On The Origin Of The Highest Redshift Gamma-Ray Bursts

TL;DR

This study investigates the origins of the highest redshift gamma-ray bursts, revealing they likely originate from Population II stars and providing insights into early universe star formation and GRB progenitors.

Contribution

It presents population synthesis models that connect high-redshift GRBs to Population II stars, clarifying their formation rates and progenitors in the early universe.

Findings

High-z GRBs mostly originate from Population II stars.

Predicted GRB rates peak at z=7 for efficient metallicity mixing.

Observed high-z GRBs are likely long GRBs from Population II collapsars.

Abstract

GRB 080913 and GRB 090423 are the most distant gamma-ray bursts (GRBs) known to-date, with spectroscopically determined redshifts of z=6.7 and z=8.1, respectively. The detection of bursts at this early epoch of the Universe significantly constrains the nature of GRBs and their progenitors. We perform population synthesis studies of the formation and evolution of early stars, and calculate the resulting formation rates of short and long-duration GRBs at high redshift. The peak of the GRB rate from Population II stars occurs at z=7 for a model with efficient/fast mixing of metals, while it is found at z=3 for an inefficient/slow metallicity evolution model. We show that in the redshift range 6<z<10 essentially all GRBs originate from Population II stars, regardless of metallicity evolution model. These stars (having small, but non-zero metallicity) are the most likely progenitors for both long GRBs (collapsars) and short GRBs (NS-NS or BH-NS mergers) at this epoch. Although the predicted intrinsic rates of long and short GRBs are similar at these high redshifts, observational selection effects lead to higher (factor of 10) observed rates for long GRBs. We conclude that the two recently observed high-z GRB events are most likely long GRBs originating from Population II collapsars.