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

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3. Optical identification procedure

3.1. SIMBAD identifications

Our first step toward optical identification was to search the SIMBAD database for positional coincidence with a catalogued object. For the entire galactic plane survey, about 20% to 30% of the sources may be readily identified with SIMBAD entries with an expected number of random spurious coincidence of about 10% of the identifications (Motch 1992). A similar identification rate was derived from the analysis of a small area in the galactic plane in Perseus (Motch et al. 1991).

In our 'full' area, 30 among 95 sources (i.e. 31%) detected at a maximum likelihood larger than 10 have SIMBAD entries within r90 the 90% ROSAT survey confidence radius. Scanning the database shows that the mean density of SIMBAD objects in a [FORMULA] [FORMULA] [FORMULA] region centered on l = [FORMULA], b = [FORMULA] is [FORMULA] 130 per square degree. With an average r90 = [FORMULA] (ML [FORMULA] 10), we expect a random match in [FORMULA] 2% of the cases. This lower false identification rate compared to former studies is due to the improved accuracy of survey X-ray sources positioning with respect to the first analysis. We thus expect 2 wrong identifications among the 30 SIMBAD proposed matches. However, the spatial density of SIMBAD entries may be locally much higher because of the inclusion of a particular catalogue (e.g. the catalogue of objects in the direction of M39 compiled by Platais, 1994). Therefore, whenever the information retrieved from the literature was not firmly conclusive we tried to obtain our own optical data.

3.2. Optical observations

All optical data were obtained at the Observatoire de Haute-Provence in the time interval from 1991 May till 1993 September. A description of the instrumentation used may be found in Motch et al. (1996a).

For each ROSAT source we produced a finding chart using the HST Guide Star Catalogue (GSC, Lasker et al. 1990). GSC data were extracted in a first step from the STARCAT facility at ESO (Pirenne et al. 1993) and later from the SIMBAD catalogue GSC browser (Preite-Martinez & Ochsenbein 1993).

In the cases where one or more rather bright GSC stars were lying close to the center of the X-ray position, with apparently a rather low probability of random coincidence, we directly started our investigation by obtaining medium resolution spectroscopy ([FORMULA] 3800 - 4300 Å ; FWHM resolution [FORMULA] 1.8 Å) of the GSC candidates. These spectra allowed the detection of re-emission in the Ca II H&K lines which is a well known signature of chromospheric and associated coronal activity (e.g. Schrijver 1983, Maggio et al. 1987). When the chromospheric activity was not found at the level expected from the intensity of the X-ray source (see below) or when no bright GSC star was conspicuous in the error circle, we obtained B and I band CCD photometry of the field. From the B-I index we could select stars exhibiting a red excess, presumably dM star candidates and blue excess objects considered as candidates for white dwarfs, cataclysmic variables and background AGNs. We then obtained low resolution spectroscopy ([FORMULA] 3500 - 7500 Å ; FWHM resolution [FORMULA] 14 Å) of the photometric candidates and of other faint sources if necessary. We tried to push our optical low resolution spectroscopic investigations at the same limiting magnitude for all sources at a given X-ray count rate in order to preserve as much as possible the completeness of the identified sample. This means that for the X-ray brightest unidentified sources we are complete down to V [FORMULA] 17-18. However, considering the sometimes relatively high number of objects encountered in the error circles and the changing instrumental conditions our optical depth is probably not quite homogeneous.

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© European Southern Observatory (ESO) 1997

Online publication: July 8, 1998