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

TL;DR

This paper introduces a method using observation time dependence of gamma-ray burst distributions to distinguish different populations, revealing a distinct sub-luminous GRB group with a higher occurrence rate.

Contribution

The study develops a novel observation time relation approach to identify and characterize sub-luminous GRBs as a separate population, independent of high-redshift selection effects.

Findings

Sub-luminous GRBs occur at a rate of about 150 Gpc^{-3} yr^{-1}.

The method can differentiate GRB populations using redshift and flux distributions.

GRB 060505 is consistent with being a sub-luminous burst.

Abstract

We show how 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 BATSE and \emph{Swift} satellites. Using \emph{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 sub-luminous GRBs are a distinct population occurring at a higher rate of order $150^{+180}_{-90} \mathrm{Gpc}^{-3}\mathrm{yr}^{-1}$. Our analysis suggests that GRB 060505 -- a relatively nearby GRB observed without any associated supernova -- is consistent with a sub-luminous population of bursts. Finally, we suggest that our relations can be used as a consistency test for some of the proposed GRB spectral energy correlations.