A catalog of 203 galaxy clusters serendipitously detected in the ROSAT PSPC pointed observations

We present a catalog of 203 clusters of galaxies serendipitously detected i n 647 ROSAT PSPC high Galactic latitude pointings covering 158 deg(2). This is one of the largest X-ray-selected cluster samples, comparable in size o nly to the ROSAT All-Sky Survey sample of nearby clusters (Ebeling et al. 1 997). We detect clusters in the inner 17.'5 of the ROSAT PSPC field of view using the spatial extent of their X-ray emission. Fluxes of detected clust ers range from 1.6 x 10(-14) to 8 x 10(-12) ergs s(-1) cm(-2) in the 0.5-2 keV energy band. X-ray luminosities range from 10(42) ergs s(-1), correspon ding to very poor groups, to similar to 5 x 10(44) ergs s(-1), correspondin g to rich clusters. The cluster redshifts range from z = 0.015 to z > 0.5. The catalog lists X-ray fluxes, core radii, and spectroscopic redshifts for 73 clusters and photometric redshifts for the remainder. Our detection met hod, optimized for finding extended sources in the presence of source confu sion, is described in detail. Selection effects necessary for a statistical analysis of the cluster sample are comprehensively studied by Monte Carlo simulations. We have optically confirmed 203 of 223 X-ray sources as cluste rs of galaxies. Of the remaining 20 sources, 19 are likely false detections arising from blends of unresolved point X-ray sources. Optical identificat ions of the remaining object are hampered by a bright nearby star. Above a flux of 2 x 10(-13) ergs s(-1) cm(-2), 98% of extended X-ray sources are op tically confirmed clusters. The number of false detections and their flux d istribution are in perfect agreement with simulations. The log N-log S rela tion for clusters derived from our catalog shows excellent agreement with c ounts of bright clusters derived from the Einstein Extended Medium Sensitiv ity Survey and ROSAT All-Sky Survey. At fainter fluxes, our log N-log S rel ation agrees with the smaller area WARPS survey. Our cluster counts appear to be systematically higher than those from a 50 deg(2) survey by Rosati et al. In particular, at a flux of 2 x 10(-13) ergs s(-1) cm(-2), we find a s urface density of clusters of 0.57 +/- 0.07 deg(-2), which is a factor of 1 .3 more than was found-by Rosati et al. This difference is marginally signi ficant at the similar to 2 sigma level. The large area of our survey makes it possible to study the evolution of the X-ray luminosity function in the high luminosity range inaccessible with other, smaller area ROSAT surveys.