The ROSAT Deep Survey II. Optical identification, photomettry and spectra of sources in the Lockman field

The ROSAT Deep Survey includes a complete sample of 50 X-ray sources with fluxes in the $0.5-2$ keV band larger than 5.5 ${10}^{-15}$ erg cm${}^{-2}$ s${}^{-1}$ in the Lockman field (Hasinger et al., Paper I). We have obtained deep broad-band CCD images of the field and spectra of many optical objects near the positions of the X-ray sources. We define systematically the process leading to the optical identifications of the X-ray sources. For this purpose, we introduce five identification (ID) classes that characterize the process in each case. Among the 50 X-ray sources, 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 objects may have an unusually large ratio of X-ray to optical flux. Keywords: surveys – galaxies: active – quasars: emission lines – galaxies: Seyfert – X-rays: galaxies