ROSAT PSPC observations of the richest (R >= 2) ACO clusters

We have compiled an X-ray catalog of optically selected rich clusters of ga laxies observed by the Position Sensitive Proportional Counter (PSPC) durin g the pointed GO phase of the ROSAT mission. This paper contains a systemat ic X-ray analysis of 150 clusters with an optical richness classification o f R greater than or equal to 2 from the Abell, Corwin, & Olowin (ACO) catal og. All clusters were observed within 45' of the optical axis of the telesc ope during pointed PSPC observations. For each cluster, we calculate: the n et 0.5-2.0 keV PSPC count rate (or 4 sigma upper limit) in a 1 Mpc radius a perture, 0.5-2.0 keV flux and luminosity, bolometric luminosity, and X-ray centroid. The cluster sample is then used to examine correlations between t he X-ray and optical properties of clusters, derive the X-ray luminosity fu nction of clusters with different optical classifications, and obtain a qua ntitative 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 held of view of the PSPC, many rich clusters were sere ndipitously observed during the GO phase of the ROSAT mission. Of the 150 c lusters in our sample, 82 were observed serendipitously and 68 were targete d observations. The overall detection rate of serendipitously observed clus ters is quite high at 76%. However, the detection rate is sensitive to the optical properties of clusters, and the details of the optical selection pr ocess. For example, all serendipitously observed Bautz-Morgan type I and ty pe I-II clusters are detected, while only 71% of Bautz-Morgan type II, type II-III, and type III clusters are detected. Beyond z approximate to 0.1, 8 3% of the observed Abell clusters are detected, compared to only 60% of sou thern ACO clusters. Dire to the long integration times in pointed PSPC obse rvations, the typical X-ray luminosity threshold for detection is quite low at similar to 10(43) ergs s(-1) for clusters within z = 0.2. This luminosi ty is more characteristic of an X-ray luminous group rather than a rich clu ster. The nondetected clusters must therefore be either highly unrelaxed sy stems or have an optical richness that is significantly overestimated due t o contamination by interloping galaxies. We show that the later possibility is more likely, since low X-ray luminosity clusters are preferentially fou nd in denser cluster environments compared with X-ray luminous clusters. Th is 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 quant itative 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 greater than or equal to 2 Abell clusters with R greater than or equal to 3 southern ACO clusters can we ob tain consistency in their X-ray properties. This indicates that the optical richness of southern ACO clusters is overestimated by about 1 richness cla ss.