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Population III stars are theoretically expected to be prominent aroundredshifts z ~ 20, consisting of mainly very massive stars with M_* >~ 10M_sun$, but there is no direct observational evidence for these objects. Theymay produce collapsar gamma-ray bursts (GRBs), with jets driven bymagnetohydrodynamic processes, whose total isotropic-equivalent energy could beas high as E_iso >~ 10^{57} erg over a cosmological-rest-frame duration of t_d>~ 10^4 s, depending on the progenitor mass. Here we calculate the afterglowspectra of such Pop. III GRBs based on the standard external shock model, andshow that they will be detectable with the Swift BAT/XRT and Fermi LATinstruments. We find that in some cases a spectral break due toelectron-positron pair creation will be observable in the LAT energy range,which can put constraints on the ambient density of the pre-collapse Pop. IIIstar. Thus, high redshift GRB afterglow observations could be unique andpowerful probes of the properties of Pop. III stars and their environments. Weexamine the trigger threshold of the BAT instrument in detail, focusing on theimage trigger system, and show that the prompt emission of Pop. III GRBs couldalso be detected by BAT. Finally we briefly show that the late-time radioafterglows of Pop. III GRBs for typical parameters, despite the largedistances, can be very bright: ~ 140 mJy at 1 GHz, which may lead to aconstraint on the Pop. III GRB rate from the current radio survey data, and~2.4 mJy at 70 MHz, which implies that Pop. III GRB radio afterglows could beinteresting background source candidates for 21 cm absorption line detections.

Population III GRB Afterglows: Constraints on Stellar Masses and External Medium Densities