A possible origin of bimodal distribution of gamma-ray bursts

We study the distribution of the durations of gamma-ray bursts (GRBs) in the unified model of short and long GRBs recently proposed by Yamazaki, Ioka, and Nakamura. Monte Carlo simulations show clear bimodal distributions, with lognormal- like shapes for both short and long GRBs, in a power-law as well as a Gaussian angular distribution of the subjets. We find that the bimodality comes from the existence of the discrete emission regions ( subjets or patchy shells) in the GRB jet. To explain other temporal properties of short and long GRBs, the subjet parameters should depend on the angle in the whole jet