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

¹ Department of Physics G. OcchialiniUniversity of Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
e-mail: r.arcodia@campus.unimib.it
² INAF–Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate ( LC), Italy
e-mail: sergio.campana@brera.inaf.it
³ INAF, IASF Milano, via E. Bassini 15, 20133 Milano, Italy

Received: 17 February 2016
Accepted: 25 March 2016

Abstract

We study the X-ray absorption of a complete sample of 99 bright Swift gamma-ray bursts (GRBs). In recent years, a strong correlation has been found between the intrinsic X-ray absorbing column density (N_H(z)) and the redshift. This absorption excess in high-z GRBs is now thought to be due to the overlooked contribution of the absorption along the intergalactic medium (IGM), 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 H_I survey and the more recent model that includes 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, 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.