of GW190425 by means of numerical relativity simulations. We showed that current laboratory and astrophysical constraints on the equation of state of dense matter do not rule out the formation of a long-lived remnant. However, the formation of a stable remnant would have produced a bright kilonova, in tension with upper limits by ZTF at the location and time of FRB 20190425A. Moreover, the ejecta would have been optically thick to radio emission for days to months, preventing a putative FRB from propagating out. Our results indicate that FRB 20190425A and GW190425 are not associated. However, we could not completely rule out the formation of a long-lived remnant, due to the incomplete coverage of the relevant sky regions. More observations of GW190425-like events, including potential upper limit, have the potential to constrain nuclear physics. To this aim, it is important that follow-up observational campaigns of gravitational wave events are informed by the properties of the source, such as their chirp mass, and we urge the LIGO-Virgo-KAGRA collaboration to promptly release them publicly.

In “Adaptive mesh refinement in binary black holes simulations” we compare three different strategies for mesh refinement: the “box-in-box” approach, the “sphere-in-sphere” approach and a local criterion for refinement based on the estimation of truncation error of the finite difference scheme. We extract and compare gravitational waveforms using the three different mesh refinement methods and compare their accuracy against a calibration waveform and demonstrate that the sphere-in-sphere approach provides the best strategy overall when considering computational cost and the waveform accuracy. Ultimately, we demonstrate the capability of each mesh refinement method in accurately simulating gravitational waves from binary black-hole systems—a crucial aspect for their application in next-generation detectors. We quantify the mismatch achievable with the different strategies by extrapolating the gravitational wave mismatch to higher resolution.

We study the ringdown signal of black holes formed in prompt-collapse binary neutron star mergers. We analyze data from 48 numerical relativity simulations. We show that the (ℓ=2,m=2) and (ℓ=2,m=1) multipoles of the gravitational wave signal are well fitted by decaying damped exponentials, as predicted by black-hole perturbation theory. We show that the ratio of the amplitude in the two modes depends on the progenitor binary mass ratio q and reduced tidal parameter Λ. Unfortunately, the numerical uncertainty in our data is too large to fully quantify this dependency. If confirmed, these results will enable novel tests of general relativity in the presence of matter with next-generation gravitational-wave observatories.

In 2311.04989 we present the extension of GR-Athena++ to general-relativistic magnetohydrodynamics (GRMHD) for applications to neutron star spacetimes. The new solver couples the constrained transport implementation of Athena++ to the Z4c formulation of the Einstein equations to simulate dynamical spacetimes with GRMHD using oct-tree adaptive mesh refinement. We consider benchmark problems for isolated and binary neutron star spacetimes demonstrating stable and convergent results at relatively low resolutions and without grid symmetries imposed. The code correctly captures magnetic field instabilities in non-rotating stars with total relative violation of the divergence-free constraint of 10⁻¹⁶. It handles evolutions with a microphysical equation of state and black hole formation in the gravitational collapse of a rapidly rotating star. For binaries, we demonstrate correctness of the evolution under the gravitational radiation reaction and show convergence of gravitational waveforms. We showcase the use of adaptive mesh refinement to resolve the Kelvin-Helmholtz instability at the collisional interface in a merger of magnetised binary neutron stars. GR-Athena++ shows strong scaling efficiencies above 80% in excess of 100,000 CPU cores and excellent weak scaling is shown up to ∼5×10⁵ CPU cores in a realistic production setup. GR-Athena++ allows for the robust simulation of GRMHD flows in strong and dynamical gravity with exascale computers.

In 2311.00031 we study out-of-thermodynamic equilibrium effects in neutron star mergers with 3D general-relativistic neutrino-radiation large-eddy simulations. During merger, the cores of the neutron stars remain cold (a few MeV) and out of thermodynamic equilibrium with trapped neutrinos originating from the hot collisional interface between the stars. However, within ∼2−3 milliseconds matter and neutrinos reach equilibrium everywhere in the remnant. Our results show that dissipative effects, such as bulk viscosity, if present, are only active for a short window of time after the merger.