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The relationship between X-ray and optical absorbers in active galactic nuclei


 We present a large compilation of reddening estimates from broad-line Balmer decrements for active galactic nuclei (AGNs) with measured X-ray column densities. The median reddening is E(B − V) ≈ 0.77 ± 0.10 for type-1 to type-1.9 AGNs with reported column densities. This is notably higher than the median reddening of AGNs from the SDSS. We attribute this to the selection bias of the SDSS towards blue AGNs. For other AGNs, we find evidence of a publication bias against reporting low column densities. We find a significant correlation between NH and E(B − V) but with a large scatter of ±1 dex. On average, the X-ray columns are consistent with those predicted from E(B − V) for a solar neighbourhood dust-to-gas ratio. We argue that the large scatter of column densities and reddenings can be explained by X-ray column density variability. For AGNs with detectable broad-line regions (BLRs) that have undergone significant changes of Seyfert type (‘changing-look’ AGNs), we do not find any statistically significant differences in NH or E(B − V) compared to standard type-1 to type-1.9 AGNs. There is no evidence for any type-1 AGNs being Compton thick. We also analyse type-2 AGNs and find no significant correlation between NH and narrow-line region reddening. We find no evidence for a previously claimed anticorrelation. The median column density of LINERs is 22.68 ± 0.75 compared to a column density of 22.90 ± 0.28 for type-2 AGNs. We find the majority of low column density type-2 AGNs are LINERs, but NH is probably underestimated because of scattered light.