HYDRODYNAMIC TIMESCALES AND TEMPORAL STRUCTURE OF GAMMA-RAY BURSTS

We calculate the hydrodynamic timescales for a spherical ultrarelativi stic shell that is decelerated by the ISM and discuss the possible rel ations between these timescales and the observed temporal structure in gamma-ray bursts. We suggest that the bursts' duration is related to the deceleration time, the variability is related to the ISM inhomogen eities, and that precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reason able, not fine-tuned, astrophysical parameters. The difference between Newtonian and relativistic reverse shocks may lead to the observed bi modal distribution of bursts' durations.