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 $\log ({\mathrm{N}}_{\mathrm{H},\mathrm{X}}/{\mathrm{N}}_{\mathrm{HI}})\sim 0.5$) 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 $\log ({\mathrm{N}}_{\mathrm{H},\mathrm{X}}/{\mathrm{N}}_{\mathrm{HI}})>0$) 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.

16 pages, 5 figures, 1 table, accepted by ApJ

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• A Global Photoionization Response to Prompt Emission and Outliers: Different Origin of Long Gamma-ray Bursts?