The Cambridge-Cambridge ROSAT Serendipity Survey .5. Catalogue and optical identifications

We report the results of a medium-depth X-ray survey of 20 ROSAT Position Sensitive Proportional Counter (PSPC) fields. 123 X-ray sources were 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 Sensitivity Survey (EMSS) and the deepest ROSAT surveys. Optical identifications of 110 of these sources have revealed 68 QSOs, 12 narrow-emission-line 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 from the EMSS [L(X) proportional to(1 + z)(2.55+/-0.1)], the latter being rejected at greater than the 3 sigma level as a model for the current sample. We present the optical spectra and measurements of the emission lines. The equivalent-width distributions are consistent with those 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.