High-redshift gamma-ray bursts from population III progenitors

Detection of gamma-ray bursts (GRBs) from redshifts z greater than or similar to 7 would open a new window into the earliest epoch of cosmic star formation. We construct separate star formation histories at high redshifts for normal ( Population I and II) stars and for predominantly massive ( Population III) stars. Based on these separate histories, we predict the GRB redshift distribution to be observed by the Swift mission. Regardless of whether Population III progenitors are able to trigger GRBs, we find that a fraction, similar to 10%, of all bursts detected by Swift will originate at z greater than or similar to 5. This baseline contribution is due to Population I/II star formation, which must have extended out to high redshifts in rare massive galaxies that were enriched by heavy elements earlier than the typical galaxies. In addition, we consider the possible contribution of Population III progenitors to the observable GRB rate. Population III stars are viable progenitors for long-duration GRBs that are triggered by the collapsar mechanism, as long as they can lose their outer envelope through mass transfer to a companion star in a close binary. We find that the likelihood of Population III binaries to satisfy the conditions required by the collapsar mechanism could be enhanced significantly relative to Population I/II binaries. If Population III binaries are common, Swift will be the first observatory to probe Population III star formation at redshifts z greater than or similar to 7.