Quiescent times in gamma-ray-bursts: evidence of a dormant inner engine

We perform a statistical analysis of the temporal structure of long Gamma-Ray-Bursts (GRBs). First we consider a sample of bursts in which a long quiescent time is present. Comparing the pre-quiescence with the post-quiescence emission we show that they display similar temporal structures, hardness ratios and emitted powers, but, on the average, the post-quiescence emission is roughly twice as long as the pre-quiescence emission. We then consider a sample of long and bright GRBs. We show that the duration of each emission period is compatible with the duration of an active period computed in various inner engine models. At the contrary, if the inner engine is assumed to be always active, i.e. also during the quiescent times, in several cases the total duration of the burst largely exceeds the theoretical durations. Our analysis therefore does not support the interpretation of long quiescent times in terms of stochastic modulation of a continuous wind. Instead the quiescent times can be interpreted as dormancy periods of the inner engine. Before and after a dormancy period the inner engine produces similar emissions.