The large two-degree field of view of the ROSAT PSPC made it ideal as a survey instrument. Over the entire four-year pointed phase of the ROSAT program, approximately 18% of the sky was covered to varying degrees of sensitivity. While the sky coverage is not complete, each observation had an exposure typically a factor of 100 longer than that achieved during the six-month ROSAT all-sky survey. The ROSAT PSPC pointed data is now all in the public domain and provides a valuable resource for discovering new interesting X-ray sources and undertaking survey programs. The value of such X-ray survey programs to provide insight into the nature of source populations has been demonstrated from surveys made using Einstein and EXOSAT to investigate, e.g., extragalactic populations (Maccacaro et al. 1988, ApJ, 326,680; Giommi et al. 1991, ApJ, 378,77) and stellar coronae (Pallavicini et al. 1981, ApJ, 248,279), to name just two. The increased sensitivity of ROSAT provided a major new capability for these studies.
WGACAT is a point source catalog generated from all ROSAT PSPC pointed observations. This catalog has been generated by N. E. White (HEASARC/GSFC), P. Giommi (SDC/ASI), and L. Angelini (HEASARC/GSFC) and is a private research effort not related to the official catalogs generated by the ROSAT project.
The WGACAT was first made publicly available in November 1994 through the HEASARC Online Service, with a first minor revision in March 1995 (WGACAT95, hereafter White, Giommi, and Angelini, 1994, IAUCirc, 6100). It was generated using all public ROSAT PSPC pointed data then available, corresponding to 75% of the entire set, and featured ~68000 detections of which 62000 were unique sources. The current version, released in May 2000, is also the final and complete version of WGACAT. This includes the processing of the remaining sequences not included in the WGACAT95. WGACAT (May 2000) contains 88,000 detections, with more than 84,000 individual sources obtained from 4160 sequences.
The catalog was generated using an optimized sliding cell detect algorithm in XIMAGE (first developed for the EXOSAT project). The inner and outer parts of the images were run separately, to maximize the sensitivity to source detection. This method is very sensitive in finding point sources, but can also find spurious sources where there is extended emission. We have visually inspected each detection, removed the obvious spurious cases, and assigned a quality flag to each detection.