The Kα complex of He-like iron with dielectronic satellites

It is shown that the dielectronic satellites (DES) dominate X-ray spectral formation in the 6.7-keV K alpha complex of Fe XXV at temperatures below that of maximum abundance in collisional ionization equilibrium T-m. Owing to their extreme temperature sensitivity, the DES are excellent spectral diagnostics for T < T-m in photoionized, collisional or hybrid plasmas; whereas the forbidden, intercombination and resonance lines of Fe XXV are not. A diagnostic line ratio GD(T) is defined including the DES and the lines, with parameters from new relativistic atomic calculations. The DES absorption resonance strengths may be obtained from differential oscillator strengths, possibly to yield the Fe XXIV/Fe XXV column densities. The DES contribution to highly ionized Fe should be of interest for models of redward broadening of K<alpha> features, ionized accretion discs, accretion flows and Ka temporal-temperature variability in AGN.