Gamma Ray Bursts and Cosmic Ray Origin

Abstract

This paper presents the theoretical basis of the fireball/blast wave model, and some implications of recent results on GRB source models and cosmic-ray production from GRBs. BATSE observations of the prompt gamma-ray luminous phase, and Beppo-SAX and long wavelength afterglow observations of GRBs are briefly summarized. Derivation of spectral and temporal indices of an adiabatic blast wave decelerating in a uniform surrounding medium in the limiting case of a nonrelativistic reverse shock, both for spherical and collimated outflows, is presented as an example of the general theory. External shock model fits for the afterglow lead to the conclusion that GRB outflows are jetted. The external shock model also explains the temporal duration distribution and clustering of peak energies in prompt spectra of long-duration GRBs, from which the redshift dependence of the GRB source rate density can be derived. Source models are reviewed in light of the constant energy reservoir result of Frail et al. that implies a total GRB energy of a few times 10^{51} ergs and an average beaming fraction of ~1/500 of full sky. Paczynski's isotropic hypernova model is ruled out. The Vietri-Stella model two-step collapse process is preferred over a hypernova/collapsar model in view of the X-ray observations of GRBs and the constant energy reservoir result. Second-order processes in GRB blast waves can accelerate particles to ultra-high energies. GRBs may be the sources of UHECRs and cosmic rays with energies above the knee of the cosmic ray spectrum. High-energy neutrino and gamma-ray observations with GLAST and ground-based gamma-ray telescopes will be crucial to test GRB source models.