Einstein Observatory Images of Clusters of Galaxies

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 z<0.1, demonstrating that the large majority of these optically selected clusters are not the results of chance superpositions. For 163 clusters, we measure their X-ray surface brightness profiles and determine their core radii. For ~230 clusters, we then use either our measured core radii and β values, or mean values derived for this sample, to measure central gas densities and gas masses. We use estimated or measured cluster gas temperatures, along with the derived gas-density profiles, to estimate total cluster masses, under the assumptions that the gas is isothermal and in hydrostatic equilibrium. We also present contour plots of the X-ray emission, which we use to classify the cluster morphology. We find the percentage of clusters with substructure in their X-ray images is about 40%, with no significant change in this percentage as a function of X-ray luminosity. This implies that a large fraction of all present epoch clusters are still undergoing subcluster mergers. Based on our analysis of surface brightness profiles, we find that most clusters have core radii in the range from 0.1 to 0.3 Mpc (for H₀=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.