THE CAMBRIDGE-CAMBRIDGE ROSAT SERENDIPITY SURVEY .5. CATALOG AND OPTICAL IDENTIFICATIONS

We report the results of a medium-depth X-ray survey of 20 ROSAT Posit ion Sensitive Proportional Counter (PSPC) fields. 123 X-ray sources we re detected down to a flux limit of S (0.5-2 keV) > 2 x 10(-14) erg s( -1) cm(-2) lying between that of the Einstein Extended Medium Sensitiv ity Survey (EMSS) and the deepest ROSAT surveys. Optical identificatio ns of 110 of these sources have revealed 68 QSOs, 12 narrow-emission-l ine X-ray-luminous galaxies (NLXGs), 24 stars, two BL Lac objects, two galaxies and two clusters. CCD imaging reveals the possible presence of galaxy groups or clusters at the positions of a further four X-ray sources. The number-redshift and log N-log S relations of the 68 QSOs are in better agreement with the faster rate of cosmological evolution for X-ray QSOs derived from ROSAT deep surveys [L(X) proportional to( 1 + z)(3.34+/-0.1), z(max) = 1.79] than with the evolution obtained fr om the EMSS [L(X) proportional to(1 + z)(2.55+/-0.1)], the latter bein g rejected at greater than the 3 sigma level as a model for the curren t sample. We present the optical spectra and measurements of the emiss ion lines. The equivalent-width distributions are consistent with thos e of QSO samples selected in other wavebands. We find no evidence for an inverse correlation between line width and X-ray spectral slope as recently reported for other ROSAT QSO samples.