2007ApJ...662.1111L

Swift BAT has detected ∼200 long-duration GRBs, with redshift measurements for ∼50 of them. We derive the luminosity function (Φ^HL) and the local event rate (ρ^HL₀) 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 ∼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 ρ^LL₀ is ∼325⁺³⁵²_–177/Gpc3/yr, which is much higher than ρ^HL₀(1.12^+0.43_–0.20/Gpc3/yr). This rate is ∼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°. 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.