A ROSAT medium-sensitivity Galactic plane survey at 180 degrees <1<280 degrees 180°≲l≲280°

We have performed a moderately deep soft X-ray (0.1–2 keV) survey of the Galactic plane using pointed observations with the ROSAT Position Sensitive Proportional Counter (PSPC). The survey is more than an order of magnitude more sensitive than previous X-ray surveys near the Galactic plane. The data consist of nine fields each of ≈10 ks exposure, pointed at positions on or very close to the Galactic plane (ǀbǀ<0°.3) in the longitude range 180°≲l≲280°. This region has relatively low X-ray absorbing material out to distances of several hundred pc and presents fewer source-confusion problems than at other longitudes. The total sky area surveyed was 2.5 deg[superscript 2]; this yielded 93 sources, 89 of which were detected in the ‘hard’(0.4–2.0 keV) band. Nine sources were detected in both ‘soft’(0.1–0.4 keV) and hard bands. In the hard band, the survey coverage is ≳90 per cent for sources brighter than 0.002 count s[superscript −1](∼2×10[superscript −14] erg cm[superscript −2] s[superscript −1]), but falls steeply below this value, with the weakest sources being ∼0.001 count s[superscript −1]. The median limiting flux is ≈0.0013 count s[superscript −1](∼1.3×10[superscript −14]erg cm[superscript −2] s[superscript −1]). There are 64 sources with hard-band count rates > 0.002 count s[superscript −1]. We present the catalogue of X-ray sources and the number–flux relations (log N–log S). Eighteen sources have possible identifications from the SIMBAD data base. We have searched the Tycho-2 and USNO-A2.0 catalogues to find all possible optical counterparts brighter than ∼ 19th magnitude, and attempt to classify these on the basis of log (f[subscript X]/f[subscript opt]) versus optical colour diagrams and near-infrared photometry from the 2MASS Second Incremental Data Release. Hence, we have found the majority of these sources to be consistent with being late-type main-sequence stars, as previous studies have proposed from incompletely identified surveys. Comparison of the measured number–flux relations with predictions of Galactic (stellar) and extragalactic populations supports the view that the population of young stars in the plane is denser than previously thought.