Excess Ionization and Soft X-ray Emission from Cooling Flow Clusters
  X-ray spectroscopy of cooling-flow clusters reveal an unexpected deficit of soft X-ray emission lines from gas at ~1/3 of the ambient plasma temperature, across a wide range of X-ray luminosities and virial temperatures. We propose excess ionization from either a population of suprathermal electrons or photoionization by X-ray continuum emission from hot gas or a central AGN as a means of decoupling the thermal state of the gas from its emission line spectrum. The former effect is thought to operate in the solar corona. Because they generally become important at some fixed fraction of the cluster gas temperature, such mechanisms could in principle explain both the universality and temperature dependence of the emission line suppression, properties which none of the present-day models based on gas heating can explain. Ultimately these models cannot explain the observations; however, they have attractive and robust features which could be useful in elucidating a final solution to the soft X-ray deficit. 
S. Peng Oh