Self-consistent picture of the mass ejection from a one second-long binary neutron star merger leaving a short-lived remnant in general-relativistic neutrino-radiation magnetohydrodynamic simulation

TL;DR

This study presents a detailed general-relativistic simulation of a binary neutron star merger, revealing the dynamics of mass ejection and the formation of a black hole with a massive torus, highlighting the roles of magnetorotational instability and turbulence.

Contribution

First detailed simulation demonstrating post-merger mass ejection mechanisms and ejecta properties in a neutron star merger with a short-lived remnant.

Findings

Confirmation of dynamical mass ejection with fast and mildly relativistic components.

Identification of post-merger mass ejection from the torus driven by magnetorotational instability.

Distinct electron fraction and velocity distributions in different ejecta components.

Abstract

We perform a general-relativistic neutrino-radiation magnetohydrodynamic simulation of a one second-long binary neutron star merger on Japanese supercomputer Fugaku using about $72$ million CPU hours with $20,736$ CPUs. We consider an asymmetric binary neutron star merger with masses of $1.2$ and $1.5M_\odot$ and a `soft' equation of state SFHo. It results in a short-lived remnant with the lifetime of $\approx 0.017$\,s, and subsequent massive torus formation with the mass of $\approx 0.05M_\odot$ after the remnant collapses to a black hole. For the first time, we confirm that after the dynamical mass ejection, which drives the fast tail and mildly relativistic components, the post-merger mass ejection from the massive torus takes place due to the magnetorotational instability-driven turbulent viscosity and the two ejecta components are seen in the distributions of the electron fraction and velocity with distinct features.