DETECTION STATISTICS OF ABELL AND ACO CLUSTERS OF GALAXIES IN THE ROSAT ALL-SKY SURVEY

The X-ray emission of the clusters of galaxies compiled by Abell and A bell, Corwin and Olowin (ACO) is investigated utilizing the ROSAT XRT' s All-Sky Survey (RASS). X-ray confirmed subsamples of different stati stical quality are extracted by setting thresholds in the optical-X-ra y source separation obtained in positional cross correlations of the o ptical and the X-ray sample in order to discriminate against chance co incidences. The analysis is performed for an X-ray sample of almost un iform exposure corresponding to a flux limit of about 7 . 10(-13) erg/ cm2sec in the ROSAT energy range between 0.1 and 2.4 keV. For both the Abell and the ACO catalogue the chance of detecting a rich (Abell ric hness class greater-than-or-equal-to 1) cluster as a RASS source of 22 % (for a sample almost free of chance coincidences) is about two to th ree times higher than for a nominal poor cluster. From the distance de pendence of the RASS detection rate we conclude that Abell's and ACO's main compilation might be affected by selection effects and thus inco mplete already at redshifts as low as z almost-equal-to 0. 14. We also investigated ACO's supplementary sample, the RASS-detection distribut ions for which we found, surprisingly, to show many similarities to th e ones for regular clusters in the Abell and ACO catalogues at only sl ightly lower X-ray detection rates.