Astron. Astrophys. 323, 853-875 (1997)
2. Cross-correlation of the ROSAT all-sky survey source list with SIMBAD OB stars
2.1. The ROSAT source list
The source list considered here arises from the first ROSAT all-sky
survey data reduction as completed by 1991 October by the Scientific
Analysis System Software (SASS; Voges et al. 1992). This software
provides for each source the position, count rates and hardness ratios
HR1 and HR2 defined as
![[EQUATION]](img8.gif)
![[EQUATION]](img9.gif)
where (A-B) is the raw count rate in the energy range A to B in
keV. Because of spacecraft problems no data were available for about
5% of the sky located between ecliptic longitudes 41
and 49 and ecliptic
longitudes 221 and 229 .
The accumulation by 2 wide strips along the
great scan circles yields variations of sensitivity perpendicular to
the strip and overlapping at high ecliptic latitudes may produce
multiple detection of the same source in adjacent strips. The lists of
sources derived from each strip were then merged into a single master
list totaling about 15,000 sources at
20 . The merging process
assumed that a source detected in two or more strips was the same when
the difference in position was less than a minimum value of one survey
sky pixel ( ) or less than the combined
positional errors. The strip oriented analysis does not allow an easy
estimate of the sensitivity of detection in a given part of the sky
but allows quick detection. The errors on ROSAT X-ray positions have
two different origins. First, the statistical uncertainty with which
the centroid of the X-ray image is positioned on the pixel grid by the
Maximum Likelihood source detection algorithm. Second, the systematic
error in the knowledge of the satellite attitude for each photon
collected in scan mode. The analysis of the subsample of the 13 known
high mass X-ray binaries (HMXBs) detected in the RGPS by the SASS (see
Sect. 2.2 and Table 10), led to the conclusion that the
systematic attitude error to apply to survey
positions was and that
the 95% confidence radius could be expressed as
![[EQUATION]](img17.gif)
where is the Maximum Likelihood error.
2.2. The SIMBAD database
The master list used for the cross-correlation was extracted from
the SIMBAD database in 1990 and not later updated. SIMBAD early type
stars are mostly recognized from HD, CD, BD, HR and SAO spectral
information and associated literature. We list in Table 1 the
distribution in spectral types of the SIMBAD stars located within 20
from the galactic plane, subdwarfs excluded.
Stars having general spectral types 'OB' mostly originate from the
Luminous Star (LS) catalogues compiled by the Hamburger Sternwarte and
Warner and Swasey Observatories for both hemispheres (Hardorp et al.
1959, Stephenson & Sanduleak 1971). Another important group of
'OB' stars without precise spectral classification arises from the
Vatican Emission line Star (VES) catalogue (McConnell & Coyne
1983). Most of the stars classified as 'OB' are in fact earlier than
B3 (Slettebak & Stock 1957). Because of the heterogeneousness of
the various OB catalogues, it is difficult to quantify with accuracy
the completeness in magnitude of our input sample. The Luminous Star
catalogue which gathers most of the faintest early type stars is
apparently complete down to B 12 over the whole
galactic plane.
![[TABLE]](img20.gif)
Table 1. Distribution in spectral types of the OB SIMBAD stars ( 20 )
We first extracted all SIMBAD entries which had an associated error
circle overlapping a wide box centered on each
ROSAT source. This rather large search area allows the correlation of
candidate OB/X-ray sources with objects having inaccurate coordinates
such as some variable stars, or with supernova remnants. In a second
step we selected from the main correlation list all sources having a
match within with a star of spectral type O or
B, retaining for each selected OB/X-ray association the entire list of
possible candidates extracted in the wide box.
The restriction on angular distance allows to eliminate many spurious
OB/X-ray matches since the 90% confidence ROSAT radius is usually less
than (Voges et al. 1992, corresponding to a 95%
confidence radius in the range of ) and optical
positions of OB stars are known to better than
. Thirteen sources which were thought to have a
likelier identification than the proposed OB star were removed by hand
from the correlation list (see Table 2). This happened for
instance when a known active corona (RS CVn binary, pre-main sequence
star) or a supernova remnant was also present in the ROSAT error
circle. Pre-main sequence stars and RS CVn binaries are known to be
sometimes bright soft X-ray sources with luminosities up to
1031 erg s-1 (e.g. Montmerle et al. 1983,
Walter & Bowyer 1981). Keeping in mind that our aim was to select
only promising accreting candidates leaving doubtful identifications
for a later study we decided to ignore these cases for the moment. On
similar grounds, compact OB associations (i.e. groups of OB stars
located within few arcminutes and appearing blended at the X-ray
spatial resolution of the RASS) were not considered in this analysis
because of the difficulty in assessing an individual X-ray to
bolometric luminosity ratio for such objects. The B2V star HD 63177 (V
= 8.31) tentatively associated with RX J0744.9-5257 was also taken off
from the final correlation list since optical follow-up observations
have shown the presence of a cataclysmic variable
away from the candidate B
star (Motch et al. 1996a).
![[TABLE]](img27.gif)
Table 2. List of OB/X-ray associations removed from the main correlation list on the basis of the existence of a possible alternative identification. Count rates are taken from the SASS analysis of the survey and the optical information listed is entirely extracted from SIMBAD as in 1990
Finally, for the sake of comparison, we keep aside the group of 13
known massive X-ray binaries detected during the ROSAT all-sky survey
at (see Table 10).
Of these, 9 were listed in the OB SIMBAD catalogues and retrieved
correctly during the above selection process while 4 were added using
data extracted from the literature (He 3 -640, Cen X-3, 1H1909+096,
EXO 2030+375). The final list of OB/ROSAT source associations
contained a total of 237 sources entries split as shown in
Table 3.
![[TABLE]](img29.gif)
Table 3. The final OB/RGPS correlation list
2.3. The / diagnosis for OB stars
All stars of spectral type earlier than B5
emit soft X-rays (0.2-4.0 keV) with a roughly constant X-ray to
bolometric luminosity ratio independent of the luminosity class and
age (e.g. Long & White 1980; Pallavicini et al. 1981). Sciortino
et al. (1990) show that the mean value of /
for O stars is close to 10-6.46 with
about 10% of these early type stars having /
in the range of 10-6
-10-5.5. From ROSAT survey data Meurs et al. (1992) find a
mean log ( / ) of -6.8
0.5 for 43 O3 to B2.5 stars with no strong
difference between OB and OBe stars. Using ROSAT PSPC pointed
observations, Cassinelli et al. (1994) find that the
/ ratio decreases sharply
with spectral types later than B1 and could reach
10-9 at B3V.
The physical origin of the X-ray emission is still a matter of
debate. Several authors assumed a picture stretched from the solar
case in which a hot corona located close to the stellar surface lies
below the cooler, high speed velocity wind (see e.g. Cassinelli et al.
1981, Waldron 1984). Alternatively, Lucy & White (1980) proposed
that blobs of high density formed in the expanding wind may produce
shocks and consequently X-rays.
2.4. Computation of the X-ray to bolometric luminosity ratio
Interstellar extinction may alter quite significantly the ratio of
X-ray to optical flux measured from these stars. Depending on the
softness of the assumed intrinsic X-ray spectrum the decrease of the
0.1-2.4 keV PSPC count rate with increasing interstellar column
density may be quicker or slower than that of the optical flux. For
instance, the overall PSPC count rate produced by a T = 106
K thin thermal spectrum (Raymond & Smith 1977) will be steeply
decreasing with column density and at =
1021 cm-2 it will be 100
times more dimmed than any optical flux crossing the same interstellar
medium. On the opposite, the PSPC count rate resulting from a power
law energy distribution with a photon index of 1, typical for Be/X-ray
systems, will decrease less rapidly with than
optical radiation.
In order to correct for interstellar extinction we computed the
colour excess E(B-V) from the spectral types and magnitudes listed in
SIMBAD. Intrinsic B-V were taken from Johnson (1966) and absolute
magnitude and bolometric corrections are from Deutschman et al. (1976)
and Humphreys & McElroy (1984). The corresponding column density
was then used to compute the PSPC count rate to un-absorbed flux
conversion factor assuming a 107 K thin thermal spectrum
(e.g. Pallavicini et al. 1981). Finally, the X-ray luminosity was
estimated using the distance derived from the colour excess and
spectral type together with the catalogue V or B magnitudes.
Obviously, there exist several possible temperatures for normal OB
X-ray emission. Chlebowski, Harnden & Sciortino (1989) find
temperatures in the range of 3 to 9 106 K from Einstein
data and Cassinelli et al. (1994) reach similar conclusions from ROSAT
observations. By intentionally assuming a temperature on the hot side
of the distribution we avoid a systematic overestimation of the X-ray
luminosity when correcting for interstellar absorption. Only in the
rare case of a soft component (e.g. 2 106 K) seen at very
low do we expect overestimation of the
un-absorbed X-ray emission. If the intrinsic spectrum is actually
described by a power law distribution of photon index 1-2 as expected
for young accreting neutron stars (e.g. White et al. 1983) the effect
on the un-absorbed luminosities is not large since we overestimate
them by at most a factor of 2 at low and
underestimate them by the same factor at =
1022 cm-2. Finally we note that with a
107 K thin thermal energy distribution, the absorbed X-ray
count rate to optical flux ratio remains constant within a factor of 2
for a large range of and that therefore, the
estimated X-ray to bolometric luminosity ratio is relatively
unaffected by errors on the intervening column density.
Another problem arises from the unavoidable incompleteness of the
data listed in SIMBAD. Several stars lack precise spectral types
and/or magnitudes. Keeping in mind our concern to select the most
obvious accreting candidates, in order not to overestimate the X-ray
to bolometric luminosity ratio we used as default value B0 for all
stars without subtypes and luminosity class III for all stars lacking
appropriate information. When the B-V colour was not available we
arbitrarily assumed B-V = 0.0. These default assumptions imply that we
may have missed a fraction of the low /
candidates. In the few cases when a range of
spectral types and luminosity classes was available, we used the
average value. For early type stars, the difference in intrinsic
colours and bolometric corrections between two consecutive spectral
types or luminosity classes is always small compared to other
uncertainties.
We show in Fig. 1 the distribution of all X-ray/OB star
identifications having spectral types earlier than or equal to B5 and
located within r95, the 95% confidence radius of the ROSAT
position, in the / versus
HR2 diagram. The hardness ratio HR2 is more sensitive to the intrinsic
shape of the spectrum while the softer HR1 is rather an indicator of
the photoelectric absorption. Most OB stars cluster at
/ between 10-7
and 10-6 as expected from previous studies carried out with
the Einstein satellite (Pallavicini et al. 1981, Sciortino et al.
1990). Simulations show that indeed the slight variation in energy
range from Einstein (0.2-4.0 keV) to ROSAT (0.1-2.4 keV) does not
significantly change the X-ray luminosities derived from the two
satellites. Similar values of /
were also derived from a subsample of ROSAT
survey data (Meurs et al. 1992). We did not investigate possible
differences between OB and OBe stars in our sample. Most normal OB
stars have HR2 values comprised between -0.7 and +0.3 which correspond
to thin thermal temperatures in the range of 3 106 K to 3
107 K in agreement with those usually reported for normal
OB stars. In contrast, the known massive X-ray binaries detected in
the galactic plane exhibit larger luminosity ratios and a much harder
HR2 which probably reflects the intrinsically harder power law-like
energy distribution of accreting neutron stars and also maybe to a
lower extent the often large interstellar and intrinsic column
densities. We note, however, that luminous soft X-ray components were
detected in some massive X-ray binaries accreting from the wind of the
primary (e.g. 4U 1700-37; Haberl et al. 1994) or through Roche lobe
overflow (e.g. LMC X-4; Dennerl 1991). This last source is located in
a direction of low interstellar absorption and exhibits HR1 = 0.36
0.01; HR2 = -0.20 0.01.
Because of the sometimes large orbital phase dependent circumstellar
absorption, the presence of a soft X-ray excess, although clearly
detectable from the relative strength of the hard (2-10 keV) and soft
(0.1-2 keV) un-absorbed components, does not necessarily imply a very
soft value of HR1 or HR2 in the ROSAT band (e.g. 4U 1700-37; Haberl et
al. 1994). On the other hand, Be/X-ray systems seem to lack a soft
component (Haberl 1994).
![[FIGURE]](img32.gif) |
Fig. 1. Distribution of all X-ray/OB star identifications having spectral types earlier or equal to B5 and located within the 95% confidence radius in the / versus HR2 diagram. Open circles represent the ROSAT OB identifications and crosses represent the known high mass X-ray binaries detected during the ROSAT all-sky survey and located at absolute galactic latitudes below 20 . The size of the symbols is inversely proportional to the error on HR2. While most ROSAT OB star identifications cluster in the narrow range of / ratio similar to that derived from Einstein observations, all established X-ray binaries appear with clear X-ray excess and much harder X-ray spectra than normal OB stars
|
By contrast, it can be seen on Fig. 2 that B6 to B9 stars
exhibit a much larger /
ratio than earlier types and also a somewhat larger scatter. This
behaviour was already noticed by Meurs et al. (1992). Obviously these
late B stars do not obey the same relation as hotter types. The range
of HR2, however, is comparable to that observed in earlier B stars.
Einstein observations of nearby A type stars by Schmitt et al. (1985)
showed that none of these stars had any detectable X-ray emission.
Nevertheless, the Einstein observatory detected several A type stars
in the Pleiades cluster (Micela et al. 1990) and more recently several
field A stars were detected in the ROSAT all-sky survey (Schmitt et
al. 1993). These evidences led to the common assumption that the X-ray
emission sometimes associated with late B or A stars was originating
from an optically undetectable G-M type companion. However, ROSAT HRI
observations seem to question this explanation and may point toward
intrinsic X-ray emission at least in some late B stars (Berghöfer
& Schmitt 1994). In spite of the high /
ratio the actual un-absorbed X-ray luminosities
all remain below 2 1032 erg s-1 and only four
stars exhibit X-ray luminosity in excess of 6 1031
erg s-1. Inspection of the optical maps of these four
stars suggests the presence of likelier optical counterparts in the
ROSAT error circle. Therefore, considering the absence of good
candidates displaying X-ray luminosities above the level at which an
interpretation in terms of an optically unseen late type companion
star becomes untenable and the rather large expected number of
spurious matches resulting from the size of the entry catalogue, we
decided not to investigate the late B stars for the moment.
Consequently, in the following, we shall only consider stars earlier
than B6 or those having the general 'OB' type designation totaling
15895 stars.
![[FIGURE]](img34.gif) |
Fig. 2. Distribution of all X-ray/OB star identifications having spectral types B6 to B9 and located within the 95% confidence radius in the / versus HR2 diagram. The size of the symbols is inversely proportional to the error on HR2. Late B stars do not follow the same / relation as earlier spectral types (see Fig. 1) and exhibit a large scatter. At the count statistics typical of survey data, their hardness ratios HR2, however, are not essentially different from those of O-B5 stars
|
2.5. Rate of spurious matches
We show in Fig. 3 the histogram of X-ray to optical distances
for the 128 stars earlier than B6 associated with an X-ray source. The
shape of the distribution shows without ambiguity that several OB
stars were indeed detected during the ROSAT survey. The average 95%
confidence radius for all OB star matches in the original
cross-correlation list is 34. 5. With a total of 15895 OB stars
earlier than B6 and a mean survey source density of
one per square degree in the galactic plane
(Motch et al. 1991b), roughly independent of galactic latitude and
longitude (Voges 1992), we expect 5 spurious matches among the 108
spatial coincidences within r95 and 10 spurious matches
among the 17 associations found in the range of 1 to 1.8
r95, this latter value corresponding to the
limit. The estimated number of real OB/X-ray
associations within and outside r95 is 103 and 7
respectively, thus confirming the validity of the statistics used for
the computation of error radii.
![[FIGURE]](img36.gif) |
Fig. 3. Histogram of the distances between the SASS and SIMBAD positions for the 128 stars earlier than B6 associated with a ROSAT all-sky survey source. X-ray to optical distances are expressed in units of the 95% confidence error radius
|
Because the number of non X-ray detected OB stars is much larger
than the number of OB stars detected in the survey, we expect most
spurious OB/RGPS source associations to be with non detected early
type stars and therefore to be preferentially found in the high
/ samples. Consequently,
the recognition of a genuine accreting source requires additional
information which may be the X-ray spectral or temporal behaviour
and/or follow-up optical searches for alternative optical
counterparts, essentially active coronae in the galactic plane. For
the B6-B9 stars, we expect 6 spurious spatial
coincidences among the 79 located within the 95% confidence radius
implying that these random associations cannot explain the
systematically higher /
ratio exhibited by these late B stars.
2.6. Selection of the candidate stars
Using the computed luminosity ratio and optical / X-ray positional
information we selected three different sets of candidate OB/X-ray
binaries as defined in Table 4. The use of the 95% confidence
radius was guided by the fact that the expected number of spurious
matches within r95 ( 5) was about the
same as the number of true associations expected to be located outside
this radius. However, when referring to the accuracy of the X-ray
positions we will mention the 90% confidence radius in order to be
consistent with commonly used conventions in X-ray astronomy. The
lower limit of / = 3
10-6 is the maximum ratio observed
for normal OB stars with the Einstein satellite (Sciortino et al.
1990). As a final criterion we removed 3 stars displaying X-ray
luminosities below 1031 erg s-1 since as
already argued above, these luminosities may be radiated by the most
active late type coronae. The 3 stars (HD 37272, HD 180939 and HD
68518) all have B5V spectral types and /
in the range 4-6 10-6.
![[TABLE]](img39.gif)
Table 4. Definition of the three groups of OB/X-ray binary candidates
For comparison, all but one known massive X-ray binaries detected
by ROSAT fall into candidate group 1. None appears in group 2 and the
only one listed in group 3 is 4U 1700-37 / HD 153919 which consists of
a neutron star accreting in the wind of a hot and luminous O6.5Iaf+
star (Walborn 1973; Haberl et al. 1994).
We then searched the literature in order to check for possible
errors in the spectral type listed in each SIMBAD object header and
more generally with the aim to find evidences for an alternative
explanation to the observed X-ray emission such as a referenced late
type companion or a white dwarf.
As a final check we analyzed interactively each remaining source
with the dedicated Extended Scientific Analysis System (EXSAS)
developed at MPE (Zimmermann et al. 1992). Whereas SASS operated on
distinct survey strips, EXSAS has the capability to use all photons
detected from a given region of the sky (1
1 merged fields in our
case) thus yielding improved background determination and source
detection. For most sources, count rates, positions and hardness
ratios derived from the interactive analysis were fully consistent
with those given by the SASS output. We note that some of the
positions computed by EXSAS were or more away
from the SASS determinations but still compatible within the errors.
This slight change of X-ray position together with the use of refined
optical coordinates extracted from the Guide Star Catalogue (GSC;
Lasker et al. 1990) for 'LS' stars explains the difference between
X-ray to optical distances listed in Table 5 and those appearing
in individual finding charts and later Tables.
![[TABLE]](img41.gif)
Table 5. The list of candidate OB/X-ray binaries scheduled for optical and X-ray follow-up observations. This table summarizes the preliminary values of spectral type, magnitude and star distance used as input of the automatic process in order to estimate / and un-absorbed . The input information was extracted from the SIMBAD header and results of the SASS analysis of the survey (see Sects. 2.1 and 2.4). When a range of spectral types and luminosity classes was available, we used the average value listed here. d is the distance between the SASS position and that of the associated OB star as read from the SIMBAD header. Errors listed here only arise from the PSPC count rate and do not take into account uncertainties on the detailed spectral type, interstellar absorption and distance. The horizontal lines divide the three groups of candidate stars defined in Table 4
However, in a few instances, the EXSAS process derived count rates
significantly lower than those given by SASS. In four cases (CPD -59
2854, LS III +58 47, HD 313343 and LS 4287), the SASS source was
hardly or even not recovered at all by EXSAS and therefore dropped
from the final list. These discrepancies could be related to the
different data analyzed by the two processes. Survey spectra were
accumulated for each source and light curves were systematically
checked for variability and for the presence of features
characteristic of a survey artifact.
The final list of SASS/SIMBAD OB/X-ray candidates containing 24
entries is printed in Table 5.
© European Southern Observatory (ESO) 1997
Online publication: May 26, 1998
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