Detection of X-ray clusters of galaxies by matching RASS photons and SDSS galaxies within GAVO

A new method for a simultaneous search for clusters of galaxies in X-ray photon maps and optical galaxy maps is described. The merging of X-ray and optical data improves the source identification so that a large amount of telescope time for spectroscopic follow-up can be saved. The method appears thus ideally Suited for the analysis of the recently proposed wide-angle X-ray missions like DUO and ROSITA. As a first application, clusters are extracted from the 3rd version of the ROSAT All-Sky Survey and the Early Date Release of the Sloan Digital Sky Survey (SDSS). The time-consuming computations are performed within the German Astrophysical Virtual Observatory (GAVO). On a test area of 140 square degrees, 75 X-ray clusters are detected down to an X-ray flux limit of 3-5x10(-13) erg s(-1) cm(-2) in the ROSAT energy band 0.1-2.4 keV. The Clusters have redshifts zless than or equal to0.5. The survey thus fills the gap between traditional large-area X-ray surveys and serendipitous X-ray cluster searches based on pointed observations, and has the potential to yield about 4000 X-ray clusters after completion of SDSS.