Magnetized hypermassive neutron star collapse: a central engine for short gamma-ray bursts

Abstract

A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron star binary. In the latest magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes `delayed' collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasi-steady accretion rate ~10 solar mass/s; the lifetime of the torus is ~10 ms. The torus has a temperature \sim 10^{12} K, leading to copious neutrino-antineutrino thermal radiation. Therefore, the collapse of an HMNS is a promising scenario for generating short-duration gamma-ray bursts and an accompanying burst of gravitational waves and neutrinos.