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Effects of Departures from Ionization Equilibrium on Cooling-Flow X-Ray Spectra

X-ray spectra of galaxy cluster cooling cores have traditionally been modeled under the assumption that the cooling plasma is in ionization equilibrium. Departures from ionization equilibrium could affect the expected line-emission spectra from these objects and consequently the physical quantities inferred from such spectra. We have determined the elemental ionization fractions for six astrophysically abundant elements for temperatures from 104 to 108 K, assuming both equilibrium and nonequilibrium plasma models, and using cooling function values derived for both solar and one-third solar abundance. The magnitude of deviations from ionization equilibrium in a cooling plasma increases with decreasing atomic number: it is noticeable for C and O and becomes negligible for Si and S. The results of our recalculation of ionization fractions were used to derive X-ray emissivities for cluster cooling cores. Comparisons between luminosities produced by helium-like 2P1 → 1S0 transitions and hydrogen-like Lyα transitions indicate that nonequilibrium ionization significantly affects (by 10%-20%) only low-Z elements, and that the determination of the cooling rate via spectral analysis remains unchanged.