Passage of a Gamma-Ray Burst through a Molecular Cloud: Cloud Ionization Structure

Abstract

We have constructed a model for the passage of gamma-ray burst radiation through a dense molecular cloud. In our calculations we take into account the main radiation–cloud material interaction processes: the ionization of H and He atoms, the ionization of metal ions with the emission of Auger electrons, the ionization and photodissociation of H\({}_{2}\) molecules, the absorption of ultraviolet radiation in the H\({}_{2}\) lines of the Lyman and Werner bands, and the vaporization of dust grains. The ionization of metal ions by X-ray radiation determines the gas ionization fraction in the region where the gas is predominantly neutral. The photoionization of the inner electron shells of ions is accompanied by the emission of Auger electrons, giving rise to metal ions in a high ionization state. In particular, the column densities of Mg, Si, and Fe ions in the ionization states I–IV are much lower than the column densities of these ions in the ionization state V or higher. The photoionization of metal ions by ultraviolet radiation occurs only at distances smaller than the dust vaporization radius and for neutral atoms with an ionization threshold below 13.6 eV. The results of our calculations have confirmed the previously made assumption that the ionization of He atoms plays an important role in the absorption of radiation in the X-ray wavelength range. For a low metallicity, \(\textrm{[M/H]}\leq{-}1\), the role of helium atoms is dominant.