Magnetorotational instability in relativistic hypermassive neutron stars

TL;DR

This paper demonstrates the occurrence of magnetorotational instability in hypermassive neutron stars through 3D general-relativistic magnetohydrodynamic simulations, showing its role in magnetic field amplification and influencing black hole formation.

Contribution

First direct simulation evidence of MRI in hypermassive neutron stars, including analysis of growth rates and structures, linking MRI to merger outcomes and gamma-ray burst mechanisms.

Findings

MRI causes rapidly-growing, periodic structures in HMNS interiors.

Growth time and wavelength match analytical predictions.

MRI accelerates collapse of HMNS into black holes.

Abstract

A differentially rotating hypermassive neutron star (HMNS) is a metastable object which can be formed in the merger of neutron-star binaries. The eventual collapse of the HMNS into a black hole is a key element in generating the physical conditions expected to accompany the launch of a short gamma-ray burst. We investigate the influence of magnetic fields on HMNSs by performing three-dimensional simulations in general-relativistic magnetohydrodynamics. In particular, we provide direct evidence for the occurrence of the magnetorotational instability (MRI) in HMNS interiors. For the first time in simulations of these systems, rapidly-growing and spatially-periodic structures are observed to form with features like those of the channel flows produced by the MRI in other systems. Moreover, the growth time and wavelength of the fastest-growing mode are extracted and compared successfully with analytical predictions. The MRI emerges as an important mechanism to amplify magnetic fields over the lifetime of the HMNS, whose collapse to a black hole is accelerated. The evidence provided here that the MRI can actually develop in HMNSs could have a profound impact on the outcome of the merger of neutron-star binaries and on its connection to short gamma-ray bursts.