Collapse and black hole formation in magnetized, differentially rotating neutron stars

TL;DR

This paper develops a numerical relativity code to simulate magnetized, differentially rotating neutron stars, revealing how magnetic effects induce collapse into black holes or stable configurations, with implications for gamma-ray burst origins.

Contribution

It introduces a coupled Einstein-Maxwell-MHD evolution code and applies it to study the collapse of magnetized neutron stars, highlighting magnetic braking and instabilities as key processes.

Findings

Magnetic braking and MRI cause hypermassive neutron stars to collapse into black holes.

Non-hypermassive models can form stable, rotating stars with surrounding tori.

Simulations suggest a link between neutron star collapse and gamma-ray burst mechanisms.

Abstract

The capacity to model magnetohydrodynamical (MHD) flows in dynamical, strongly curved spacetimes significantly extends the reach of numerical relativity in addressing many problems at the forefront of theoretical astrophysics. We have developed and tested an evolution code for the coupled Einstein-Maxwell-MHD equations which combines a BSSN solver with a high resolution shock capturing scheme. As one application, we evolve magnetized, differentially rotating neutron stars under the influence of a small seed magnetic field. Of particular significance is the behavior found for hypermassive neutron stars (HMNSs), which have rest masses greater the mass limit allowed by uniform rotation for a given equation of state. The remnant of a binary neutron star merger is likely to be a HMNS. We find that magnetic braking and the magnetorotational instability lead to the collapse of HMNSs and the formation of rotating black holes surrounded by massive, hot accretion tori and collimated magnetic field lines. Such tori radiate strongly in neutrinos, and the resulting neutrino-antineutrino annihilation (possibly in concert with energy extraction by MHD effects) could provide enough energy to power short-hard gamma-ray bursts. To explore the range of outcomes, we also evolve differentially rotating neutron stars with lower masses and angular momenta than the HMNS models. Instead of collapsing, the non-hypermassive models form nearly uniformly rotating central objects which, in cases with significant angular momentum, are surrounded by massive tori.