On The Lack of Time Dilation Signatures in Gamma-ray Burst Light Curves

TL;DR

This study uses simulations to show that observed gamma-ray burst durations do not increase with redshift as expected due to cosmological time dilation, due to observational biases and signal-to-noise effects.

Contribution

It demonstrates that observational biases explain the lack of observed time dilation in GRB durations and predicts future measurements could reveal this effect.

Findings

Observed GRB durations do not increase with redshift.

Signal-to-noise bias causes shorter observed durations at high redshift.

Predicted that future multi-pulsed GRB studies may detect time dilation.

Abstract

We examine the effects of time dilation on the temporal profiles of gamma-ray burst (GRB) pulses. By using prescriptions for the shape and evolution of prompt gamma-ray spectra, we can generate a simulated population of single pulsed GRBs at a variety of redshifts and observe how their light curves would appear to a gamma-ray detector here on Earth. We find that the observer frame duration of individual pulses does not increase as a function of redshift as one would expect from the cosmological expansion of a Friedman-Lemaitre-Robertson-Walker Universe. In fact, the duration of individual pulses is seen to decrease as their signal-to-noise decreases with increasing redshift, as only the brightest portion of a high redshift GRB's light curve is accessible to the detector. The results of our simulation are consistent with the fact that a systematic broadening of GRB durations as a function of redshift has not materialized in either the Swift or Fermi detected GRBs with known redshift. We show that this fundamental duration bias implies that the measured durations and associated Eiso estimates for GRBs detected near an instrument's detection threshold should be considered lower limits to their true values. We conclude by predicting that the average peak-to-peak time for a large number of multi-pulsed GRBs as a function of redshift may eventually provide the evidence for time dilation that has so far eluded detection.