On the origin of bimodal duration distribution of Gamma Ray Bursts

The modified version of a bullet model for gamma ray bursts is studied. The central engine of the source produces multiple sub-jets that are contained within a cone. The emission of photons in the source frame of a sub-jet either takes part in an infinitesimally thin shell, or during its expansion for a finite time. The analysis of the observed profiles of GRBs taken by BATSE leads us to the conclusion that the latter possibility is much more favored. We also study the statistical distribution of GRBs, in the context of their bimodality of durations, taking into account the detector's capability 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.