An X-ray Baldwin effect for the narrow Fe Kα lines observed in active galactic nuclei

Abstract

The majority of active galactic nuclei (AGN) observed by XMM–Newton reveal narrow Fe Kα lines at ∼6.4 keV, due to emission from cold (neutral) material. There is an X-ray Baldwin effect in type I AGN, in that the equivalent width (EW) of the line decreases with increasing luminosity, with weighted linear regression giving EW∝ L[superscript −0.17±0.08] (Spearman Rank probability of >99.9 per cent).With current instrumental capabilities it is not possible to determine the precise origin for the narrow line, with both the broad-line region and putative molecular torus being possibilities. A possible explanation for the X-ray Baldwin effect is a decrease in the covering factor of the material forming the fluorescence line