Abstract
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.
Highlights
There are two main observational signatures of reprocessed radiation in the X-ray spectra of active galactic nuclei (AGN): a Compton hump peaking around 30 keV and a fluorescent iron Kα line
The existence of an anti-correlation between the equivalent width (EW) of the narrow core of the iron Kα line and the luminosity of the continuum in type I active galactic nuclei has been confirmed in recent years by several studies carried out with XMM-Newton, Chandra and Suzaku
Several works have proposed the decrease of the covering factor of the molecular torus with the luminosity as a possible explanation for the X-ray Baldwin effect
Summary
There are two main observational signatures of reprocessed radiation in the X-ray spectra of active galactic nuclei (AGN): a Compton hump peaking around 30 keV and a fluorescent iron Kα line. While the broad component is observed in only ∼35–45% of bright nearby AGN (e.g., de La Calle Pérez et al 2010), the narrow (FWHM ∼ 2000 km s−1, Shu et al 2010) core of the iron line has been found to be almost ubiquitous (e.g., Nandra et al 2007; Singh et al 2011) This component peaks at 6.4 keV (e.g.,Yaqoob & Padmanabhan 2004), which points to the line being produced in cold neutral material. Chandra/HEG observations (Shu et al 2010) have shown that the normalization of the X-ray Baldwin effect is lower (i.e., the average EW of the narrow Fe Kα component is smaller) than that obtained by previous works performed using XMM-Newton.
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