The Wide‐AngleROSATPointed X‐Ray Survey of Galaxies, Groups, and Clusters. I. Method and First Results

We have embarked on a survey of ROSAT PSPC archival data with the aim ofdetecting all significant surface brightness enhancements due to sources in theinnermost 15 arcmin of the PSPC field of view (0.5-2.0 keV). This project ispart of the Wide Angle ROSAT Pointed Survey (WARPS) and is designed primarilyto measure the low luminosity, high redshift, X-ray luminosity function ofgalaxy clusters and groups. The approach we have chosen for source detection[Voronoi Tessellation and Percolation (VTP)] represents a significant advanceover conventional methods and is particularly suited for the detection andaccurate quantification of extended and/or low surface brightness emission. Inan extensive optical follow-up programme we are identifying galaxies, groupsand clusters at redshifts ranging from z~0.1 to z~0.7. We present first resultsfor an initial 17.2 deg^2 at detected fluxes > 3.5 x 10^-14 erg s^-1 cm^-2. Wefind the sky density of extended objects to be in the range 2.8 to 4.0 (+- 0.4)deg^-2. A comparison with a point source detection algorithm demonstrates thatour VTP approach typically finds 1-2 more objects deg^-2 to this detected fluxlimit. The surface brightness limit of the WARPS cluster survey is ~1x10^-15erg sec^-1 cm^-2 arcmin^-2, approximately 6 times lower than the EMSS. TheWARPS LogN-LogS shows a significant excess over previous measurements for S > 8x 10^-14 erg sec^-1 cm^-2.