2013ApJ...776...96W

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. Here, we test this scenario by using the intervening Mg II absorption as a tracer of the neutral hydrogen column density of the intervening clouds. We identify a connection between the large X-ray column density (and large column density ratio of log(N_H,X/N_HI ∼0.5) and large neutral hydrogen column density probed by the Mg II doublet ratio (DR). In addition, GRBs with large X-ray column density (and large ratio of log(N_H,X/N_HI > 0) tend to have multiple saturated intervening absorbers with DR < 1.2. These results therefore indicate an additional contribution from the intervening system to the observed X-ray column density in some GRBs, although the contribution from the host galaxy alone cannot be excluded based on this study.