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 N_H and E(B-V) but with a large scatter of +/- 1 dex (without
accounting for uncertainties in measurements). 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 N_H 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 analyze
type-2 AGNs and find no significant correlation between N_H and narrow-line
region reddening. The median column density of LINERs is 22.68 +/- 0.75
compared to a column density of 22.90 +/- 0.28 for type-2s. We find the
majority of low column density type-2s are LINERs, but N_H is probably
underestimated because of scattered X-ray light diluting the photoelectric
edge.
