A SIZE-DURATION TREND FOR GAMMA-RAY BURST PROGENITORS

Gamma-ray bursts (GRBs) show a bimodal distribution of durations, separated at a duration of similar to 2 s. Observations have confirmed the association of long GRBs with the collapse of massive stars. The origin of short GRBs is still being explored. We examine constraints on the size of emission regions in short and long GRBs detected by Fermi/GBM. We find that the transverse extent of emission regions during the prompt phase, R, and the burst duration, T-90, are consistent with the relation R similar to c x T-90, for both long and short GRBs. We find the characteristic transverse extent for the prompt emission region to be similar to 2 x 10(10) cm and similar to 4 x 10(11) cm for short and long GRBs, respectively.