Binary neutron star mergers: a jet engine for short gamma-ray bursts

TL;DR

This paper uses full general relativistic magnetohydrodynamic simulations to demonstrate that binary neutron star mergers can produce collimated, mildly relativistic jets potentially explaining short gamma-ray bursts, regardless of initial magnetic field confinement.

Contribution

It shows that even with interior-only magnetic fields, neutron star mergers can launch jets, expanding understanding of gamma-ray burst progenitors.

Findings

Jets are launched approximately 60 ms after merger.

Jets are collimated and mildly relativistic.

Magnetic fields confined to interiors can still produce jets.

Abstract

We perform magnetohydrodynamic simulations in full general relativity (GRMHD) of quasi-circular, equal-mass, binary neutron stars that undergo merger. The initial stars are irrotational, $n=1$ polytropes and are magnetized. We explore two types of magnetic-field geometries: one where each star is endowed with a dipole magnetic field extending from the interior into the exterior, as in a pulsar, and the other where the dipole field is initially confined to the interior. In both cases the adopted magnetic fields are initially dynamically unimportant. The merger outcome is a hypermassive neutron star that undergoes delayed collapse to a black hole (spin parameter $a/M_{\rm BH} \sim 0.74$) immersed in a magnetized accretion disk. About $4000M \sim 60(M_{\rm NS}/1.625M_\odot)$ ms following merger, the region above the black hole poles becomes strongly magnetized, and a collimated, mildly relativistic outflow --- an incipient jet --- is launched. The lifetime of the accretion disk, which likely equals the lifetime of the jet, is $\Delta t \sim 0.1 (M_{\rm NS}/1.625M_\odot)$ s. In contrast to black hole--neutron star mergers, we find that incipient jets are launched even when the initial magnetic field is confined to the interior of the stars.