A Narrow Short-Duration GRB Jet from a Wide Central Engine

TL;DR

This study demonstrates that neutron star mergers can produce highly collimated short gamma-ray burst jets through hydrodynamic collimation by surrounding ejecta, without requiring a highly directed engine or magnetic flux.

Contribution

It reveals that dense ejecta from neutron star mergers can naturally collimate relativistic jets, expanding the understanding of jet formation in short GRBs.

Findings

Jets with ~10° opening angles are produced.

A dense surrounding medium enhances collimation.

Differences in outflow angles between merger types are predicted.

Abstract

We use two-dimensional relativistic hydrodynamic numerical calculations to show that highly collimated relativistic jets can be produced in neutron star merger models of short-duration gamma ray bursts without the need for a highly directed engine or a large net magnetic flux. Even a hydrodynamic engine generating a very wide sustained outflow on small scales can in principle produce a highly collimated relativistic jet, facilitated by a dense surrounding medium which provides a cocoon surrounding the jet core. An oblate geometry to the surrounding gas significantly enhances the collimation process. Previous numerical simulations have shown that the merger of two neutron stars produces an oblate, expanding cloud of dynamical ejecta. We show that this gas can efficiently collimate the central engine power much as the surrounding star does in long-duration GRB models. For typical short-duration GRB central engine parameters, we find jets with opening angles of order 10 degrees in which a large fraction of the total outflow power of the central engine resides in highly relativistic material. These results predict large differences in the opening angles of outflows from binary neutron star mergers versus neutron star-black hole mergers.