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

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.Comment: 15 pages, 9 figures, accepted for publication in Ap