AGN dust tori: the X-ray-infrared connection
  We have combined the CLASXS Chandra survey in Lockman with the Spitzer SWIRE
survey data to study the X-ray-infrared connection for AGN. The sample consists
of 401 X-ray-sources, of which 306 are detected by Spitzer, and a further 257
AGN candidates detected through their dust torus, but not by Chandra. For X-ray
sources the X-ray hardness ratio has been modelled in terms of a power-law with
absorption N(H). The optical and infrared data have been modelled in terms of
our well-established optical galaxy and QSO templates, and infrared templates
based on radiative transfer models.
  Our estimate of the N(H) distribution is consistent with other studies, but
we do find a higher proportion of low absorption objects at z < 0.5 than at z >
0.5. While we find only one X-ray AGN with N(H) > 10^{24} cm^{-2}, we argue
that 10 objects with torus luminosity apparently exceeding the bolometric X-ray
to 3 \mu m luminosity are strong candidates for being heavily absorbed in
X-rays. We also estimate that at least half of the infrared-detected AGN dust
tori which are undetected in X-rays are likely to be Compton thick. Our
estimate of the total number of Compton-thick objects corresponds to > 20%$ of
the combined SWIRE-CLASXS sample (and with an upper limit of 39 %).
  The range of dust covering factors is 1-100 %, with a mean of 40 %, ie a Type
2 fraction of 40 %. Measured by the ratio of dust torus luminosity to X-ray or
(for Type 1 objects) optical luminosity, the covering factor appears to
decrease towards intermediate AGN luminosity, in contradiction to estimates
based on ratios of narrow-line and broad-line spectra, but may increase again
at low AGN luminosity.
