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B. Margon, S. F. Anderson, P. Szkody (Univ. Washington), W. Voges, J. Trümper, Th. Boller (MPE), J. Annis (FNAL), R. C. Nichol (CMU), N. A. Bahcall, J. E. Gunn, R. S. J. Kim, M. A. Strauss (Princeton), F. J. Castander (Obs. Midi-Pyr), J. Brinkmann (APO), SDSS Collaboration
Many problems in X-ray astronomy are limited by the relatively small number of objects in uniform optically-identified samples, especially when rare subclasses are considered, or subsets isolated to search for evolution or correlations between wavebands. We describe a new program aimed to yield ~104 fully characterized X-ray source identifications---a sample about an order of magnitude larger than earlier efforts. The technique employs X-ray data from the ROSAT All Sky Survey (RASS), and optical IDs from the Sloan Digital Sky Survey (SDSS); the two surveys are very well-matched in sensitivity. Optical objects in the SDSS photometric catalogs are positionally cross-correlated with RASS X-ray sources. Priorities for follow-on SDSS optical spectra of candidate counterparts are automatically assigned, using an algorithm based on the known ratios of fx/fopt for various classes of X-ray emitters, and with SDSS photometric parameters for optical morphology, magnitude, colors, etc. (plus FIRST radio information) serving as proxies for object class.
During SDSS commissioning, this approach has already yielded hundreds of plausible optical IDs (some recoveries, but most new) for RASS sources; all have uniform RASS X-ray data, accurate 5-color optical SDSS photometry, and high quality SDSS optical spectroscopy. Identifications are found for a range of extragalactic and Galactic classes, including AGN, clusters of galaxies, galaxies with radio or optical emission, bright galaxies, bright normal stars, dMe's, CV's etc. The SDSS/RASS approach is demonstrated to be especially efficient for AGN. We present early results emphasizing a sample of ~102.5 such RASS/SDSS-selected quasars, Seyferts, and BL~Lacs, obtained from an initial area encompassing only a few percent of the ultimate joint SDSS/RASS sky coverage.