Very Different X-Ray-to-Optical Column Density Ratios in γ-Ray Burst Afterglows: Ionization in GRB Environments

Abstract

We compare the H I column densities from Lyalpha absorption to the metal column densities from X-ray absorption in gamma-ray burst (GRB) afterglows. Eight bursts of 17 entering the sample have significant extragalactic X-ray absorptions. Of these we find a range in metallicities from subsolar to a few hundred times the solar value. There is a complete lack of correlation between the X-ray and optical column densities. This, and the large range and extreme values of these ``metallicities,'' suggests that the column densities derived by one or both methods are not a reliable indication of the true total column densities toward GRBs. Ionization of the GRB's gas cloud to large distances along the line of sight seems the most likely cause. From the lower limit on the total column density and the UV luminosity of the GRBs, we derive a maximum distance to the majority of the gas surrounding GRBs of ~3 pc, suggesting that the gas probed by optical afterglow spectra is not the cloud in which the burst occurs. This is an encouraging result for the use of GRB optical afterglows as probes of the interstellar medium (ISM) in their host galaxies, as the ISM observed is less likely to be strongly affected by the GRB or its progenitor.