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We study the X-ray absorption of a complete sample of 99 bright Swiftgamma-ray bursts. Over the last few years, a strong correlation between theintrinsic 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 theoverlooked contribution of the absorption along the intergalactic medium, bymeans of both intervening objects and the diffuse warm-hot intergalactic mediumalong 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 theGalactic absorption model on the N_H(z) distribution. Therefore, we derive theintrinsic absorbing column densities using two different Galactic absorptionmodels, the Leiden Argentine Bonn HI survey and the more recent model includingmolecular hydrogen. We find that, if on the one hand the new Galactic modelconsiderably affects the single column density values, on the other hand thereis no drastic change in the distribution as a whole. It becomes clear that thecontribution of Galactic column densities alone, no matter how improved, is notsufficient to change the observed general trend and it has to be considered asa second-order correction. The cosmological increase of N_H(z) as a function ofredshift persists and, in order to explain the observed distribution, it isnecessary to include the contribution of both the diffuse intergalactic mediumand the intervening systems along the line of sight of the GRBs.

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