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 currently-explored standard fireball model of GRBs, to ‘high-latitude’ synchrotron emission from a collision of two conical shells. This high latitude emission does not explain the observed spectral softening. In contrast, the temporal behaviour and the spectral evolution during the fast-decline phase agree with the predictions of the cannonball model of GRBs. Subject headings: γ rays: burst-radiation mechanisms: non-thermal X-rays: flare