The variable and non-variable X-ray absorbers in Compton-thin type-II
  Active Galactic Nuclei
  We have conducted an extensive X-ray spectral variability study of a sample
of 20 Compton-thin type II galaxies using broad band spectra from XMM-Newton,
Chandra, and Suzaku. The aim is to study the variability of the neutral
intrinsic X-ray obscuration along the line of sight and investigate the
properties and location of the dominant component of the X-ray-obscuring gas.
The observations are sensitive to absorption columns of $N_{\rm H} \sim
10^{20.5-24} {\rm cm^{-2}}$ of fully- and partially-covering neutral and/or
lowly-ionized gas on timescales spanning days to well over a decade. We
detected variability in the column density of the full-covering absorber in
7/20 sources, on timescales of months-years, indicating a component of
compact-scale X-ray-obscuring gas lying along the line of sight of each of
these objects. Our results imply that torus models incorporating clouds or
overdense regions should account for line of sight column densities as low as
$\sim$ a few $\times 10^{21}$ cm$^{-2}$. However, 13/20 sources yielded no
detection of significant variability in the full-covering obscurer, with upper
limits to ${\Delta}N_{\rm H}$ spanning $10^{21-23}$ cm$^{-2}$. The dominant
absorbing media in these systems could be distant, such as kpc-scale dusty
structures associated with the host galaxy, or a homogeneous medium along the
line of sight. Thus, we find that overall, strong variability in full-covering
obscurers is not highly prevalent in Compton-thin type IIs, at least for our
sample, in contrast to previous results in the literature. Finally, 11/20
sources required a partial-covering, obscuring component in all or some of
their observations, consistent with clumpy near-Compton-thick compact-scale
gas.
