Collapse of magnetized hypermassive neutron stars in general relativity: Disk evolution and outflows

TL;DR

This paper investigates the collapse of magnetized hypermassive neutron stars into black holes, focusing on disk evolution and outflows, which are relevant to understanding short gamma-ray burst engines.

Contribution

It presents the first full general relativity simulations of hypermassive neutron star collapse with detailed disk and outflow analysis, using a novel computational approach.

Findings

Outflows are suppressed with a stiff equation of state.

Outflows are sensitive to initial magnetic field configurations.

The resulting black hole-disk systems are potential short GRB central engines.

Abstract

We study the evolution in axisymmetry of accretion disks formed self-consistently through collapse of magnetized hypermassive neutron stars to black holes. Such stars can arise following the merger of binary neutron stars. They are differentially rotating, dynamically stable, and have rest masses exceeding the mass limit for uniform rotation. However, hypermassive neutron stars are secularly unstable to collapse due to MHD-driven angular momentum transport. The rotating black hole which forms in this process is surrounded by a hot, massive, magnetized torus and a magnetic field collimated along the spin axis. This system is a candidate for the central engine of a short-hard gamma-ray burst (GRB). Our code integrates the coupled Einstein-Maxwell-MHD equations and is used to follow the collapse of magnetized hypermassive neutron star models in full general relativity until the spacetime settles down to a quasi-stationary state. We then employ the Cowling approximation, in which the spacetime is frozen, to track the subsequent evolution of the disk. This approximation allows us to greatly extend the disk evolutions and study the resulting outflows, which may be relevant to the generation of a GRB. We find that outflows are suppressed when a stiff equation of state is assumed for low density disk material and are sensitive to the initial magnetic field configuration.