Author: Romain Hascoët, Andrei Beloborodov, Frederic Daigne and Robert Mochkovitch
Abstract: The variability timescale observed during the prompt gamma-ray emission and the peak time of the optical afterglow provide constraints on the Lorentz factor of GRB ejecta. Combining these two methods and taking into account selection effects, we (i) constrain the Lorentz factor distribution and (ii) derive stringent upper limits on the ambient density of long GRBs. We discuss the implications of these results for the nature of circum-burst medium and GRB progenitors. We also propose an explanation for the absence of bright bursts with low Lorentz factors.