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

Abstract

We study the absorption of a gamma-ray burst afterglow in a dense molecular cloud in the X-ray wavelength range. We report the results of numerical simulations of the propagation of the gamma-ray burst radiation in the cloud for various gas densities, metallicities, and distances from the gamma-ray burst progenitor star and the cloud. We consider a sample of 45 gamma-ray bursts with known redshifts in which the isotropic-equivalent gamma-ray energy is approximately equal to the value adopted in our numerical simulations. For these gamma-ray bursts, we have analyzed the Swift/XRT energy spectra of their afterglows at late times, $t \geq 4 \times 10^3$ s. It is shown that the hydrogen column densities estimated using the absorption model in which the ionization of metal ions is not taken into account and the solar metallicity is used are a factor of 1-3 smaller than the actual values - if the molecular cloud is located close to the gamma-ray burst progenitor star. If the gas-dust cloud is located at a distance of $R \geq 10$ pc from the source of the gamma-ray burst or the gas metallicity is $[M/H] \leq -1$, then the effect of the cloud ionization structure on the absorption of the afterglow is minor.