2001MNRAS.327L..42O

It is shown that the dielectronic satellites (DES) dominate X-ray spectral formation in the 6.7-keV Kα complex of Fexxv 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 Fexxv 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 Kα features, ionized accretion discs, accretion flows and Kα temporal-temperature variability in AGN.