The rapid decline of the prompt emission in Gamma-Ray Bursts

Many gamma ray bursts (GRBs) have been observed with the Burst-Alert and X-Ray telescopes of the SWIFT satellite. The successive `pulses' of these GRBs end with a fast decline and a fast spectral softening, until they are overtaken by another pulse, or the last pulse's decline is overtaken by a less rapidly-varying `afterglow'. The fast decline-phase has been attributed, in the standard fireball model of GRBs, to `high-latitude' synchrotron emission from a collision of two conical shells. This interpretation does not agree with the observed spectral softening. The temporal behaviour and the spectral evolution during the fast-decline phase agree with the predictions of the cannonball model of GRBs.