Cocoon breakout and escape from the ejecta of neutron star mergers

TL;DR

This study uses relativistic hydrodynamic simulations to show that only a small fraction of the cocoon from a neutron star merger jet escapes the ejecta, impacting the observability of associated electromagnetic signals.

Contribution

It provides the first detailed analysis of cocoon breakout dynamics in neutron star merger ejecta, highlighting differences from collapsar scenarios.

Findings

Only 0.5-5% of cocoon mass escapes the ejecta.

Escaped cocoon has an opening angle of 20-30 degrees.

Most of the cocoon remains trapped and hidden by ejecta.

Abstract

The cocoon is an inevitable product of a jet propagating through ambient matter, and takes a fair fraction of the jet energy. In short gamma-ray bursts, the ambient matter is the ejecta from the merger of neutron stars, expanding with a high velocity $\sim 0.2 c$, in contrast to the static stellar envelope in collapsars. Using 2D relativistic hydrodynamic simulations with the ejecta density profile as $\rho \propto r^{-2}$, we find that the expansion makes a big difference; only 0.5--5\% of the cocoon mass escapes from (faster than) the ejecta, with an opening angle $20^{\circ}$--$30^{\circ}$, while it is $\sim 100\%$ and spherical in collapsars. We also analytically obtain the shares of mass and energies for the escaped and trapped cocoons. Because the mass of the escaped cocoon is small and the trapped cocoon is concealed by the ejecta and the escaped cocoon, we conclude that it is unlikely that the cocoon emission was observed as a counterpart to the gravitational wave event GW170817.