2003ApJ...598L..79I

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/cm3) 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/cm3 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/cm3 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.