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

TL;DR

This paper models the afterglows of Population III gamma-ray bursts, demonstrating their detectability with current instruments and their potential to reveal properties of early stars and their environments.

Contribution

It provides the first detailed calculations of Pop. III GRB afterglows, linking observable features to progenitor and environmental parameters, and explores detection prospects.

Findings

Pop. III GRB afterglows are detectable with Swift and Fermi.

Spectral breaks due to pair creation can constrain ambient densities.

Radio afterglows could be very bright, aiding in rate estimates and background studies.

Abstract

Population III stars are theoretically expected to be prominent around redshifts z ~ 20, consisting of mainly very massive stars with M_* >~ 10 M_sun$, but 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 >~ 10^{57} erg over a cosmological-rest-frame duration of t_d >~ 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 BAT/XRT and Fermi 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: ~ 140 mJy at 1 GHz, which may lead to a constraint 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 be interesting background source candidates for 21 cm absorption line detections.