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.