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 (λEdd). Using Chandra/High Energy Grating results of Shu et al. and bolometric corrections we study the relation between EW and λEdd, and find that log EW = ( − 0.13 ± 0.03)log λEdd + 1.47. We explore the role of the known positive correlation between the photon index of the primary X-ray continuum Γ and λEdd 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–λEdd correlation cannot account for the whole X-ray Baldwin effect, unless a strong dependence of Γ on λEdd, 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–λEdd relation is critical to assess whether the trend plays a leading or rather a marginal role in the X-ray Baldwin effect.