3426$ [@Croom:2002vn]. The slope of the Baldwin effect for most of these lines has been shown to be steeper for those originating from higher ionization species (e.g., [@Dietrich:2002fk]). The origin of the Baldwin effect is still unknown, although several possible explanations have been put forward, and it might be different for lines originating in different regions of the AGN. For the lines produced in the BLR it might be related to the lower ionization and photoelectric heating in the BLR gas of more luminous objects (e.g., [@Netzer:1992fk]).

In the X-rays, @Iwasawa:1993ys using [*Ginga*]{} observations of 37 AGN, found the first evidence of an anti-correlation between the equivalent width of the iron K$\alpha$ line and the $2-10\rm\,keV$ luminosity ($EW \propto L_{\,\mathrm{X}}^{-0.20\pm0.04}$). This trend is usually called the [*X-ray Baldwin effect*]{} or the [*Iwasawa-Taniguchi effect*]{}. Using [*ASCA*]{} observations, @Nandra:1997ve confirmed the existence of such an anti-correlation, and argued that most of the effect could be explained by variations of the broad component of the iron K$\alpha$ line with the luminosity. The advent of [*XMM-Newton*]{}, [*Chandra*]{} and [*Suzaku*]{} made however clear that most of the observed X-ray Baldwin effect is due to the narrow core of the iron K$\alpha$ line (e.g., [@Page:2004kx], [@Shu:2010zr], 