THE ROSAT DEEP SURVEY - II - OPTICAL-IDENTIFICATION, PHOTOMETRY AND SPECTRA OF X-RAY SOURCES IN THE LOCKMAN FIELD

The ROSAT Deep Survey includes a complete sample of 50 X-ray sources w ith fluxes in the 0.5 - 2 keV band larger than 5.5 10(-15) erg cm(-2) s(-1) in the Lackman field (Hasinger et al., Paper I). We have obtaine d deep broad-band CCD images of the field and spectra of many optical objects near the positions of the X-ray sources. We define systematica lly the process leading to the optical identifications of the X-ray so urces. For this purpose, we introduce five identification (ID) classes that characterize the process in each case. Among the 50 X-ray source s, we identify 39 AGNs, 3 groups of galaxies, 1 galaxy and 3 galactic stars. Four X-ray sources remain unidentified so far; two of these obj ects may have an unusually large ratio of X-ray to optical flux.