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 ∼1 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 Å. 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 ∼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 σ level above what one would expect from a random distribution; only QSOs are overrepresented in these error circles at greater than 4 σ 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 Δz). There appears to be a supercluster at z ∼ 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.