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 , i.e. , where the cooling time is less than the age of the cluster , in cooling flow clusters can not be in simple steady-state , i.e. , \dot { M } is not a constant at all temperatures .
Based on the measured gas emissivity , the gas can only be in steady-state if there exists a steady heating mechanism that scales as H ( T ) \propto T ^ { \alpha } where \alpha = 1 - 2 .
That is , a heating mechanism that preferentially targets the hottest and highest entropy gas , which seems very unlikely .
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 .
Such a temperature dependent \dot { M } is required by the XMM-Newton RGS data and will produce an increasing amount of intermediate temperature gas which will then be reheated during the next heating cycle .
We show the compatibility of this model for the cooling flow cluster A2052 .
The present paper strengthens the moderate cooling flow model , which can accommodate the unique activities observed in cooling flow clusters .