Soft X-Ray Absorption by Fe0+ to Fe15+ in Active Galactic Nuclei

A full set of calculations is presented for inner-shell n = 2-3 photoexcitation of the 16 iron charge states: Fe0+ (Fe I) through Fe15+ (Fe XVI). The blend of the numerous absorption lines arising from these excitations (mainly 2p-3d) forms an unresolved transition array (UTA), which has been recently identified as a prominent feature between 16 and 17 Å in the soft X-ray spectra of active galactic nuclei (AGNs). Despite the blending within charge states, the ample separation between the individual-ion features enables precise diagnostics of the ionization range in the absorbing medium. Column density and turbulent velocity diagnostics are also possible, albeit to a lesser accuracy. An abbreviated set of atomic parameters useful for modeling the Fe 2p-3d UTA is given. It is shown that the effects of accompanying photoexcitation to higher levels (n ≥ 4), as well as the associated photoionization edges, may also be relevant to AGN spectra.