Luminosity-dependent unification of Active Galactic Nuclei and the X-ray
  Baldwin effect
  The existence of an anti-correlation between the equivalent width (EW) of the
narrow core of the iron Kalpha line and the luminosity of the continuum (i.e.
the X-ray Baldwin effect) in type-I active galactic nuclei has been confirmed
over the last years by several studies carried out with XMM-Newton, Chandra and
Suzaku. However, so far no general consensus on the origin of this trend has
been reached. Several works have proposed the decrease of the covering factor
of the molecular torus with the luminosity (in the framework of the
luminosity-dependent unification models) as a possible explanation for the
X-ray Baldwin effect. Using the fraction of obscured sources measured by recent
X-ray and IR surveys as a proxy of the half-opening angle of the torus, and the
recent Monte-Carlo simulations of the X-ray radiation reprocessed by a
structure with a spherical-toroidal geometry by Ikeda et al. (2009) and
Brightman & Nandra (2011), we test the hypothesis that the X-ray Baldwin effect
is related to the decrease of the half-opening angle of the torus with the
luminosity. Simulating the spectra of an unabsorbed population with a
luminosity-dependent covering factor of the torus as predicted by recent X-ray
surveys, we find that this mechanism is able to explain the observed X-ray
Baldwin effect. Fitting the simulated data with a log-linear L_{2-10keV}-EW
relation, we found that in the Seyfert regime (L_{2-10keV}< 10^44.2 erg s^-1)
luminosity-dependent unification produces a slope consistent with the
observations for average values of the equatorial column densities of the torus
of log N_H^T > 23.1. In the quasar regime (L_{2-10 keV}> 10^44.2 erg s^-1) a
decrease of the covering factor of the torus with the luminosity slower than
that observed in the Seyfert regime (as found by recent hard X-ray surveys) is
able to reproduce the observations for 23.2 < log N_H^T < 24.2.
