External Shock Model for Gamma-Ray Bursts during the Prompt Phase

Abstract

The hard X-ray and gamma-ray phenomenology of gamma-ray bursts (GRBs) can be explained by an external shock model where a single relativistic blast wave interacts with the surrounding medium. Besides reproducing the generic spectral behavior of GRB profiles, the external shock model provides quantitative fits to the peak flux distribution, the > 1 s t_{50} duration distribution, and the distribution of the peaks E_pk of the vF_v spectra of GRBs measured with BATSE. The apparent paradox between a relativistic beaming scenario and the empirical finding that E_pk values are preferentially measured within the triggering range of a GRB detector is resolved by this model when blast wave physics and detector triggering criteria are taken into account. Some surprising implications follow, namely that the fireball event rate is ~1 per 10^4 years per Milky Way galaxy for unbeamed sources, and proportionally more if fireball outflows are collimated. This is ~ 3 orders of magnitude larger than normally quoted. Most of the clean and dirty fireball transients are undetected due to telescope sensitivity and design limitations. Strongly variable GRB time histories with good radiative efficiencies are possible because of the strongly enhanced emissions when a blast wave interacts with density inhomogeneities located nearly along the line-of-sight to the observer. Arguments against short timescale variability in an external shock model are answered, and difficulties in an internal shock/colliding shell model are mentioned.