Multicolor photometric observations of candidate optical counterparts to ROSAT faint X-ray sources in a 1 square degree field of the BATC Survey

We present optical candidates for 75 X-ray sources in a similar to1 deg(2) overlapping region with the 1997 medium-deep ROSAT survey by Molthagen et al. These candidates are selected using the multicolor CCD imaging observations made for the T329 field of the Beijing-Arizona-Taiwan-Connecticut (BATC) Sky Survey, which uses the NAOC 0.6/0.9 m Schmidt telescope with 15 intermediate-band filters covering the wavelength range 3360 - 9745 Angstrom. These X-ray sources are relatively faint (CR << 0.2 s(-1)) and thus are mostly not included in the ROSAT Bright Source Catalogue; they also remain as X-ray sources without optical candidates in a previous identification program carried out by the Hamburg Quasar Survey. Within their position error circles, almost all of the X-ray sources are observed to contain one or more spatially associated optical candidates down to the magnitude m(V) similar to 23.1. We have classified 149 of 156 detected optical candidates with 73 of the 75 X-ray sources with a new method that predicts a redshift for nonstellar objects, which we have termed the SED-based Object Classification Approach. These optical candidates include 31 QSOs, 39 stars, 37 starburst galaxies, 42 galaxies, and seven "just''- visible objects. Twenty-eight X-ray error circles have only one visible object in them: nine QSOs, three normal galaxies, eight starburst galaxies, six stars, and two of the just-visible objects. We have also cross-correlated the positions of these optical objects with NASA Extragalactic Database, the FIRST radio source catalog, and the Two Micron All Sky Survey. Separately, we have also SED-classified the remaining 6011 objects in our field of view. Optical objects are found at the 6.5 sigma level above what one would expect from a random distribution; only QSOs are overrepresented in these error circles at greater than 4 sigma frequency. We estimate redshifts for all extragalactic objects and find a good correspondence between our predicted redshift and the measured redshift (a mean error of 0.04 in Deltaz). There appears to be a supercluster at z similar to 0.3 - 0.35 in this direction, including many of the galaxies in the X-ray error circles that are found in this redshift range.Astronomy & AstrophysicsSCI(E)2ARTICLE52579-259712