The WGA Catalog of ROSAT Sources

The large field of view of the ROSAT PSPC made it an ideal instrument to find a large number of serendipitous X-ray sources. WGACAT is a catalog generated from all the ROSAT PSPC pointed observations available in the HEASARC public archive. The catalog resulted from an independent research effort by HEASARC staff (White and Angelini) and BeppoSAX Science Data Center staff (Giommi) with the aim of creating a catalog of sources detected by ROSAT in its pointed phase. The goals were to: 1) identify the detected sources, 2) ensure their timely observation by currently active X-ray missions e.g., ASCA, BeppoSAX and RXTE, 3) to search for objects which show exceptional time variability and spectral properties, and 4) to provide an independent check of the detection technique used in the official ROSAT project (SASS) processing. The catalog was created using the source detection algorithm within XIMAGE on all the public ROSAT PSPC pointed data available in the summer of 1994 (~70%) of the entire set. A total of ~62,000 unique sources were listed in the first version of WGACAT, made publicly available in November 1994 through the HEASARC on-line service (White et al. 1994, BAAS 185, 41.11).

A comparison with the project-generated ROSAT catalog reveals that WGACAT is much more sensitive in the outer area of the PSPC (where much more sky coverage is contained) and slightly less sensitive in the central regions. So the two catalogs are very complementary. In the past year the remaining PSPC fields have been processed and an updated version of the catalog will be released in May 2000. This update also includes a comprehensive quality check of the validity of the sources found. The updated WGACAT contains 88,000 sources and was made possible thanks to an ADP grant awarded in 1998. The ultimate aim of this project (ADP PI L. Angelini) is to provide a unified object-based catalog of X-ray sources (called XCAT) that merges all the ROSAT, EXOSAT, ASCA and Einstein catalogs.

The richness of WGACAT has stimulated many research projects and resulted in more than 40 papers in the Ap J. Singh et al. (1995, ApJ 455, 456) identified a sample of very soft X-ray sources that included white dwarfs, cataclysmic variables (Szkody et al. 1995, ApJ 455, L43), stars and a group of sources that are probably Seyferts I with a very step spectrum (Singh & Laurence 2000, MNRAS 312, 308). Israel et al. (1997, ApJ 484, L141; 1997, ApJ 474, L53; 1998, MNRAS 298, 502) discovered a number of X-ray pulsators by searching the WGACAT lightcurves. Fiore et al. (1998, ApJ 492, 79), Elvis et al. (1998, ApJ 492, 91), and Padovani et al. (1997, MNRAS, 286, 415) used WGACAT to shown: 1) radio-loud quasars commonly exhibit low-energy X-ray cutoffs that increase with redshift rather than with luminosity, indicating evolution with cosmic epoch, and 2) X-ray emission of the flat-spectrum radio quasars and BL Lacs is generated by the same mechanism but occurs in radio galaxies of lower power.

Gamma Ray Bursts
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