Cosmological Evolution of the Absorption of γ-Ray Burst X-Ray Afterglows

X-ray absorption of γ-ray burst (GRB) afterglows is prevalent yet poorly understood. X-ray-derived neutral hydrogen column densities (N _H) of GRB X-ray afterglows show an increase with redshift, which might give a clue for the origin of this absorption. We use more than 350 X-ray afterglows with spectroscopic redshift (z) from the Swift X-ray Telescope repository as well as over 100 Lyα absorption measurements in z > 1.6 sources. The observed trend of the average optical depth τ at 0.5 keV is consistent with both a sharp increase of host N _H(z), and an absorbing diffuse intergalactic medium (IGM), along with decreasing host contribution to τ. We analyze a subsample of high-z GRBs with N _H derived both from the X-ray afterglow and the Lyα line. The increase of X-ray-derived N _H(z) is contrasted by no such increase in the Lyα-derived column density. We argue that this discrepancy implies a lack of association between the X-ray and Lyα absorbers at high z. This points toward the X-ray absorption at high z being dominated by an intervening absorber, which lends credibility to an absorbing IGM contribution.