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

The majority of active galactic nuclei (AGN) observed by XMM-Newton reveal narrow Fe Ka 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 -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.