Dynamical Evolution of an Ultra-relativistic Fireball Colliding with a Freely Expanding Gas

TL;DR

This paper models the hydrodynamical interaction between an ultra-relativistic fireball and expanding gas, revealing shell formation and potential early gamma-ray burst emissions.

Contribution

It provides an analytical and numerical study of the shell evolution and links the shock emissions to gamma-ray burst phenomena.

Findings

Formation of a thin shell during fireball-gas collision

Bright X-ray and gamma-ray emissions from shocked gas

Breakout and photospheric emissions contribute to early gamma-ray bursts

Abstract

We investigate the hydrodynamical evolution of an ultra-relativistic fireball colliding with a freely expanding gas. The hydrodynamical interaction of the fireball and the gas results in the formation of a geometrically thin shell. We study the dynamical evolution of the shell by an analytical way and perform a numerical simulation equipped with an adaptive mesh refinement to investigate the internal structure of the shell. The shocked gas can give rise to bright emission in the X-ray and gamma-ray energy range. We propose that the breakout emission from the forward shock and the photospheric emission from the reverse-shocked fireball contribute to early gamma-ray emission from gamma-ray bursts.