A redshift–observation time relation for gamma-ray bursts: evidence of a distinct subluminous population

Abstract

We show that the redshift and peak flux distributions of gamma-ray bursts (GRBs) have an observation time dependence that can be used to discriminate between different burst populations. We demonstrate how observation time relations can be derived from the standard integral distributions and that they can differentiate between GRB populations detected by both the Burst and Transient Source Experiment (BATSE) and Swift satellites. Using Swift data, we show that a redshift–observation time relation (log Z–log T) is consistent with both a peak flux–observation time relation (log P–log T) and a standard log N–log P brightness distribution. As the method depends only on rarer small-z events, it is invariant to high-z selection effects. We use the log Z–log T relation to show that subluminous GRBs are a distinct population occurring at a higher rate of the order of 150+ 180− 90 Gpc− 3 yr− 1. Our analysis suggests that GRB 060505 – a relatively nearby GRB observed without any associated supernova – is consistent with a subluminous population of bursts. Finally, we show that our relations can be used as a consistency test for some of the proposed GRB spectral energy correlations.