Narrow Iron Kα Lines in Active Galactic Nuclei: Evolving Populations?

Abstract

We assemble a sample consisting of 66 active galactic nuclei (AGNs) from the literature and from the XMM-Newton archive in order to investigate the origin of the 6.4 keV narrow iron Kα line (NIKAL). The X-ray Baldwin effect of the NIKAL is confirmed in this sample. We find that the equivalent width (EW) of the NIKAL is more strongly inversely correlated with the Eddington ratio (E) than with the 2-10 keV X-ray luminosity. Our sample favors the dusty torus origin, with the X-ray Baldwin effect being caused by the changing opening angle of the dusty torus. The EW-E relation can be derived from a toy model of the dusty torus. If the unification scheme is valid in all AGNs, we can derive the Baldwin effect from the ratio of type II AGNs to the total population given by Chandra and Hubble Space Telescope deep surveys. Thus, the evolution of populations could be reflected in the NIKAL's Baldwin effect.