Properties of the X-ray brightest Abell-type clusters of galaxies   (XBACs) from ROSAT All-Sky Survey data --- I. 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 ($|b| \geq 20^{\circ}$), the nominal redshift range of the ACO catalogue of $z \leq 0.2$, and X-ray fluxes above $5.0 \times 10^{-12}$ erg cm$^{-2}$ s$^{-1}$ in the 0.1 -- 2.4 keV band. Due 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. 
H. Ebeling, W. Voges, H. Boehringer, A.C. Edge, J.P. Huchra, U.G.   Briel