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Astron. Astrophys. 318, 111-133 (1997)

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1. Introduction

Starting in 1990 July the ROSAT X-ray satellite has performed during six months the first soft X-ray all-sky survey ever made with an imaging instrument. At this occasion, the position sensitive proportional counter (PSPC) was moved in the focal plane of the X-ray telescope (XRT). This detector offers an energy range of 0.1-2.4 keV and an energy resolution of about 45% at 1 keV. In survey mode, the satellite was scanning the sky in great circles perpendicular to the solar direction thus covering the whole celestial sphere in six months time. The spin period of the satellite was synchronized with its orbital motion. Any part of the sky was continuously seen during a maximum detector crossing time of 32 s every 96 min. The time interval during which a given source is visible mainly depends upon its ecliptic latitude and the cumulative exposure time varies between 300 sec and 40,000 sec for the equatorial and polar regions respectively. The mean flux sensitivity is [FORMULA] 2 10-13 erg cm-2 s-1 (0.1-2.4 keV). Taking into account the varying point spread function across the PSPC field of view, the final spatial resolution of the survey is of the order of [FORMULA] with a capability to localize point sources with a 1 [FORMULA] accuracy close to [FORMULA]. A review of the main characteristics of the ROSAT satellite and instrumentation can be found in Trümper (1983) and Pfeffermann et al. (1986) while the ROSAT all-sky survey (RASS) is described in Voges (1992).

As part of the RASS the plane of the Galaxy was also surveyed entirely. The analysis of the region of the sky located below absolute galactic latitude [FORMULA], known as the ROSAT Galactic Plane Survey (RGPS) is the scope of a distinct project (Motch et al. 1991). Over 14,000 XRT survey sources are found in the Milky Way, most of these being unknown before the launch of ROSAT.

As for studies at high galactic latitudes, the RGPS source catalogue will serve as basis for statistical analysis of various kinds of X-ray emitters. The galactic plane is suited for studies of active coronae, OB stars and cataclysmic variables for instance. It will also be extremely useful as finder for follow-up observations of particularly interesting objects in the forthcoming years. The RGPS may also specifically address several pending questions of high astrophysical interest such as the origin of the hard X-ray galactic ridge emission, the nature of supersoft X-ray sources and the evolutionary status of accreting binary sources for example.

Compared with previous X-ray surveys of the Galaxy, the RGPS offers a much larger area and improved sensitivity and spatial resolution. The Einstein observatory mapped only [FORMULA] 2.5% of the Galactic Plane (Hertz & Grindlay 1984, 1988) at a flux limit similar to that of ROSAT while the previous Milky Way surveys performed with collimating instruments (e.g., HEAO: Nugent et al. 1983, Wood et al. 1984, EXOSAT: Warwick et al. 1985) had a sensitivity a factor 10 to 100 lower than that of the RGPS.

Obviously, the identification of this large amount of X-ray sources is far beyond the nowadays possibilities. Accordingly, we used three different paths for exploring this unprecedented amount of new sources. The first and easiest step was the cross correlation of RGPS source positions with astronomical catalogues, mostly extracted from the SIMBAD database. The historical development of astronomy has favoured galactic studies and the number of Milky Way objects held in catalogues is rather large. With the positional accuracy of the survey, up to 30% of RGPS sources may be readily identified with SIMBAD objects with only limited rate of false coincidence (see e.g. Motch et al. 1991, Voges 1992). The second step consisted in selecting potentially interesting objects for optical identification at the telescope. Using the observational RASS X-ray properties of identified classes of X-ray sources together with other available information at a different wavelength such as the Guide Star Catalogue of the Hubble Space Telescope (Lasker et al. 1990) we could extract small subsets of sources having a higher probability to be identified with a given kind of X-ray emitter than in the overall survey installment. This approach led to several interesting results such as the discovery of a luminous supersoft source in the Galaxy (Motch, Hasinger & Pietsch 1994) and of a new class of X-ray soft intermediate polars (Haberl et al. 1994, Haberl & Motch 1995). The third step was the selection of several areas located in particular directions of the Galaxy. Systematic optical identification of RGPS sources in these small sized regions allows to estimate at least statistically the X-ray content of the Milky Way as seen by ROSAT.

Several groups working in both hemispheres participate into the optical identification project in collaboration with scientists at the Max-Planck-Institut für Extraterrestrische Physik. We report here on the efforts carried out by one of the optical groups to optically identify ROSAT survey sources in a field located in the galactic plane. In this paper we present the details of the identification strategy and discuss the statistical properties of the sample which are likely to be representative of the overall properties of the galactic ROSAT X-ray sky. We also report on a few particularly interesting objects discovered in this field. The observed X-ray population of active coronae is compared with that predicted by stellar population models having resolution in age, folded with the most recent X-ray luminosity functions and the applications of such studies are shortly reviewed. We also put constraints on the contribution to the galactic plane X-ray emission of an hypothetical population of old neutron stars accreting from the interstellar medium. Observational details and source finding charts are presented in a companion paper (Motch et al. 1996a).

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998