The Optical and X-ray Afterlows of Γ-ray Bursts Do Not Probe the Same Gas: Ionisation in Grb Environments

We compare the H I column densities from Lyα absorption to the metal column densities from soft X-ray absorption in gamma-ray burst (GRB) afterglows. Of the eight bursts for which useful measures are obtained we find a range in metallicities from sub-solar to a few hundred times the solar value. The lack of correlation in the column densities, and the large range and extreme values of these ‘metallicities’, suggest that the column densities derived by one or both methods are not a reliable indication of the true total column densities towards GRBs. Ionisation of the GRB’s gas cloud to large distances along the line of sight seems the most likely interpretation of these results. 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. Subject headings: gamma rays: bursts – X-rays: general – X-rays: galaxies – quasars: absorption lines