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4. The X-ray data

The ROSAT All-Sky Survey was conducted from July 1990 until January 1991. During the survey the satellite scanned the sky in circles perpendicular to the direction of the Sun. Any particular position in the sky was in the tex2html_wrap_inline1402 field of view of the Position Sensitive Proportional Counter (PSPC) for about 30 seconds once every 90 minutes, during at least 2 days (depending on the ecliptic latitude). The PSPC is sensitive in the energy range tex2html_wrap_inline1404. For a detailed description of the satellite and the PSPC we refer to Trümper (1983) and Pfeffermann et al. (1988), and for a description of the All-Sky Survey to Voges (1992).

The X-ray count rates have been derived as described in Chapter 2 of Piters (1995). A short summary will be given here. We selected a region around the position of the source, and two background regions on the same ecliptic longitude (i.e. on the same survey scans). From the number of counts in these three regions, and their effective exposure times we derived the most probable source count rate and its uncertainty, using a maximum-likelihood method based on Poisson statistics. A tex2html_wrap_inline1406 upper limit to the count rate is determined if the probability tex2html_wrap_inline1408 that the counts in the source region are only background counts is larger than 0.025. With this threshold value we expect only 0.5 false detections in our sample (determined by the sum of tex2html_wrap_inline1408 over all detections).

The resulting count rates and upper limit values are given in Table 5 (Col. 2). We detected 86 X-ray sources out of the total of 162 stars for which X-ray data were available.

The conversion of count rate tex2html_wrap_inline1412 to flux density (tex2html_wrap_inline1414) at Earth tex2html_wrap_inline1416 is given by
equation817
where tex2html_wrap_inline1418 is the energy-conversion factor, derived from the hydrogen column density tex2html_wrap_inline1420, and from the probability distribution of the hardness ratio q(h), following the procedure as described in Chapter 2 of Piters (1995). This method assumes that the X-ray spectrum can be described by a single-temperature Mewe & Gronenschild (Mewe et al. 1985) spectrum which is subject to galactic absorption (Morrison & McCammon 1983). From the probability distribution of the hardness ratio (defined as the ratio of the source count rate in the high-energy band -- channels 41 to 240, tex2html_wrap_inline1424 -- and the total source count rate) we derive a probability distribution for the temperature. Because every temperature is associated with a value for tex2html_wrap_inline1418 the most probable value for tex2html_wrap_inline1418 and its uncertainty interval can be calculated from this distribution of temperature. The most probable value for the hardness ratio and its uncertainty are listed in Col. 3 of Table 5, and the most probable value for tex2html_wrap_inline1418 and its uncertainty interval are listed in Col. 4 of Table 5. For nearby stars in the galactic plane (distance less than 200 pc and galactic latitude between tex2html_wrap_inline1432 and tex2html_wrap_inline1434) we derived tex2html_wrap_inline1420 from Paresce (1984), while for more distant stars we estimated tex2html_wrap_inline1420 from the interstellar reddening E(V-B), as derived in Sect. 3, using the relation E(B-V) = 2.39 E(V-B)-0.17E(V-B)2 and the expression tex2html_wrap_inline1444 (Bohlin et al. 1978; close to the relation recently derived by Predehl & Schmitt 1995). The spread around this relationship is about 30%. The distance is derived from the parallax or, if the parallax is not known, from the distance modulus. The adopted tex2html_wrap_inline1420 values are listed in Table 5, Col. 5.

For each star detected in the ROSAT survey we derived the X-ray flux density at the stellar surface, tex2html_wrap_inline1448, the X-ray luminosity tex2html_wrap_inline1450, and the normalised X-ray flux density tex2html_wrap_inline1452 from the flux density at the detector, tex2html_wrap_inline1416:
eqnarray825
The effective temperatures tex2html_wrap_inline1290 have been taken from Table 2, the bolometric correction BC from Hayes (1978), who gives these corrections as a function of the effective temperature. Johnson's apparent visual magnitudes tex2html_wrap_inline1258 have been obtained from the extinction corrected Walraven brightness tex2html_wrap_inline1118 and colour tex2html_wrap_inline1120 using Eq. (3 (click here)). The stellar radii R* have been derived from the surface gravity, using the relation tex2html_wrap_inline1468. The mass M is calculated as a function of effective temperature from the mass-spectral type relation, as given by Schmidt-Kaler (1982), combined with the spectral type-temperature relation as given by Hayes (1978). The numerical constants are based on the solar parameters used by Oranje et al. (1982). The values for tex2html_wrap_inline1472, tex2html_wrap_inline1474 and tex2html_wrap_inline1476 and their uncertainties are listed in Table 5, Cols. 6 to 8. These uncertainties are dominated by the uncertainties in the source count rate tex2html_wrap_inline1412 and in the hydrogen column density tex2html_wrap_inline1420, the latter being caused by the relatively large uncertainties in the distance and in E(V-B).

Acknowledgements

We thank ms. L. Spijkstra for obtaining the Walraven photometry at La Silla, and Dr. A. van Genderen for his help with the data reduction. The ROSAT All-Sky Survey data result from the hardware and software efforts of many people in the ROSAT team at MPE. It is a pleasure to acknowledge their dedicated work and continuing support. We are grateful to Dr. J.W. Pel for his help in deriving effective temperatures and surface gravity values and for providing still unpublished data on theoretical colours as used in Sect. 3.


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