On the role of the Γ-λEdd relation on the X-ray Baldwin effect in active galactic nuclei

The X-ray Baldwin effect is the inverse correlation between the equivalent width (EW) of the narrow component of the iron K alpha line and the X-ray luminosity of active galactic nuclei (AGN). A similar trend has also been observed between Fe K alpha EW and the Eddington ratio (lambda(Edd)). Using Chandra/High Energy Grating results of Shu et al. and bolometric corrections we study the relation between EW and lambda(Edd), and find that log EW = ( - 0.13 +/- 0.03)log lambda(Edd) + 1.47. We explore the role of the known positive correlation between the photon index of the primary X-ray continuum Gamma and lambda(Edd) on the X-ray Baldwin effect. We simulate the iron K alpha line emitted by populations of unabsorbed AGN considering three different geometries of the reflecting material: toroidal, spherical-toroidal and slab. We find that the Gamma-lambda(Edd) correlation cannot account for the whole X-ray Baldwin effect, unless a strong dependence of Gamma on lambda(Edd), such as the one recently found by Risaliti et al. and Jin et al., is assumed. No clear correlation is found between EW and Gamma. We conclude that a good understanding of the slope of the Gamma-lambda(Edd) relation is critical to assess whether the trend plays a leading or rather a marginal role in the X-ray Baldwin effect.