Iron K$\alpha$ emission in type-I and type-II Active Galactic Nuclei
  The narrow Fe K$\alpha$ line is one of the main signatures of the
reprocessing of X-ray radiation from the material surrounding supermassive
black holes, and it has been found to be omnipresent in the X-ray spectra of
active galactic nuclei (AGN). In this work we study the characteristics of the
narrow Fe K$\alpha$ line in different types of AGN. Using the results of a
large Suzaku study we find that Seyfert 2s have on average lower Fe K$\alpha$
luminosities than Seyfert 1s for the same 10-50 keV continuum luminosity.
Simulating dummy Sy1s and Sy2s populations using physical torus models of X-ray
reflected emission, we find that this difference can be explained by means of
different average inclination angles with respect to the torus, as predicted by
the unified model. Alternative explanations include differences in the
intensities of Compton humps or in the photon index distributions. We show that
the ratio between the flux of the broad and narrow Fe K$\alpha$ line in the
6.35-6.45 keV range depends on the torus geometry considered, and is on average
$<25\%$ and $<15\%$ for type I and type II AGN, respectively. We find evidence
of absorption of the narrow Fe K$\alpha$ line flux in Compton-thick AGN, which
suggests that part of the reflecting material is obscured. We estimate that on
average in obscured AGN the reflected radiation from neutral material is seen
through a column density which is 1/4 of that absorbing the primary X-ray
emission. This should be taken into account in synthesis models of the CXB and
when studying the luminosity function of heavily obscured AGN. We detect the
first evidence of the X-ray Baldwin effect in Seyfert 2s, with the same slope
as that found for Seyfert 1s, which suggests that the mechanism responsible for
the decrease of the equivalent width with the continuum luminosity is the same
in the two classes of objects.
