The ROSAT all-sky survey (RASS; Voges 1992) was performed from 1990 July till 1991 February and was carried out with the X-ray telescope (XRT) and the Position Sensitive Proportional Counter (PSPC, Pfeffermann et al. 1986). The survey mapped as many as 60000 new sources in the soft X-ray band (0.1-2.4 keV, Maximum Likelihood, ML 10) down to limiting fluxes of the order of a few 10-13erg cm-2 s-1. At this detection threshold, there are about 1.45 10-2 spurious sources per square degree. Recently, the 18811 RASS sources having a PSPC count rate larger than 0.05cts s-1 and ML 15 have been published (Voges et al. 1996). The study of the RASS point source content at low galactic latitudes (20) is the scope of a dedicated project, the ROSAT Galactic Plane Survey (RGPS: Motch et al. 1991b). About 15000 RASS sources are located in this 40 wide strip of the sky.
In the galactic plane area, already 20% to 30% of RGPS sources may be identified with high confidence on the basis of positional coincidence with objects, mostly stars, catalogued in the SIMBAD database. Adding cross-correlation with relatively bright stellar catalogues such as Guide Star Catalogue entries allows the identification of about 50% of RGPS sources (Motch et al. 1997a).
The number of remaining unidentified sources is however still too large to allow their systematic identification at the telescope. In order to tackle this problem, two paths of investigations were chosen; i) selection of sources over the whole galactic sky using criteria on their X-ray characteristics and optical content of the error box and ii) selection of sample areas at judicious positions for complete optical identification.
The X-ray selected approach has led to the discovery of a number of interesting objects, mostly soft sources which have escaped previous surveys carried out at higher energies and furthermore without imaging instrumentation or large sky coverage. Among the most noticeable results are the discovery of galactic supersoft sources (Ögelman et al. 1993; Motch et al. 1994; Beuermann et al. 1995), a new class of soft intermediate polars (Haberl & Motch 1995), and more recently the identification of a couple of isolated neutron stars presumably accreting from the interstellar medium (Walter et al. 1996; Haberl et al. 1997).
On the other hand, systematic optical identification of RGPS sources in sample areas has led to the conclusion that active coronae dominate by number the point source content in the galactic plane strip (up to 85% of the total; Motch et al. 1997a). In contrast, the stellar fraction identified at high galactic latitude is much lower (; Zickgraf et al. 1997). The modelling of this stellar population and ages derived from Lithium lines clearly show that the stars detected in the RASS are quite young, mostly younger than 1 Gyr (Guillout et al. 1996a,b). Another important result from this study is derived from the small number of sources remaining unidentified. In particular, this implies that the space density of isolated neutron stars accreting from the interstellar medium is at least a factor 10 smaller than expected.
This paper is divided in two parts. First we show how X-ray and optical characteristics of ROSAT sources may be used to build samples with enhanced probabilities of identification with a given class of object. For that purpose we cross-identified the ROSAT all sky survey bright source list with SIMBAD. Second, we report on optical observations of 93 selected RGPS sources. The source list contains various kinds of X-ray selected sub samples out of which several identifications not repeated here were already published.
Our identification programme of X-ray selected sources in the galactic plane is continuing and we expect to provide more RGPS source identifications in a future paper.
Copyright The European Southern Observatory (ESO)