The Cosmic Dispersion Measure from Gamma-Ray Burst Afterglows: Probing the Reionization History and the Burst Environment

Abstract

We show a possible way to measure the column density of free electrons along the light path, the so-called dispersion measure (DM), from the early [~415(ν/1 GHz)-2(DM/105 pc cm-3) s] radio afterglows of the gamma-ray bursts. We find that the proposed Square Kilometer Array can detect bright radio afterglows around the time ~103(ν/160 MHz)-2 s to measure the intergalactic DM (6000 pc cm-3 at redshift z > 6) up to z ~ 30, from which we can determine the reionization history of the universe and identify the missing warm-hot baryons if many DMs can be measured. At low z, the DM in the host galaxy may reach ~105 pc cm-3 depending on the burst environment, which may be probed by the current detectors. Free-free absorption and diffractive scattering may also affect the radio emission at a high density.