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

Understand

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.