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 Kα line and the X-ray luminosity of active galactic nuclei (AGN). A similar trend has also been observed between Fe Kα EW and the Eddington ratio (λ_Edd). Using Chandra/High Energy Grating results of Shu et al. and bolometric corrections we study the relation between EW and λ_Edd, and find that log EW = ( - 0.13 ± 0.03)log λ_Edd + 1.47. We explore the role of the known positive correlation between the photon index of the primary X-ray continuum Γ and λ_Edd on the X-ray Baldwin effect. We simulate the iron Kα line emitted by populations of unabsorbed AGN considering three different geometries of the reflecting material: toroidal, spherical-toroidal and slab. We find that the Γ-λ_Edd correlation cannot account for the whole X-ray Baldwin effect, unless a strong dependence of Γ on λ_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 Γ. We conclude that a good understanding of the slope of the Γ-λ_Edd relation is critical to assess whether the trend plays a leading or rather a marginal role in the X-ray Baldwin effect.