POPULATION III GAMMA-RAY BURST AFTERGLOWS: CONSTRAINTS ON STELLAR MASSES AND EXTERNAL MEDIUM DENSITIES

Population (Pop.) III stars are theoretically expected to be prominent around redshifts z similar to 20, consisting of mainly very massive stars with M-* greater than or similar to 100 M-circle dot, though there is no direct observational evidence for these objects. They may produce collapsar gamma-ray bursts (GRBs), with jets driven by magnetohydrodynamic processes, whose total isotropic-equivalent energy could be as high as E-iso greater than or similar to 10(57) erg over a cosmological-rest-frame duration of t(d) greater than or similar to 10(4) s, depending on the progenitor mass. Here, we calculate the afterglow spectra of such Pop. III GRBs based on the standard external shock model and show that they will be detectable with the Swift Burst Alert Telescope (BAT)/XRT and Fermi Large Area Telescope (LAT) instruments. We find that in some cases a spectral break due to electron-positron pair creation will be observable in the LAT energy range, which can put constraints on the ambient density of the pre-collapse Pop. III star. Thus, high-redshift GRB afterglow observations could be unique and powerful probes of the properties of Pop. III stars and their environments. We examine the trigger threshold of the BAT instrument in detail, focusing on the image trigger system, and show that the prompt emission of Pop. III GRBs could also be detected by BAT. Finally we briefly show that the late-time radio afterglows of Pop. III GRBs for typical parameters, despite the large distances, can be very bright: similar or equal to 140 mJy at 1 GHz, which may lead to a constraint on the Pop. III GRB rate from the current radio survey data, and similar or equal to 2.4 mJy at 70 MHz, which implies that Pop. III GRB radio afterglows could be interesting background source candidates for 21 cm absorption line detections.