The dependence of gamma-ray burst X-ray column densities on the model for Galactic hydrogen

TL;DR

This study examines how different models of Galactic hydrogen absorption affect the observed X-ray column densities in gamma-ray bursts, finding that Galactic models influence individual measurements but do not alter the overall redshift trend, which still requires intergalactic medium contributions for explanation.

Contribution

The paper compares two Galactic hydrogen absorption models and demonstrates that model choice impacts individual N_H(z) values but not the overall redshift dependence, emphasizing the need to consider intergalactic absorption effects.

Findings

Galactic absorption model choice affects individual N_H(z) measurements.

The redshift dependence of N_H(z) remains consistent across models.

Intergalactic medium contributions are necessary to explain the observed N_H(z) distribution.

Abstract

We study the X-ray absorption of a complete sample of 99 bright Swift gamma-ray bursts. Over the last few years, a strong correlation between the intrinsic X-ray absorbing column density (N_H(z)) and the redshift was found. This absorption excess in high-z GRBs is now thought to be due to the overlooked contribution of the absorption along the intergalactic medium, by means of both intervening objects and the diffuse warm-hot intergalactic medium along the line of sight. In this work we neglect the absorption along the IGM, because our purpose is to study the eventual effect of a radical change in the Galactic absorption model on the N_H(z) distribution. Therefore, we derive the intrinsic absorbing column densities using two different Galactic absorption models, the Leiden Argentine Bonn HI survey and the more recent model including molecular hydrogen. We find that, if on the one hand the new Galactic model considerably affects the single column density values, on the other hand there is no drastic change in the distribution as a whole. It becomes clear that the contribution of Galactic column densities alone, no matter how improved, is not sufficient to change the observed general trend and it has to be considered as a second-order correction. The cosmological increase of N_H(z) as a function of redshift persists and, in order to explain the observed distribution, it is necessary to include the contribution of both the diffuse intergalactic medium and the intervening systems along the line of sight of the GRBs.