Low-luminosity gamma-ray bursts as a distinct GRB population: a firmer case from multiple criteria constraints

The intriguing observations of Swift/BAT X-ray flash XRF 060218 and the BATSE-BeppoSAX gamma-ray burst GRB 980425, both with much lower luminosity and redshift compared to other observed bursts, naturally lead to the question of how these low-luminosity (LL) bursts are related to high-luminosity (HL) bursts. Incorporating the constraints from both the flux-limited samples observed with CGRO/BATSE and Swift/BAT and the redshift-known GRB sample, we investigate the luminosity function for both LL- and HL-GRBs through simulations. Our multiple criteria, including the log N − log P distributions from the flux-limited GRB sample, the redshift and luminosity distributions of the redshift-known sample, and the detection ratio of HLand LL- GRBs with Swift/BAT, provide a set of stringent constraints to the luminosity function. Assuming that the GRB rate follows the star formation rate, our simulations show that a simple power law or a broken power law model of luminosity function fail to reproduce the observations, and a new component is required. This component can be modeled with a broken power, which is characterized by a sharp

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