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