Hydrodynamic Time Scales and Temporal Structure of

We calculate the hydrodynamic time scales for a spherical ultra-relativistic shell that is decelerated by the ISM and discuss the possible relations between these time scales and the observed temporal structure in -ray bursts. We suggest that the bursts ' duration is related to the deceleration time, the variability is related to the ISM inhomogeneities and precursors are related to internal shocks within the shell. Good agreement can be achieved for these quantities with reasonable, not ned tuned, astrophysi-cal parameters. The dierence between Newtonian and relativistic reverse shocks may lead to the observed bimodal distribution of bursts ' durations. Subject heading: gamma-rays: bursts- hydrodynamics-relativity The Astrophyiscal Journal Letters, in Press 1