Testing the gamma-ray burst blast-wave model: A primer

The discovery of optical gamma-ray burst (GRB) counterparts with measurable redshifts has shown that the sources are indeed cosmological in origin. Th e 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 Lor entz boosting of the radiation makes it appear in the gamma-ray regime. The observed X-ray and optical counterparts match some predictions of an exter nal shock model, in which the blast wave collides with the surrounding ambi ent medium. However, recent studies of burst time histories suggest that th e gamma-ray active phase of bursts cannot result from external shocks. Feat ures found in the time-resolved spectra of gamma-ray bursts also suggest th at their emission mechanism is not optically thin synchrotron. In this pape r, we will review the more important gamma-ray burst observations, discuss briefly the development of the synchrotron shock model for GRBs, and propos e that this model cannot currently explain the prompt emission of gamma-ray bursts.