Quiescent times in gamma-ray bursts - I. An observed correlation between the durations of subsequent emission episodes

Although more than 2000 astronomical gamma-ray bursts (GRBs) have been dete cted, the precise progenitor responsible for these events is unknown. The t emporal phenomenology observed in GRBs can significantly constrain the diff erent models. Here we analyse the time histories of a sample of bright, lon g GRBs, searching for the ones exhibiting relatively long (more than 5 per cent of the total burst duration) 'quiescent times', defined as the interva ls between adjacent episodes of emission during which the gamma-ray count r ate drops to the background level. We find a quantitative relation between the duration of an emission episode and the quiescent time elapsed since th e previous episode. We suggest here that the mechanism responsible for the extraction and the dissipation of energy has to take place in a metastable configuration, such that the longer the accumulation period, the higher the stored energy available for the next emission episode.