Hydrodynamical interaction of mildly relativistic ejecta with an ambient medium

TL;DR

This paper models the hydrodynamical interaction between mildly relativistic ejecta and the surrounding medium, exploring the resulting shell formation, emission properties, and potential link to underluminous gamma-ray bursts.

Contribution

It introduces a semi-analytical model of ejecta-medium interaction and compares it with numerical simulations, providing insights into emission mechanisms and gamma-ray burst origins.

Findings

Formation of a thin shell due to shock interaction

Development of an emission model for optically thick shells

Potential explanation for underluminous gamma-ray bursts

Abstract

Hydrodynamical interaction of spherical ejecta freely expanding at mildly relativistic speeds into an ambient cold medium is studied in semi-analytical and numerical ways to investigate how ejecta produced in energetic stellar explosions dissipate their kinetic energy through the interaction with the surrounding medium. We especially focus on the case in which the circumstellar medium is well represented by a steady wind at a constant mass-loss rate having been ejected from the stellar surface prior to the explosion. As a result of the hydrodynamical interaction, the ejecta and circumstellar medium are swept by the reverse and forward shocks, leading to the formation of a geometrically thin shell. We present a semi-analytical model describing the dynamical evolution of the shell and compare the results with numerical simulations. The shell can give rise to bright emission as it gradually becomes transparent to photons. while it is optically thick. We develop an emission model for the expected emission from the optically thick shell, in which photons in the shell gradually diffuse out to the interstellar space. Then, we investigate the possibility that radiation powered by the hydrodynamical interaction is the origin of an underluminous class of gamma-ray bursts.