The luminosity and redshift distributions of short-duration GRBs

Using the BATSE peak flux distribution we rederive the short GRBs luminosity function and compare it with the observed redshift distribution of long bursts. We show that both distributions are compatible with the assumption that short as well as long bursts follow the star formation rate. In this case the difference between the two observed distributions can be interpreted as arising mostly from differences in the detector's sensitivity to long and short bursts, while the local rate of short bursts is 0.1 h(70)(3) Gpc(-3) yr(-1). We also consider the possibility that short GRBs may be associated with binary neutron star mergers and estimate the effect of the merging time delay on the luminosity function and redshift distribution. We find that in this case the local rate of short GRBs is similar to 0.8 h(70)(3) Gpc(-3) yr(-1). Assuming that all binary merging systems lead to short GRBs, we find a typical jet opening angle of 1.6 degrees.