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
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
. 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 upper limit to the count rate is determined
if the probability
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
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 to flux density
(
) at Earth
is given by
where is the energy-conversion factor, derived from the hydrogen
column density
, 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,
-- and the total source count rate) we derive a
probability distribution for the temperature. Because every temperature is
associated with a value for
the most probable value for
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
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
and
) we derived
from
Paresce (1984), while for more distant stars we estimated
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
(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
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, , the X-ray luminosity
, and the
normalised X-ray flux density
from the flux
density at the detector,
:
The effective temperatures 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
have been obtained from the extinction corrected Walraven brightness
and colour
using Eq. (3 (click here)). The stellar
radii R* have been derived from the surface gravity, using the relation
. 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
,
and
and their uncertainties are listed in Table 5,
Cols. 6
to 8.
These uncertainties are dominated by the uncertainties in the source count rate
and in the hydrogen column density
, 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.