X-ray emission lines from cooling flows

Techniques employing individual X-ray line strengths to estimate the rate at which gas cools in the intracluster medium are discussed. It is demonstrated that departures from ionization equilibrium in the cooling plasma are not important at X-ray temperatures. Nonrestrictive upper limits are obtained for four cooling flow clusters. The results favor the existence of substantial cooling flows and argue against the dominance of conduction or other heat sources.