  We examine recent developments in the cluster cooling flow scenario following
recent observations by Chandra and XMM-Newton. We show that the distribution of
gas emissivity verses temperature determined by XMM-Newton gratings
observations demonstrates that the central gas in cooling flow clusters cannot
be in simple steady-state. Combining this result with the lack of spectroscopic
evidence for gas below one-third of the ambient cluster temperature is strong
evidence that the gas is heated intermittently. While the old steady-state
isobaric cooling flow model is incompatible with recent observations, a
"moderate cooling flow model", in which the gas undergoes intermittent heating
that effectively reduces the age of a cooling flow is consistent with
observations. Most of the gas within cooling flows resides in the hottest gas,
which is prevented from cooling continuously and attaining a steady-state
configuration. This results in a mass cooling rate that decreases with
decreasing temperature, with a much lower mass cooling rate at the lowest
temperatures. The present paper strengthens the moderate cooling flow model,
which can accommodate the unique activities observed in cooling flow clusters.
