SIMBAD references

A significant correlation between the Fe Kα line energy and the X-ray spectral slope has been discovered among radio-quiet active galactic nuclei. The ionization stage of the bulk of the Fe Kα-emitting material is not the same in all active galactic nuclei and is related to the shape of the X-ray continua. Active galactic nuclei with a steep X-ray spectrum tend to have a fluorescence Fe Kα line from highly ionized material. In the narrow-line Seyfert 1 galaxies with a steeper X-ray spectrum (Γ_X≳2.1), the Fe Kα line originates from highly ionized material. In the Seyfert 1 galaxies and quasars with a flatter X-ray spectrum (Γ_X≲2.1), the bulk of the Fe Kα emission arises from near-neutral or weakly ionized material. The correlation is an important observational characteristic related to the accretion process in radio-quiet active galactic nuclei and is driven by a fundamental physical parameter that is likely to be the accretion rate relative to the Eddington rate.