Gamma-ray burst prompt emission: Implications from shock acceleration theory

Abstract

The principal paradigm for gamma-ray bursts (GRB) suggest that the prompt transient gamma-ray signal arises from multiple shocks internal to the relativistic expansion. This paper illustrates some properties of diffusive acceleration at relativistic shocks that pertain to GRB models, providing interpretation of the BATSE/EGRET data. Using a standard Monte Carlo simulation, computations of the spectral shape, and the range of spectral indices are presented, as functions of the shock speed, magnetic field obliquity and the type of particle scattering. It is clear that while quasi-parallel, relativistic shocks with fields approximately normal to the shock plane can efficiently accelerate particles, highly oblique and perpendicular ones cannot unless the particle diffusion is almost isotropic, i.e., extremely close to the Bohm limit. Accordingly, an array of distribution indices should be present in the burst population, as is exhibited by the EGRET data, and even mildly relativistic internal shocks require strong field turbulence in order to model the >100 MeV observations. In addition, recent spectral fitting to burst data in the BATSE band is discussed, providing probes of the efficiency of injection of non-thermal electrons.