A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: The Catalog and log N-log S

We describe the selection of an X-ray flux-limited sample of bright clusters of galaxies in the southern hemisphere, based on the first analysis of the ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an identification of candidate clusters in RASS1, after which their X-ray fluxes are remeasured using the steepness-ratio technique. This method is better suited than the RASS1 standard algorithm to measuring flux from extended sources. The final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so that as a result of the distribution of NH, its effective flux limit varies between ~3 and 4×10−12 ergs cm-2 s-1 over the selected area. This covers the δ<2.°5 part of the south Galactic cap region (b<-20°), excluding patches of low RASS1 exposure time and of the Magellanic Clouds area, for a total of 8235 deg2. One hundred and thirty candidate sources fulfill our selection criteria for bona fide clusters of galaxies in this area. Of these, 101 are Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of these clusters, 126 (97%) have a redshift, and for these we compute an X-ray luminosity. 20% of the cluster redshifts come from new observations, as part of the ESO Key Program survey of RASS clusters that is currently under completion. Considering the intrinsic biases and incompletenesses introduced by the flux selection and source identification processes, we estimate the overall completeness to be better than 90%. The observed number count distribution, log N-log S, is well fitted by a power law with slope α=1.34±0.15 and normalization A=11.87±1.04 sr-1 (10-11 ergs cm-2 s-1)α, in good agreement with other measurements.