Gamma-ray Bursts *

The origin of GRBs, bursts of 0.1 MeV—1 MeV photons lasting for a few seconds, remained unknown for over 20 years, primarily because GRBs were not detected prior to 1997 at wave-bands other than γ-rays [1]. The isotropic distribution of bursts over the sky suggested that GRB sources lie at cosmological distances, and general phenomenological considerations were used to argue that the bursts are produced by the dissipation of the kinetic energy of a relativistic expanding fireball (see [2] for review). Adopting the cosmological fireball hypothesis, it was shown that the physical conditions in the fireball dissipation region allow Fermi acceleration of protons to energy > 10eV [3,4], and that the average rate at which energy is emitted as γ-rays by GRBs is comparable to the energy generation rate of UHECRs in a model where UHECRs are produced by a cosmological distribution of sources [3]. Based on these two facts, it was suggested that GRBs and UHECRs have a common origin (see [5] for a recent review). In the last two years, afterglows of GRBs have been discovered in X-ray, optical, and radio wave bands [6]. Afterglow observations confirmed the cosmological origin of the bursts, through the redshift determination of several GRB host-galaxies, and confirmed standard model predictions of afterglows that result from the collision of an expanding fireball with its surrounding medium [2]. These observations therefore provide strong support for the GRB model of UHECR production. In this review, UHECR and neutrino production in GRBs is discussed in the light of recent GRB and UHECR observations. The fireball model is briefly described in §2.1, and proton acceleration in GRB fireballs is discussed in §2.2. Implications of recent afterglow