Search for X-ray occultations in Active Galactic Nuclei
  Recent time-resolved spectral studies of a few Active Galactic Nuclei in hard
X-rays revealed occultations of the X-ray primary source probably by Broad Line
Region (BLR) clouds. An important open question on the structure of the
circumnuclear medium of AGN is whether this phenomenon is common, i.e. whether
a significant fraction of the X-ray absorption in AGN is due to BLR clouds.
Here we present the first attempt to perform this kind of analysis in a
homogeneous way, on a statistically representative sample of AGN, consisting of
the ~40 brightest sources with long XMM-Newton and/or Suzaku observations. We
describe our method, based on a simple analysis of hardness-ratio light curves,
and its validation through a complete spectroscopic analysis of a few cases. We
find that X-ray eclipses, most probably due to clouds at the distance of the
BLR, are common in sources where the expected occultation time is compatible
with the observation time, while they are not found in sources with longer
estimated occultation times. Overall, our results show that occultations by BLR
clouds may be responsible for most of the observed X-ray spectral variability
at energies higher than 2 keV, on time scales longer than a few ks.
