Evidence of Contribution of Intervening Clouds to GRB's X-ray Column Density

Abstract

The origin of excess of X-ray column density with respect to optical extinction in Gamma-ray bursts (GRBs) is still a puzzle. A proposed explanation of the excess is the photoelectric absorption due to the intervening clouds along a GRB's line-of-sight. We here test this scenario by using the intervening \ion{Mg}{2} absorption as a tracer of the neutral hydrogen column density of the intervening clouds. We identify a connection between large X-ray column density (and large column density ratio of $\mathrm{\log(N_{H,X}/N_{HI})}\sim0.5$) and large neutral hydrogen column density probed by the \ion{Mg}{2} doublet ratio (DR). In addition, GRBs with large X-ray column density (and large ratio of $\mathrm{\log(N_{H,X}/N_{HI})}>0$) tend to have multiple saturated intervening absorbers with $\mathrm{DR<1.2}$. These results therefore indicate an additional contribution of the intervening system to the observed X-ray column density in some GRBs, although the contribution of the host galaxy alone cannot be excluded based on this study.