Einstein ObservatoryImages of Clusters of Galaxies

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Abstract
We present the X-ray characteristics of a sample of 368 clusters of galaxies with redshifts less than 0.2 observed with the Einstein Imaging Proportional Counter. For each cluster, we measure the 0.5-4.5 keV counting rate and compute the 0.5-4.5 keV source luminosity, as well as the bolometric luminosity within fixed metric radii. We detect 85% of Abell clusters with zH0=50 km s-1 Mpc-1), with more massive "single" clusters having larger core radii. The β values determined from the slope of the surface brightness profiles fall in the narrow range from 0.4 to 0.8, with β values increasing with cluster gas temperatures. No change in the value of β is found in the surface brightness profiles for individual clusters as a function of distance from the cluster center. We compare the β values derived from the surface brightness profiles with the corresponding β values calculated from the gas temperatures and cluster velocity dispersions. We argue that much of the discrepancy between the values of β derived from these two methods results from overestimates of the cluster velocity dispersion due to cluster substructure. Finally, we compare the gas mass to the cluster virial mass and find, for an isothermal gas, that, within a fixed metric radius of 1 Mpc, the gas mass fraction increases as a function of X-ray luminosity from 10% to 20% of the total cluster mass.