The ROSAT all-sky survey catalogue of optically bright OB-type stars

ASTRONOMY & ASTROPHYSICS SEPTEMBER 1996, PAGE 481 SUPPLEMENT SERIES Astron. Astrophys. Suppl. Ser. 118, 481-494 (1996) The ROSAT all-sky survey catalogue of optically bright OB-type stars T.W. Berghöfer1 , J.H.M.M. Schmitt1 and J.P. Cassinelli2 1 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstr, 1, D-85740 Garching, Germany University of Wisconsin - Madison, Department of Astronomy, 475 North Charter Street, Madison, WI 53706-1582, U.S.A. 2 Received January 2; accepted February 6, 1996 Abstract. — For the detailed statistical analysis of the X-ray emission of hot stars we selected all stars of spectral type O and B listed in the Yale Bright Star Catalogue and searched for them in the ROSAT All-Sky Survey. In this paper we describe the selection and preparation of the data and present a compilation? of the derived X-ray data for a complete sample of bright OB stars. Key words: stars: early-type — X-rays: stars 1. Introduction The discovery of hot stars as sources of soft X-ray emission (Seward et al. 1979; Harnden et al. 1979) was one of the first results obtained with the Einstein Observatory (Giacconi et al. 1979). Subsequently, a larger number of bright hot stars have been observed with the Einstein satellite. These observations showed that many O stars as well as early B stars are soft X-ray emitters. Chlebowski et al. (1989) provided a compilation of all available Einstein data for O stars, whereas Grillo et al. (1992) analysed in detail the B stars observed with the Einstein Observatory. In the case of (optically) bright O stars the Einstein observations are almost complete to a limiting visual magnitude V = 6.5. However, in the case of the B stars, the Einstein data do not allow to study X-ray properties on the basis of any complete sample of stars. For a detailed discussion of X-ray emission of hot stars we refer to Hillier et al. (1993), Cassinelli et al. (1994), and Berghöfer & Schmitt (1994). The purpose of this paper is to present the X-ray data of OB stars provided by the ROSAT All-Sky Survey (RASS, Voges 1992) in a complete and systematic fashion. Here we describe our method to extract the X-ray data from the RASS and to derive the X-ray properties for our complete sample of OB stars. For an astrophysical discussion of the detailed analysis of the derived X-ray properties of OB stars we refer to the accompanying paper (Berghöfer et al. 1995). Send offprint requests to: T.W. Berghöfer Tables 2 and 3 are also available in electronic form at the CDS via anonymous ftp 130.79.128.5 ? 2. ROSAT All-Sky Survey data (selection and preparation) In 1990 the ROSAT X-ray satellite performed the first imaging all-sky survey in the soft X-ray band (0.1−2.4 keV). A detailed description of the ROSAT satellite can be found in Trümper (1983) and Trümper et al. (1991). Detailed information about the detector, the Position Sensitive Proportional Counter (PSPC), is provided by Pfeffermann et al. (1996). During the ROSAT All-Sky Survey (RASS), the sky was scanned along great circles roughly perpendicular to the direction to the Sun and through the ecliptic poles. Due to the Earth’s motion relative to the Sun, the whole sky was covered in a 6 month period. The survey was carried out in such a way that scan and orbital period were identical. The RASS data on an individual source thus consist of a series of “snapshots” each of which typically lasts about 20 seconds. A more detailed discussion of the RASS data is given by, e.g., Belloni et al. (1994). Here we only want to point out that the observational technique of the RASS led to different exposure times of the sky; close to the ecliptic plane the RASS observations lasted typically 2 days, whereas the vicinity of the ecliptic poles was observed during the entire period of RASS observations. Thus, effective exposure times range between several hundreds seconds and 40000 s, the actual values for the objects discussed here, which are located near the galactic plane, range between 140 s and 1600 s. 482 T.W. Berghöfer et al.: The ROSAT all-sky survey catalogue of optically bright OB-type stars Table 1. Sample stars without any RASS exposure HR 38 809 899 1134 1200 4679 4806 4814 Sp. Type B2V B9Vn B9.5V B5IV B9.5V B2.5V B1Ia B8V V / mag 6.73 5.77 5.56 5.00 6.86 4.04 6.25 6.26 HR 5034 5035 5151 5207 5217 5230 5400 8723 Sp. Type B2.5Vn B3V B0.5III B9V B5III B9V B7IV B7III V / mag 6.18 4.53 6.00 5.25 5.89 5.71 6.35 5.74 2.1. Selection of the OB-stars and detection in the RASS In order to perform a detailed statistical analysis of the X-ray emission of OB stars on the basis of a complete sample of stars we selected all stars of spectral type O and B listed in the Yale Bright Star Catalogue (BSC, Hoffleit & Warren 1991); we did not include those O and B stars in binary orbits with WR stars. The BSC contains all bright stars and is complete to a limiting visual magnitude V = 6.5. In total we selected 1838 stars. Since the RASS is almost complete (sky coverage 99%), only 16 of our sample stars (∼0.9%, listed in Table 1) are not covered by any RASS observations and, therefore, had to be removed from further analysis. For every program star we selected out of the whole RASS data a 400 square centered on the star’s optical position and ran a standard source detection algorithm on these fields. We used a maximum likelihood technique (Cruddace et al. 1988) with the option to search for an X-ray source near a given position (the optical position of our sample stars). The complete data reduction of the RASS data was carried out with the EXSAS software package (EXSAS user’s guide 1994) running under ESO-MIDAS. source, we observe an offset of 67.700); we do not have any definite explanation for the observed larger offsets in the case of some of our sample stars with higher count rates (e.g., for ζ Pup about 450 counts were collected during the RASS observations) and speculate that they result from systematic uncertainties in the satellite’s attitude. Another possible explanation is that some of our program stars may have X-ray emitting visual companions (note that we do not claim this for the case of ζ Pup!) which remained unresolved by the RASS observations, thus, leading to larger offsets of the X-ray source relative to the optical position of the given OB star. For a more accurate determination of the maximal offset we compare the mean number of sources per deg2 found in the RASS (≈60000 sources/40000 deg2 = 1.5 sources/deg2 ) to the number of detected sources N (∆) within a given maximal offset ∆ per total “error” area (N (∆) · π · ∆2 ). If we allow a maximal offset of 7500(cf. Fig. 1), we detect 216 OB stars in the RASS data and we expect only ∼0.5 background objects among this sample. This is significantly lower than the number of detected OB stars and sufficiently small for the purpose of our analysis. Therefore, we treated stars with no X-ray counterparts or X-ray counterparts with position offsets (...truncated)