Investigating signatures of cosmological time dilation in duration measures of prompt gamma-ray burst light curves

TL;DR

This study examines how gamma-ray burst durations evolve with redshift, finding trends consistent with cosmological time dilation but limited by observational biases and sample size, leading to inconclusive evidence.

Contribution

It provides a comprehensive analysis of duration measures across redshifts, accounting for instrumental effects and selection biases, and discusses physical and observational reasons for observed duration distributions.

Findings

Duration measures increase with redshift following a power-law.

Observed distributions are broadly consistent with cosmological time dilation.

Instrumental effects and sample limitations hinder definitive conclusions.

Abstract

We study the evolution with redshift of three measures of gamma-ray burst (GRB) duration ($T_{\rm 90}$, $T_{\rm 50}$ and $T_{\rm R45}$) in a fixed rest frame energy band for a sample of 232 Swift/BAT detected GRBs. Binning the data in redshift we demonstrate a trend of increasing duration with increasing redshift that can be modelled with a power-law for all three measures. Comparing redshift defined subsets of rest-frame duration reveals that the observed distributions of these durations are broadly consistent with cosmological time dilation. To ascertain if this is an instrumental effect, a similar analysis of Fermi/GBM data for the 57 bursts detected by both instruments is conducted, but inconclusive due to small number statistics. We then investigate under-populated regions of the duration redshift parameter space. We propose that the lack of low-redshift, long duration GRBs is a physical effect due to the sample being volume limited at such redshifts. However, we also find that the high-redshift, short duration region of parameter space suffers from censorship as any Swift GRB sample is fundamentally defined by trigger criteria determined in the observer frame energy band of Swift/BAT. As a result, we find that the significance of any evidence for cosmological time dilation in our sample of duration measures typically reduces to $<2\sigma$.