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 primarily to measure the low-luminosity, high-redshift, X-ray luminosity function of galaxy clusters and groups. The approach we have chosen for source detection (Voronoi Tessellation and Percolation [VTP]) represents a significant advance over conventional methods, and it is particularly 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 much as 15%. In an extensive optical follow-up program, we are identifying 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 its calibration using simulated and real data. We present first results for an initial 91 fields (17.2 deg(2)) at detected fluxes greater than 3.5 x 10(-14) ergs s(-1) cm(-2) (the WARPS-I survey). We find the sky 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 demonstrates that our VTP approach typically finds 1-2 more objects deg(-2) to this detected flux limit, suggesting that the conventional method fails to detect a significant fraction of extended objects. The surface brightness limit of the WARPS cluster survey is similar to 1 x 10(-15) ergs s(-1) cm(-2) arcmin(-2), approximately 6 times lower than the Extended Medium Sensitivity Survey (EMSS). The WARPS Log N-Log S (which currently represents a lower limit) shows a significant excess over previous 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 extended sources as well as new detections of low surface brightness systems in the WARPS.