An Unexpectedly Swift Rise in the Gamma-ray Burst Rate

TL;DR

This paper presents evidence that the gamma-ray burst rate increases more rapidly with redshift than star formation rates suggest, indicating additional evolutionary factors beyond star formation history.

Contribution

It provides the first empirical evidence of enhanced gamma-ray burst evolution at high redshift, suggesting factors like metallicity influence GRB occurrence beyond star formation.

Findings

GRB rate at z~4 is about 4 times higher than expected from star formation.

New estimate of Swift GRB energetics used to construct the sample.

Supports theories involving metallicity effects on GRB production.

Abstract

The association of long gamma-ray bursts with supernovae naturally suggests that the cosmic GRB rate should trace the star formation history. Finding otherwise would provide important clues concerning these rare, curious phenomena. Using a new estimate of Swift GRB energetics to construct a sample of 36 luminous GRBs with redshifts in the range z=0-4, we find evidence of enhanced evolution in the GRB rate, with ~4 times as many GRBs observed at z~4 than expected from star formation measurements. This direct and empirical demonstration of needed additional evolution is a new result. It is consistent with theoretical expectations from metallicity effects, but other causes remain possible, and we consider them systematically.