a major role in the X-ray Baldwin effect, so far no quantitative estimation of this effect has been performed.

In this work we quantify for the first time the influence of the decrease of the covering factor of the torus with the luminosity on the equivalent width of the iron K$\alpha$ line. Using the Monte-Carlo spectral simulations of a torus with a variable half-opening angle ($\theta_{\mathrm{OA}}$) recently presented by @Ikeda:2009nx and @Brightman:2011oq, together with the most recent and comprehensive observations in different energy bands of the decrease of $f_{\mathrm{obs}}$ with the luminosity, we show that this mechanism is able to explain the X-ray Baldwin effect. The paper is organized as follows. In Sect.\[Sec:Simulations\] we present in detail our spectral simulations, and in Sect.\[sect:XBEslope\] we compare the slopes and intercepts obtained by our simulations with those measured by high sensitivity [*Chandra*]{}/HEG observations, as they provide the best energy resolution available to date. In Sect.\[Sec:discussion\] we discuss our results, and in Sect.\[Sec:summary\] we present our conclusions.

Simulations {#Sec:Simulations}
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The relation between $\theta_{\mathrm{OA}}$ and $L_{\mathrm{X}}$
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As a proxy of the relationship between the half-opening angle of the molecular torus $\theta_{\mathrm{OA}}$ (see Fig.\[fig:tor\_CFL1\]) and the intrinsic X-ray luminosity of AGN we used 