2000ApJS..127..283C

The discovery of optical gamma-ray burst (GRB) counterparts with measurable redshifts has shown that the sources are indeed cosmological in origin. The energy source and emission mechanism remain a mystery. A common aspect of many GRB models is the collision of two compact objects, typically neutron stars. The energy released forms a relativistic blast wave that eventually converts its kinetic energy into radiation via synchrotron shocks. The Lorentz boosting of the radiation makes it appear in the gamma-ray regime. The observed X-ray and optical counterparts match some predictions of an external shock model, in which the blast wave collides with the surrounding ambient medium. However, recent studies of burst time histories suggest that the gamma-ray active phase of bursts cannot result from external shocks. Features found in the time-resolved spectra of gamma-ray bursts also suggest that their emission mechanism is not optically thin synchrotron. In this paper, we will review the more important gamma-ray burst observations, discuss briefly the development of the synchrotron shock model for GRBs, and propose that this model cannot currently explain the prompt emission of gamma-ray bursts.