1 Introduction

The crucial observation of an X-ray transient following a cosmic gamma ray burst (GRB) was made by the Italian-Dutch satellite Beppo-SAX (Costa et al. 1997). Several GRBs with X-ray and optical afterglows have subsequently been found. In several cases, it has been possible to identify a remote galaxy as a likely host to the burst source, and to derive its distance. The huge luminosities, and the very short time scales involved in GRBs, may possibly be explained by merging, or collapsing, compact objects such as neutron stars or black holes (e.g. Paczynski 1986). However, the observed energy spectra of the bursts, and the details of the energy transport and conversion, are still to be explained. Models involving relativistic fireballs and jets have been proposed (e.g. Dar 1998). To date no theory has been able to explain all of the observed GRB phenomena, and it has become a challenge for modern astrophysics.