Low-luminosity gamma-ray bursts as a unique population: Luminosity function, local rate, and beaming factor

Swift BAT has detected similar to 200 long-duration GRBs, with redshift measurements for similar to 50 of them. We derive the luminosity function ( Phi(HL)) and the local event rate ( rho(HL)(0)) of the conventional high-luminosity ( HL) GRBs by using the z-known Swift GRBs. Our results are generally consistent with that derived from the CGRO BATSE data. However, the fact that Swift detected a low-luminosity ( LL) GRB, GRB 060218, at z = 0.033 within similar to 2 years of operation, together with the previous detection of the nearby GRB 980425, suggests a much higher local rate for these LL-GRBs. We explore the possibility that LL-GRBs are a distinct GRB population from the HL-GRBs. We find that rho(LL)(0) is similar to 325(-177)(+352) Gpc(-3) yr(-1), which is much higher than rho(HL)(0) (1.12(-0.20)(+0.43) Gpc(-3) yr(-1)). This rate is similar to 0.7% of the local Type Ib/c SNe. Our results, together with the finding that less than 10% of Type Ib/ c SNe are associated with off-beam GRBs, suggest that LL-GRBs have a beaming factor typically less than 14, or a jet angle typically wider than 31 degrees. The high local GRB rate, small beaming factor, and low-luminosity make the LL-GRBs distinct from the HL-GRBs. Although the current data could not fully rule out the possibility that both HL- and LL-GRBs are the same population, our results suggest that LL-GRBs are likely a unique GRB population and that the observed low-redshift GRB sample is dominated by the LL-GRBs.