Fermi Gamma-ray Space Telescope

Breakout Emission from an Ultra-relativisic Fireball Colliding with a Freely Expanding Gas

A. Suzuki
T. Shigeyama

Abstract:

The mechanism to produce highly energetic photons from gamma-ray bursts (GRBs) is one of the long-standing problems in astrophysics since its discovery. The internal shock model, in which shock waves produced in multiple shells colliding with each other are responsible for the production of gamma-ray photons, has been a leading model for the prompt emission of GRBs for the last few decades. However, the recent discovery of the photospheric component in the prompt emission of some GRBs have changed our view on the mechanism of the prompt emission. The hydrodynamical interaction between the head of an ultra-relativistic jet and the surrounding material is considered to play crucial roles in the dissipation of the kinetic energy of the jet and the production of gamma-ray photons. We investigate the hydrodynamical evolution of an ultra-relativistic fireball colliding with a massive freely expanding ejecta in spherical symmetry. As a result of the collision of the fireball with the ejecta, a forward shock powered by the fireball propagates in the ejecta and the swept gas forms a geometrically thin shell. We study the dynamical evolution of the shell analytically and numerically to clarify the internal structure of the shell. The breakout emission from the forward shock could contribute to early gamma-ray emission from GRBs.