The role of the Gamma-Eddington ratio relation on the X-ray Baldwin effect in Active Galactic Nuclei

TL;DR

This study investigates how the Gamma-Eddington ratio relation influences the X-ray Baldwin effect in AGN, finding that the correlation's slope is crucial to understanding its role in the observed inverse relationship between iron line EW and luminosity.

Contribution

It provides a detailed analysis of the Gamma-Eddington ratio relation's impact on the X-ray Baldwin effect through simulations considering different geometries of reflecting material.

Findings

Gamma-lambda_Edd correlation alone cannot fully explain the Baldwin effect.

No significant correlation found between EW and Gamma.

The slope of the Gamma-lambda_Edd relation is key to its influence on the Baldwin effect.

Abstract

The X-ray Baldwin effect is the inverse correlation between the equivalent width (EW) of the narrow component of the iron Kalpha line and the X-ray luminosity of active galactic nuclei (AGN). A similar trend has also been observed between Fe Kalpha EW and the Eddington ratio (lambda_Edd). Using Chandra/HEG results of Shu et al. (2010) and bolometric corrections we study the relation between EW and the 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 Kalpha line emitted by populations of unabsorbed AGN considering 3 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. (2009) and Jin et al. (2012), 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.