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

TL;DR

This paper introduces GR-Athena++, a new simulation code for binary neutron star mergers that accurately models neutrino transport and magnetic fields, enabling stable long-term evolution of complex merger scenarios.

Contribution

The paper presents a novel implementation of neutrino transport with a moment-based scheme in a general-relativistic MHD code, validated through extensive tests and applied to realistic merger scenarios.

Findings

Stable long-term evolution of neutron star mergers with neutrino transport.

Successful simulation of gravitational collapse with horizon excision.

Robust handling of magnetic fields and neutrino radiation in dynamic environments.

Abstract

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.