Low-Luminosity Gamma-Ray Bursts as a Unique Population: Luminosity Function, Local Rate, and Beaming Factor
Abstract
Swift BAT has detected ~200 long-duration GRBs, with redshift measurements for ~50 of them. We derive the luminosity function (ΦHL) and the local event rate (ρHL0) 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 ρLL0 is ~325+352-177 Gpc-3 yr-1, which is much higher than ρHL0 (1.12+0.43-0.20 Gpc-3 yr-1). 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2007
- DOI:
- 10.1086/517959
- arXiv:
- arXiv:astro-ph/0605200
- Bibcode:
- 2007ApJ...662.1111L
- Keywords:
-
- Gamma Rays: Bursts;
- Gamma Rays: Observations;
- Methods: Statistical;
- Astrophysics
- E-Print:
- 11 pages total with 5 figures, conclusions not changed, matched to published version, Astrophysical Journal,2007, ApJ, 662,1111