  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.
