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-1

We describe the selection of an X-ray flux-limited sample of bright cluster s of galaxies in the southern hemisphere, based on the first analysis of th e ROSAT All-Sky Survey data (RASS1). The sample is constructed starting fro m 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 exte nded 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 N-H, its effecti ve flux limit varies between similar to 3 and 4 x 10(-12) ergs cm(-2) s(-1) over the selected area. This covers the delta < 2 degrees.5 part of the so uth Galactic cap region (b < -20 degrees), excluding patches of low RASS1 e xposure time and of the Magellanic Clouds area, for a total of 8235 deg(2). One hundred and thirty candidate sources fulfill our selection criteria fo r 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 cl usters, 126 (97%) have a redshift, and for these we compute an X-ray lumino sity. 20% of the cluster redshifts come from new observations, as part of t he ESO Key Program survey of RASS clusters that is currently under completi on. Considering the intrinsic biases and incompletenesses introduced by the flux selection and source identification processes, we estimate the overal l completeness to be better than 90%. The observed number count distributio n, log N-log S, is well fitted by a power law with slope alpha = 1.34 +/- 0 .15 and normalization A = 11.87 +/- 1.04 sr(-1) (10(-11) ergs cm(-2) s(-1)) (alpha), in good agreement with other measurements.