The ROSAT Brightest Cluster Sample - IV. The extended sample

We present a low-flux extension of the X-ray-selected ROSAT Brightest Clust er Sample (BCS) published in Paper I of this series. Lice the original BCS and employing an identical selection procedure, the BCS extension is compil ed from ROSAT All-Sky Survey (RASS) data in the northern hemisphere (delta greater than or equal to 0 degrees) and at high Galactic latitudes (\b\ gre ater than or equal to 20 degrees). It comprises 99 X-ray-selected clusters of galaxies with measured redshifts z less than or equal to 0.3 (as well as eight more at z > 0.3) and total fluxes between 2.8 x 10(-12) and 4.4 x 10 (-12) erg cm(-2) s(-1) in the 0.1-2.4 keV band (the latter value being the flux limit of the original BCS). The extension can be combined with the mai n sample published in 1998 to form the homogeneously selected extended BCS (eBCS), the largest and statistically best understood cluster sample to eme rge from the PASS to date. The nominal completeness of the combined sample (defined with respect to a power-law fit to the bright end of the BCS logN-logS distribution) is relat ively low at 75 per cent (compared with 90 per cent for the high-flux sampl e of Paper I). However, just as for the original BCS, this incompleteness c an be accurately quantified, and thus statistically corrected for, as a fun ction of X-ray luminosity and redshift. In addition to its importance for improved statistical studies of the prope rties of clusters in the local Universe, the low-flux extension of the BCS is also intended to serve as a finding list for X-ray-bright clusters in th e northern hemisphere which we hope will prove useful in the preparation of cluster observations with the next generation of X-ray telescopes such as Chandra and XMM-Newton. An electronic version of the eBCS can be obtained from the following URL: h ttp ://www.ifa.hawaii.edu/similar to ebeling/clusters/BCS.html.