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X-Ray Emission from Clusters of Galaxies

We present a summary of X-ray spectral observations of ∼ 30 clusters of galaxies from HEAO-1. There exists strong correlations between X-ray luminosity, Lx, and temperature kT in the form Lx ∝ T2.3. This result combined with the Lx vs. central galaxy density relation and the viral theorem indicates that the core radius of the gas should be roughly independent of Lx or kT and that more luminous clusters have a greater fraction of their virial mass in gas. The poor correlation of kT with optical velocity dispersion, σv, seems to indicate that clusters have a variety of equations of state. There is poor agreement between X-ray imaging observations and optical and X-ray spectral measures of the polytropic index. Most clusters show Fe emission lines with a strong indication that they all have roughly 1/2 solar abundance. We discuss the evidence for cooling in the cores of several clusters based on spectral observations with the Einstein Solid State Spectrometer.