| On The Origin Of The Highest Redshift Gamma-Ray Burst GRB 080913 (2008) | |||||||||
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| GRB 080913, discovered by SWIFT, is the most distant gamma-ray burst (GRB) known to-date, with a spectroscopically determined redshift of z=6.7. The detection of a burst at such an early epoch of the Universe significantly constrains the nature of GRBs and their progenitors. To evaluate these constraints, 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 for at z=6.7 essentially all GRBs originate from Population II stars, independent of the adopted metallicity evolution model. At this epoch Population III (metal free) stars, representing the very first generation of stars, most likely have already completed their evolution, and Population I stars (representing the present population) have just begun forming. We argue that Population II stars (having small, but non-zero metallicity) are the most likely progenitors of both long GRBs (collapsars) and short GRBs (NS-NS or BH-NS mergers) in the redshift range 6. Comment: 13 pages, 5 figures, submitted to ApJ | |||||||||
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