THE WIDE-ANGLE ROSAT POINTED X-RAY SURVEY OF GALAXIES, GROUPS, AND CLUSTERS .1. METHOD AND FIRST RESULTS

We have embarked on a survey of ROSAT PSPC archival data with the aim of detecting all significant surface brightness enhancements resulting from sources in the innermost R less than or equal to 15' of the PSPC field of view in the energy band 0.5-2.0 keV. This project is part of the Wide-Angle ROSAT Pointed Survey (WARPS), and it is designed prima rily to measure the low-luminosity, high-redshift, X-ray luminosity fu nction of galaxy clusters and groups. The approach we have chosen for source detection (Voronoi Tessellation and Percolation [VTP]) represen ts a significant advance over conventional methods, and it is particul arly suited for the detection and accurate quantification of extended and/or low surface brightness emission that could otherwise be missed or wrongly interpreted. We also use energy-dependent exposure maps to estimate the fluxes of sources that can amount to corrections of as mu ch as 15%. In an extensive optical follow-up program, we are identifyi ng galaxies, groups, and clusters at redshifts ranging from z similar to 0.1 to z similar to 0.7. In this paper, we present our method and i ts calibration using simulated and real data. We present first results for an initial 91 fields (17.2 deg(2)) at detected fluxes greater tha n 3.5 x 10(-14) ergs s(-1) cm(-2) (the WARPS-I survey). We find the sk y density of extended objects to be in the range 2.8-4.0 (+/- 0.4) deg (-2). A comparison with a point-source detection algorithm demonstrate s that our VTP approach typically finds 1-2 more objects deg(-2) to th is detected flux limit, suggesting that the conventional method fails to detect a significant fraction of extended objects. The surface brig htness limit of the WARPS cluster survey is similar to 1 x 10(-15) erg s s(-1) cm(-2) arcmin(-2), approximately 6 times lower than the Extend ed Medium Sensitivity Survey (EMSS). The WARPS Log N-Log S (which curr ently represents a lower limit) shows a significant excess over previo us measurements for S greater than or similar to 8 x 10(-14) ergs s(-1 ) cm(-2). We attribute this mainly to a larger measured flux from exte nded sources as well as new detections of low surface brightness syste ms in the WARPS.