X-Ray Emission Lines from Cooling Flows

Individual X-ray emission line strengths can be used to estimate the rate at which gas cools in the intracluster medium. The authors review the technique and show that departures from ionization equilibrium in the cooling plasma are not important at X-ray temperatures. They present data from 14 observations of 7 clusters performed with the Focal Plane Crystal Spectrometer on the Einstein Observatory, in addition to a brief review of earlier results on M87. The estimates of the cooling rate Mwithin the 3×30 arc min aperture are ≈4 M_sun;yr^-1 for M87, ≈20 - 30 M_sun;yr^-1 for Centaurus and ≈200 M_sun;yr^-1 for Perseus.