On the origin of the bimodal duration distribution of gamma-ray bursts and the subjet model

The modified version of a bullet model for gamma-ray bursts (GRBs) is studied. The central engine of the source produces multiple subjets that are contained within a cone. The emission of photons in the source frame of a subjet takes place either in an infinitesimally thin shell, or during its expansion for a finite time. The analysis of the observed profiles of GRBs taken by the Burst And Transient Source Experiment leads us to the conclusion that the latter possibility is much more favoured. We also study the statistical distribution of GRBs in the context of their bimodality of durations, taking into account the capability of the detector of observing the signal above a certain flux limit. The model with shells emitting for a finite time is able to reproduce only one class of bursts, short or long, depending on the adopted physical parameters. Therefore, we suggest that the GRB bimodality is intrinsically connected with two separate classes of sources.