Observational evidence for a correlation between peak-luminosities and beaming in GRBs

We calculate the unseen-but-true GRB-event rate from a current flux-limited sample of 33 GRBs with individually measured redshifts. We consider a GRB-event rate which is proportional to the star-formation rate, in view of the GRB-SNe association in SN1998bw and GRB030329. By fitting a log-normal distribution of GRB peak-luminosities, we find a ratio of about 450 for the true-to-observed GRB-event rates. This provides an independent derivation of the GRB-beaming factor of about 500 obtained by Frail et al. (2001) from sources with standard GRB-energies. We discuss applications to GRB980425.