High resolution numerical-relativity simulations for the merger of binary magnetized neutron stars

TL;DR

This paper presents the highest-resolution magnetohydrodynamics simulations of binary neutron star mergers, revealing key magnetic field amplification mechanisms and the formation of a strongly magnetized black hole-torus system.

Contribution

It provides the first detailed resolution study of magnetic field amplification mechanisms during neutron star mergers using high-resolution simulations.

Findings

Kelvin-Helmholtz instability significantly amplifies magnetic fields.

Magnetorotational instability further amplifies magnetic fields in the hypermassive neutron star.

The merger results in a strongly magnetized black hole surrounded by an accretion torus.

Abstract

We perform high-resolution magnetohydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese supercomputer K. The neutron stars and merger remnants are covered by a grid spacing of 70\,m, which yields the highest-resolution results among those derived so far. By an in-depth resolution study, we clarify several amplification mechanisms of magnetic fields during the binary neutron star merger for the first time. First, the Kelvin-Helmholtz instability developed in the shear layer at the onset of the merger significantly amplifies the magnetic fields. A hypermassive neutron star (HMNS) formed after the merger is then subject to the nonaxisymmetric magnetorotational instability, which amplifies the magnetic field in the HMNS. These two amplification mechanisms cannot be found with insufficient-resolution runs. We also show that the HMNS eventually collapses to a black hole surrounded by an accretion torus which is strongly magnetized at birth.