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We study the > 10 ratios in the X-ray to optical column densities inferredfrom afterglow spectra of Gamma Ray Bursts due to gas surrounding theirprogenitors. We present time-evolving photoionization calculations for theseafterglows and explore different conditions for their environment. We find thathomogenous models of the environment (constant density) predict X-ray columnssimilar to those found in the optical spectra, with the bulk of the opacitybeing produced by neutral material at large distances from the burst. Thisresult is independent of gas density or metallicity. Only models assuming aprogenitor immersed in a dense (10^(2-4) cm-3) cloud of gas (with radius ~10pc), with a strong, declining gradient of density for the surroundinginterstellar medium are able to account for the large X-ray to optical columndensity ratios. However, to avoid an unphysical correlation between the size ofthis cloud, and the size of the ionization front produced by the GRB, themodels also require that the circumburst medium is already ionized prior to theburst. The inferred cloud masses are <10^6 M_solar, even if low metallicitiesin the medium are assumed (Z~0.1 Z_solar). These cloud properties areconsistent with those found in giant molecular clouds and our results support ascenario in which the progenitors reside within intense star formation regionsof galaxies. Finally, we show that modeling over large samples of GRBafterglows may offer strong constraints on the range of properties in theseclouds, and the host galaxy ISM.

An Explanation for the Different X-ray to Optical Column Densities in the Environments of Gamma Ray Bursts: A Progenitor Embedded in a Dense Medium