X-ray absorption and 9.7 um silicate feature as a probe of AGN torus
  structure
  The dusty torus plays a vital role in unifying active galactic nuclei (AGNs).
However, the physical structure of the torus remains largely unclear. Here we
present a systematical investigation of the torus mid-infrared (MIR)
spectroscopic feature, i.e., the 9.7 um silicate line, of $175$ AGNs selected
from the Swift/BAT Spectroscopic Survey (BASS). Our sample is constructed to
ensure that each of the $175$ AGNs has Spizter/IRS MIR, optical, and X-ray
spectroscopic coverage. Therefore, we can simultaneously measure the silicate
strength, optical emission lines, and X-ray properties (e.g., the column
density and the intrinsic X-ray luminosity). We show that, consistent with
previous works, the silicate strength is weakly correlated with the hydrogen
column density ($N_\mathrm{H}^\mathrm{X}$), albeit with large scatters. For
X-ray unobscured AGNs, the silicate-strength-derived $V$-band extinction and
the broad-H$\alpha$-inferred one are both small; however, for X-ray obscured
AGNs, the former is much larger than the latter. In addition, we find that the
optical type 1 AGNs with strong X-ray absorption on average show significant
silicate absorption, indicating that their X-ray absorption might not be caused
by dust-free gas in the broad-line region. Our results suggest that the
distribution and structure of the obscuring dusty torus are likely to be very
complex. We test our results against the smooth and clumpy torus models and
find evidence in favor of the clumpy torus model.
