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

We have performed a moderately deep soft X-ray (0.1-2keV) 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 ~10ks exposure, pointed at positions on or very close to the Galactic plane (|b|<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 2.5deg² this yielded 93 sources, 89 of which were detected in the `hard' (0.4-2.0keV) band. Nine sources were detected in both `soft' (0.1-0.4keV) and hard bands. In the hard band, the survey coverage is >~90 per cent for sources brighter than 0.002counts^-1 (~2×10^-14ergcm^-2s^-1), but falls steeply below this value, with the weakest sources being ~0.001counts^-1. The median limiting flux is ~0.0013counts^-1 (~1.3×10^-14ergcm^-2s^-1). There are 64 sources with hard-band count rates >0.002counts^-1. We present the catalogue of X-ray sources and the number-flux relations (logN-logS). 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_X/f_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.