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 Kline 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 th is mechanism is able to explain the observed X-ray Baldwin effect. Fitting the simulated data with a log-linear L 2−10 keV− EW relation, we found that in the Seyfert regime (L 2−10 keV≤ 10 44.2 erg s −1 ) luminosity- dependent unification produces a slope consistent with the o bservations for average values of the equatorial column densities of the torus of log N T H & 23.1, and can reproduce both the slope and the intercept for log N T H ≃ 23.2. Lower values of N T H are obtained considering the decrease of the covering factor of the torus with the luminosity extrapolated from IR observations (22.9. log N T. 23). 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 T. 24.2.