Cocoon breakout and escape from the ejecta of neutron star mergers

Hamid Hamidani, Kunihito Ioka

Published 2022-10-03Version 1

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 $r^{-2}$ density profile, 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. Considering the small mass range of the escaped cocoon, and because 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.