Properties of the X-ray-brightest Abell-type clusters of galaxies (XBACs) from ROSAT all-sky survey data .1. The sample

We present an essentially complete, all-sky, X-ray flux-limited sample of 242 Abell clusters of galaxies (six of which are double) compiled from ROSAT All-Sky Survey data. Our sample is uncontaminated in the sense that systems featuring prominent X-ray point sources such as AGN or foreground stars have been removed. The sample is limited to high Galactic latitudes (\b\ greater than or equal to 20 degrees), the nominal redshift range of the ACO catalogue oft less than or equal to 0.2, and X-ray fluxes above 5.0 x 10(-12) erg cm(-2) s(-1) in the 0.1-2.4 keV band. Owing to the X-ray flux limit, our sample consists, at intermediate and high redshifts, exclusively of very X-ray-luminous clusters. Since the latter tend to be also optically rich, the sample is not affected by the optical selection effects and, in particular, not by the volume incompleteness known to be present in the Abell and ACO catalogues for richness class 0 and 1 clusters. Our sample is the largest X-ray flux-limited sample of galaxy clusters compiled to date and will allow investigations of unprecedented statistical quality into the properties and distribution of rich clusters in the local Universe.