Are GRB optical afterglows relatively brighter at high z?

TL;DR

This study uses simulations to explore how observational biases, particularly telescope response times, affect the perceived brightness and redshift distribution of GRB optical afterglows, suggesting they may be intrinsically brighter or less extincted at high redshifts.

Contribution

It introduces a redshift-dependent optical afterglow brightness model to explain selection biases in GRB observations.

Findings

Optical afterglows may be intrinsically brighter at high redshifts.

Telescope response times influence observed redshift distributions.

High-redshift GRBs could suffer less host galaxy extinction.

Abstract

The redshift distribution of gamma-ray bursts (GRBs) is strongly biased by selection effects. We investigate, via Monte Carlo simulations, one possible selection effect that may be modifying the Swift GRB redshift distribution. We show how telescope response times to acquire a GRB redshift may, via the Malmquist effect and GRB optical afterglow brightness distribution, introduce a bias into the average of the observed redshift distribution. It is difficult to reconcile a recently reported correlated trend between telescope response time and average redshifts unless we employ a redshift-dependent optical afterglow distribution. Simulations of this selection effect suggest that GRB optical afterglows may have been either intrinsically brighter early in the Universe or suffered less local host galaxy extinction.