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 Angstrom in the soft X-ray spectra of active galactic nuclei AZ (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 greater than or equal to 4), as well as the associated photoionization edges, may also be relevant to AGN spectra.