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

TL;DR

This study models how gamma-ray burst afterglows are absorbed in dense molecular clouds, revealing that traditional estimates underestimate hydrogen column densities and that ionization effects are minimal at larger distances or low metallicities.

Contribution

It provides detailed numerical simulations of gamma-ray burst absorption in molecular clouds, accounting for various densities, metallicities, and distances, and compares these with observational data.

Findings

Hydrogen column densities are underestimated by a factor of 1-3 when ignoring ionization.

Ionization effects are minor if the cloud is beyond 10 pc or has low metallicity.

Analysis of 45 GRBs shows the model's relevance to real observations.

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.