Astron. Astrophys. 323, 853-875 (1997)

3. Follow-up observations

Each entry of the final SASS/SIMBAD list of candidate OB/X-ray binaries was then scheduled for follow-up optical observations. The main goals of the optical observations were to search for alternative identifications close to the X-ray position and to acquire better spectral classification of the OB candidate. Sources which had passed the optical investigation and which were not falling serendipitously in one of the ROSAT pointings were the scope of a dedicated AO-3 proposal.

3.1. Optical

For the southern hemisphere, optical observations were acquired mainly at ESO with the ESO-MPI 2.2 m + EFOSC2 (Buzzoni et al. 1984) during a run in 1992 from April 17 till 25. Low and medium resolution spectroscopy was obtained using respectively grism #6 ( 4500-7100 Å ; FWHM resolution 5.4 Å), #3 ( 3500-5400 Å ; FWHM resolution 3.8 Å), #7 ( 3550-4780 Å ; FWHM resolution 3.0 Å) and #9 ( 5800-7000 Å ; FWHM resolution 3.0 Å). In the imagery mode U, B, V and I frames were acquired with a pixel size of 0.332 and an exposure time of 2-5 min. In all cases, a THX #19 1024² CCD chip was used as detector. Spectra and images were corrected for bias and flat-field using standard MIDAS routines. Two-dimensional wavelength calibration was carried out using spectral lines of helium and argon lamps and the observation of standard stars allowed the flux calibration of a fraction of the spectra. Additional spectroscopy was obtained from 1994 February 10 till 14 with the ESO 1.5 m telescope and the Boller & Chivens instrument. Finally some spectra were also obtained using the Cassegrain Boller & Chivens spectrograph attached at the 1.6 m telescope of the Brazilian Laboratorio Nacional de Astrofisica (LNA; Brazopolis, Brazil). The detector used was a 770 1152 GEC CCD and the FWHM resolution was 9 Å in the range 3800-8200 Å .

Northern fields were investigated during several observing runs performed at the Observatoire de Haute-Provence, CNRS, France, between 1990 November and 1995 January. This observing programme was part of a general project aiming at the optical identification and follow-up observations of area and X-ray selected sources extracted from the ROSAT Galactic Plane Survey. All OHP spectroscopic observations were obtained with the CARELEC spectrograph (Lemaitre et al. 1990) attached at the 1.93 m telescope. Low resolution spectroscopy ( 3500-7500 Å ; FWHM resolution 14 Å) and blue and red medium resolution spectroscopy ( 3800-4300 Å ; FWHM resolution 1.8 Å , 6300-6700 Å ; FWHM resolution 1.7 Å) were acquired using a 260 Å /mm grating and a 33 Å /mm grating respectively. Spectra were calibrated in wavelength using arcs of iron, neon and helium lamps. Observations of standard stars allowed to formally calibrate in flux all spectra. However, bad meteorological conditions which prevailed in some cases and the narrow slit used for some bright stars may bias the mean flux level on occasions and therefore, the spectrophotometric flux scale should be considered with some caution. CCD images were collected using the standard camera at the 1.2 m telescope. Most of the time the Johnson B,V,and I filters were used. Depending on the CCD mounted, the field of view was either 7.1 4.4 (RCA) or 6.5 6.5 (TK512).

Spectral types of OB stars were derived comparing with spectral atlases in Jaschek & Jaschek (1987) and Walborn & Fitzpatrick (1990) and equivalent widths listed in Didelon (1982). For the later spectral types we used the atlas by Turnshek et al. (1985). Interstellar absorption was usually estimated from the 4430 and 6284 interstellar bands corrected for atmospheric effects and following the relations given in Krelowski et al. (1987) and Bromage & Nandy (1973).

3.2. X-ray

We list in Tables 6 and 7 the main X-ray characteristics of the sources derived both from survey and pointed PSPC data. Sources appear in each group ordered by increasing right ascension. All pointed X-ray data presented below were analyzed using the EXSAS package. Spectra were accumulated for each source and the light curves were checked for variability. We systematically searched all major catalogues produced by past X-ray instrumentation for a possible previous detection of the ROSAT source.

Table 6. Positions and errors resulting from the EXSAS analysis of survey and pointed PSPC observations of the OB/X-ray candidates selected for follow-up observations. We also list the difference between the X-ray pointed and optical positions (p-o) and difference between pointed and survey positions (p-s). The optical positions used here are the most accurate available for each OB target and not necessarily those used in Table 4. Column (o.a.) lists the off axis angle for the pointed observations. Source names are computed after the survey EXSAS positions. Horizontal lines divide the three groups of candidates defined in Table 4

Table 7. X-ray count rates and hardness ratios derived from survey and pointed observations for the 24 sources tentatively associated with OB/X-ray candidates and selected for follow-up observations. All data listed here were obtained using EXSAS with the exception of survey hardness ratios which are those listed in the SASS automatic analysis. For pointed observations, the maximum likelihood (ML) values are the largest of the broad (b) and hard (h) energy bands. For HD38087 count rates and exposure times are the mean and sum of pointings WG 900189 and WG 900386 whereas the ML and HRs correspond to WG900189 only. The horizontal lines divide the three groups of candidates defined in Table 4

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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