Passage of a Gamma-Ray Burst through a Molecular Cloud: The Absorption of Its Afterglow in the X-ray Wavelength Range

The absorption of a gamma-ray burst (GRB) afterglow in the X-ray wavelength range in a dense molecular cloud is investigated. We present the results of our numerical simulations of the propagation of GRB radiation in the cloud for various gas densities, metallicities, and distances from the GRB progenitor star to the cloud. We consider a sample of 45 GRBs with known redshifts in which the isotropic-equivalent gamma-ray energy corresponds to the value adopted in our numerical simulations. For these GRBs we have analyzed the Swift/XRT energy spectra of their afterglows at late times, t >= 4 x 10(3) s. We show that the hydrogen column densities estimated using the absorption model in which the ionization of metal ions is ignored and solar metallicity is used are a factor of 1-3 lower than the actual values if the molecular cloud is near the GRB progenitor star. If the gas cloud is located at a distance R >= 10 pc from the GRB source or the gas metallicity [M/H] <= -1, then the influence of the could ionization structure on the afterglow absorption is minor.