The Consequences of Gamma-ray Burst Jet Opening Angle Evolution on the Inferred Star Formation Rate

TL;DR

This paper investigates how the evolution of gamma-ray burst jet opening angles affects the inferred star formation rate, revealing that accounting for this evolution suggests higher early-universe SFRs and a significant fraction of stars producing GRBs.

Contribution

It introduces a method to estimate the star formation rate from GRB data that accounts for evolving jet opening angles, providing revised SFR estimates at high redshifts.

Findings

High redshift SFR can be up to ten times higher than canonical estimates.

Accounting for jet evolution reduces the low-redshift SFR excess.

A high fraction of stars in the early universe may produce GRBs, supporting non-canonical SFR estimates.

Abstract

Gamma-ray burst (GRB) data suggest that the jets from GRBs in the high redshift universe are more narrowly collimated than those at lower redshifts. This implies that we detect relatively fewer long GRB progenitor systems (i.e. massive stars) at high redshifts, because a greater fraction of GRBs have their jets pointed away from us. As a result, estimates of the star formation rate (from the GRB rate) at high redshifts may be diminished if this effect is not taken into account. In this paper, we estimate the star formation rate (SFR) using the observed GRB rate, accounting for an evolving jet opening angle. We find that the SFR in the early universe (z > 3) can be up to an order of magnitude higher than the canonical estimates, depending on the severity of beaming angle evolution and the fraction of stars that make long gamma-ray bursts. Additionally, we find an excess in the SFR at low redshifts, although this lessens when accounting for evolution of the beaming angle. Finally, under the assumption that GRBs do in fact trace canonical forms of the cosmic SFR, we constrain the resulting fraction of stars that must produce GRBs, again accounting for jet beaming-angle evolution. We find this assumption suggests a high fraction of stars in the early universe producing GRBs - a result that may, in fact, support our initial assertion that GRBs do not trace canonical estimates of the SFR.