GR-Athena++: General-relativistic magnetohydrodynamics simulations of neutron star spacetimes

TL;DR

This paper introduces GR-Athena++, an advanced simulation code for general-relativistic magnetohydrodynamics in neutron star spacetimes, demonstrating accurate, scalable, and robust modeling of complex astrophysical phenomena including mergers and gravitational waves.

Contribution

The paper presents the extension of Athena++ to include GRMHD with adaptive mesh refinement and coupling to Einstein equations, enabling high-resolution, scalable simulations of neutron star dynamics.

Findings

Stable and convergent results at low resolutions

Accurate capture of magnetic field instabilities

Effective handling of black hole formation and gravitational waves

Abstract

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 $10^5$ CPU cores and excellent weak scaling is shown up to $\sim 5 \times 10^5$ 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.