ROSAT PSPC Observations of the Richest (R ⩾ 2) ACO Clusters

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Abstract
We have compiled an X-ray catalog of optically selected rich clusters of galaxies observed by the Position Sensitive Proportional Counter (PSPC) during the pointed GO phase of the ROSAT mission. This paper contains a systematic X-ray analysis of 150 clusters with an optical richness classification of R ≥ 2 from the Abell, Corwin, & Olowin (ACO) catalog. All clusters were observed within 45' of the optical axis of the telescope during pointed PSPC observations. For each cluster, we calculate: the net 0.5-2.0 keV PSPC count rate (or 4 σ upper limit) in a 1 Mpc radius aperture, 0.5-2.0 keV flux and luminosity, bolometric luminosity, and X-ray centroid. The cluster sample is then used to examine correlations between the X-ray and optical properties of clusters, derive the X-ray luminosity function of clusters with different optical classifications, and obtain a quantitative estimate of contamination (i.e., the fraction of clusters with an optical richness significantly overestimated due to interloping galaxies) in the ACO catalog. Because of the large field of view of the PSPC, many rich clusters were serendipitously observed during the GO phase of the ROSAT mission. Of the 150 clusters in our sample, 82 were observed serendipitously and 68 were targeted observations. The overall detection rate of serendipitously observed clusters is quite high at 76%. However, the detection rate is sensitive to the optical properties of clusters, and the details of the optical selection process. For example, all serendipitously observed Bautz-Morgan type I and type I-II clusters are detected, while only 71% of Bautz-Morgan type II, type II-III, and type III clusters are detected. Beyond z ≈ 0.1, 83% of the observed Abell clusters are detected, compared to only 60% of southern ACO clusters. Due to the long integration times in pointed PSPC observations, the typical X-ray luminosity threshold for detection is quite low at ~ 1043 ergs s-1 for clusters within z = 0.2. This luminosity is more characteristic of an X-ray luminous group rather than a rich cluster. The nondetected clusters must therefore be either highly unrelaxed systems or have an optical richness that is significantly overestimated due to contamination by interloping galaxies. We show that the later possibility is more likely, since low X-ray luminosity clusters are preferentially found in denser cluster environments compared with X-ray luminous clusters. This contrast in cluster environment makes low X-ray luminosity clusters more susceptible to galaxy contamination than luminous systems. The fraction of clusters less luminous than an X-ray luminous group is thus a direct quantitative measure of contamination in the ACO catalog. We also find that the X-ray luminosity function of Abell clusters is not consistent with that of southern ACO clusters. Only by comparing R ≥ 2 Abell clusters with R ≥ 3 southern ACO clusters can we obtain consistency in their X-ray properties. This indicates that the optical richness of southern ACO clusters is overestimated by about 1 richness class.
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