The gamma-rays that accompanied GW170817 and the observational signature of a magnetic jet breaking out of NS merger ejecta

TL;DR

This paper presents the first relativistic MHD simulation of a magnetic jet from a neutron star merger, showing how a cocoon breakout can produce gamma-ray emission similar to GRB170817A, suggesting a new mechanism for such short GRBs.

Contribution

It introduces a novel relativistic MHD simulation of a jet from neutron star merger ejecta and links cocoon breakout to observed gamma-ray signals.

Findings

Cocoon shock breakout can produce gamma-ray emission matching GRB170817A.

Low-mass, high-velocity ejecta tail influences gamma-ray production.

Such events are expected to be common in future gravitational wave detections.

Abstract

We present the first relativistic MHD numerical simulation of a magnetic jet that propagates through and emerges from the dynamical ejecta of a binary neutron star merger. Generated by the magnetized rotation of the merger remnant, the jet propagates through the ejecta and produces an energetic cocoon that expands at mildly relativistic velocities and breaks out of the ejecta. We show that if the ejecta has a low-mass ($\sim10^{-7} M_\odot$) high-velocity ($v\sim0.85$ c) tail, the cocoon shock breakout will generate $\gamma$-ray emission that is comparable to the observed short GRB170817A that accompanied the recent gravitational wave event GW170817. Thus, we propose that this GRB, which is quite different from all other short GRBs observed before, was produced by a different mechanism. We expect, however, that such events are numerous and many will be detected in coming LIGO-Virgo runs.