A&A 376, 476-483 (2001)
DOI: 10.1051/0004-6361:20010919
J. S. Clark1,2 - P. Reig3,4 - S. P. Goodwin1,5 - V. M. Larionov6,7 - P. Blay8 - M. J. Coe9 - J. Fabregat8 - I. Negueruela10 - I. Papadakis3 - I. A. Steele11
1 - Astronomy Centre, CPES, University of Sussex, Brighton, BN1 9QH,
UK
2 - Department of Physics and Astronomy, University College London, Gower
Street, London, WC1E 6BT, England, UK
3 - Physics Department, University of Crete, PO Box 2208, 71003,
Heraklion, Greece
4 - Foundation for Research and Technology-Hellas, 71110,
Heraklion, Greece
5 - Department of Physics and Astronomy, University of Wales, Cardiff,
CF24 3YB, Wales, UK
6 - Astronomical Institute of St. Petersburg University, St. Petersburg
198904, Russia
7 - Isaac Newton Institute of Chile, St. Petersburg Branch
8 - Departmento de Astronomia, Universidad de Valencia, 46100 Burjassot,
Valencia, Spain
9 - Department of Physics & Astronomy, University of Southampton, Highfield,
Southampton, SO17 1BJ, UK
10 - Observatoire de Strasbourg, 11 rue de l'Université, 67000 Strasbourg,
France
11 - Astrophysics Research Institute, Liverpool John Moores
University, Liverpool, L41 1LD, UK
Received 24 April 2001 / Accepted 26 June 2001
Abstract
We present new optical - near-IR spectroscopic and photometric observations
of the newly discovered galactic microquasar LS 5039, which indicate a
classification for the mass donor in the system of O6.5V((f)). Optical spectroscopy and photometry
shows no variability over a timescale of years, and we find no evidence of modulation by, or emission
from the compact companion in these data. However significant
photometric variability ( mag) is present in the H and K bands
between 1995-2000. Such variability has been observed in other radio bright
X-ray binaries where it has been attributed to synchrotron emission from the
jet. However, given the non-thermal spectral index of the radio emission in
LS 5039 this explanation appears unlikely, predicting a near-IR flux
3
orders of magnitude too small to contribute significantly at such wavelengths.
Nightly optical photometry over a 21 day period between 2000 May-June reveals
variability at a level of a few hundredths of a magnitude, with no
periodicity or long term trend visible. Likewise, while the
radio lightcurves show moderate variability
(
per cent of the mean flux density) we find no evidence of periodic
modulation - Monte Carlo simulations constrain any such periodic variability
to <4 per cent modulation at 2.25 GHz. The differences in behaviour between
LS 5039 and Cygnus X-1 - the most closely related radio emitting High Mass
X-ray Binary - are likely to be a result of the weaker stellar wind and
probable greater orbital separation of LS 5039 compared to Cyg X-1.
Key words: stars: early-type - stars: individual: LS 5039 - X-rays: stars, binaries
The early type star LS 5039 has been proposed as the optical counterpart
to the X-ray source RX J1826-1450 by Motch et al. (1997, hereafter M97),
who classify it as O7V((f)) on the basis of low resolution blue end
spectroscopy. Subsequent observations by Martí et al. (1998, hereafter Ma98)
suggested that
LS 5039/RX J1826-1450 was also associated with a persistent, moderately
variable unresolved radio source with a non thermal radio spectrum of
spectral index
(adopting the convention
).
Ribó et al. (1999, henceforth R99) present a detailed timing and spectral
analysis of X-ray data obtained with the Rossi X-ray Timing Explorer
(RXTE). They find the timing analysis shows no evidence for either pulsed
emission, or periodic variability between 2-200 d, which might arise from
orbital modulation of the X-ray flux.
Furthermore, they analysed four months of daily observations
of the 8.3 and 2.25 GHz flux of LS 5039, obtained as part of a long term
monitoring campaign with the Green Bank Interferometer
(GBI), but found
no evidence for periodic variability between 2-50 d, or well defined flaring
activity in these data.
More recently Paredes et al. (2000) succesfully
resolved the radio emission as a bipolar jet, confirming LS5039 as one of
the few radio emitting High Mass X-ray Binaries (hereafter HMXB)
known in the galaxy. Given
their rather heterogeneous nature, further observations of
RX J1826-1450/LS 5039 are clearly of interest. Here we present new
optical (4100-7400 Å) and near-IR (1.5-2.2 m)
spectroscopic data obtained between 1995-2000 enabling
a more accurate spectral classification for the system. Photometric
observations from 1997-2000 are also presented. In combination with
published data and our spectroscopic observations these allow us to search
for medium-long term variability. Finally we also
undertake a more detailed analysis of the complete
8.3 and 2.25 GHz GBI datasets than has previously been published.
Optical spectroscopic observations were made with the 1.9 m telescope at
the South African Astronomical Observatory (SAAO) on 1997 June 23, the
1.3 m telescope at Mount Skinakas (Crete, Greece) on 1999 July 27 and
2000 July 20-22 and the ESO 1.52-m telescope at La Silla Observatory, Chile on
2000 September 13.
The spectrum at SAAO was taken with the ITS spectrograph +
SITe CCD + 1200 lines mm-1 (grating No 4) + 250 m slit. With this
set-up the dispersion was
Å/pixel and the wavelength coverage
4100-4900 Å. The f/7.7 Ritchey-Chretien telescope at Skinakas
observatory was equipped with a
ISA SITe CCD + 1302 lines
mm-1 grating, giving a dispersion of
Å/pixel. In the 1999 July
spectrum the slit width was 320
m, whereas in the 2000 July
spectra it was 80
m. Finally the 1.52 m La Silla Observatory telescope was
equipped with the Boller and Chivens
spectrograph + #33 holographic grating and
the Loral 38 camera. This configuration gives a dispersion of
Å/pixel (resolution of
Å at
Å).
Near-IR observations of LS 5039 were made with the United Kingdom Infrared
Telescope (UKIRT) on 1999 July 22-24, using the Cooled Grating Spectrometer
(CGS4). Observations between 1.5-2.2 microns were made
using the long focal length camera
plus the 150 line/mm grating, giving a velocity resolution of km s-1.
Note that at this resolution the wavelength range of a single spectrum is
m - hence breaks in the spectra presented in Fig. 3
are due to slight missmatches in wavelength coverage between individual
spectra. Initial data reduction was carried out at the telescope using the
CGS4DR software (Puxley et al. 1992).
This removes bad pixels, debiases, flat-fields, linearity
corrects and interleaves oversampled
scan positions. The subsequent stages of data reduction,
comprising of sky subtraction, extraction, derippling and wavelength
calibration using observations of a CuAr lamp, were carried out using
the Starlink-supported package FIGARO. Removal of telluric
features was accomplished via the procedure described by Clark & Steele
(2000).
![]() |
Figure 1: Blue end (4050-4950 Å) spectra of LS 5039 with line identifications indicated (Diffuse Interstellar bands=DIB). The comparison stars are the O6V((f)) and O7V((f)) standards from the digital atlas of Walborn & Fitzpatrick (1990; HD 101190 and 15 Mon respectively). |
Open with DEXTER |
![]() |
Figure 2: Red end (5650-7500 Å) spectrum of LS 5039. |
Open with DEXTER |
![]() |
Figure 3:
Composite K band (2.05-2.20 ![]() ![]() ![]() ![]() ![]() ![]() |
Open with DEXTER |
The optical Balmer lines are in absorption, indicating the absence of a
dense circumstellar envelope around the star, and precluding an Oe/Be
classification. The red-end spectrum is characterised by the presence of
numerous interstellar lines (we derive reddening estimates from
these in Sect. 4) and neutral helium lines in absorption
(
5876, 6406, 6679 and 7065). The blue-end of the
optical spectrum is dominated by the hydrogen Balmer series and by strong
He II lines.
The ratio between He II 4541 and He I 4471
corresponds to a spectral type O6.5, according to the relations of Mathys
(1988). The fact that He II 4686 is strongly in absorption identifies
the star as main-sequence. This can be checked by direct comparison with MK
standards. As seen in Fig. 1, the strength of the He I and He II lines in
LS 5039 falls between those of the O6V((f)) and O7V((f)) standards.
Very weak N III
4634-40-42 emission is present. The
combination of strong He II absorption and weak N III
emission is represented by ((f)) - suggesting an O6.5V((f)) classification
- slightly earlier than the classification of M97,
made with a single low resolution
3800-4600 spectrum.
Line | EW (Å) | ||
1997 | 1999 | 2000 | |
H![]() |
2.3 | 2.2 | |
HeII ![]() |
0.50 | 0.55 | |
H![]() |
2.4 | 2.4 | |
HeI ![]() |
0.50 | 0.50 | |
HeII ![]() |
0.92 | 0.90 | |
HeII ![]() |
0.95 | 1.0 | |
H![]() |
2.4 | 2.5 | |
HeI ![]() |
0.24 | 0.26 | |
H![]() |
3.6 | 3.9 | |
HeI ![]() |
0.90 | 0.85 | |
HeI ![]() |
0.50 | 0.55 |
The spectral classification of hot, luminous stars via
near-IR spectroscopy alone is far less precise than via conventional optical
classification. Nevertheless, it provides an independent test of the
optical classification. The comprehensive spectral atlas of
Hanson et al. (1997) was used for classification of
the K band spectrum
of LS 5039. The strength of H I 2.166 m and He II
2.1885
m lines (Table 2) restrict the spectral type to later than O6,
and earlier than O8, while the He I
2.113
m absorption feature further constrains the spectral type to
earlier than O7.5. Finally,
the N III emission line is absent in normal Main Sequence O6-O8
spectra and confirms the O((f)) classification, fully consistent with the
optical classification.
Unfortunately, no such comprehensive reference work exists for the
classification of OB stars via the H band; we use the preliminary work
of Blum et al. (1997) and the more extensive work of Hanson et al.
(1998),
noting that this atlas is restricted to a narrow waveband (1.66-1.72 m)
and complete only for spectral types between O7-B9.
Comparison of the strengths of the H I brackett series to
the spectra presented by Blum et al. (1997) confirm an O star
classification, while the strength of the He II transition is
consistent with a spectral type of O8 or earlier (Hanson et al. 1998).
Direct comparison to the spectrum of the similar
O7V((f)) star HD 47839 presented in Hanson et al. (1998) shows the He II feature to be stronger for LS 5039,
while the H I Brackett 11 line is somewhat weaker.
However He I absorption is of comparable strength in both spectra.
In conclusion we find that both H and K band spectra are consistent
with the optical classification of LS 5039 as O6.5V((f)),
with the K band spectrum providing the
more rigorous constraints to spectral type (i.e. no earlier than O6.5 and no
later than O7) We note that we find no evidence for a
luminous stellar companion for LS 5039 in any of the spectra, nor any
additional emission features that might be attributed to emission from a jet
or accretion disc.
Wavelength | Feature | EW |
(microns) | (microns) | (Å) |
1.573 | He II ![]() |
0.5 |
1.612 | H I ![]() |
0.8 |
1.681 | H I ![]() |
1.6 |
1.690 | ? | Blend |
1.692 | He II ![]() |
1.4 |
1.701 | He I ![]() |
1.1 |
2.059 | He I ![]() |
0.5 |
2.113 | He I ![]() |
0.1 |
2.116 | N III ![]() |
-1.1 |
2.167 | H I ![]() |
2.6 |
2.189 | He II ![]() |
2.0 |
Broadband photometric observations of LS 5039 from 1973-present
have been reported by a number of authors
(e.g. Drilling 1975; Lahulla & Hilton 1992, Ma98) and
indicate that between 1973-present LS 5039 has been remarkably stable
(e.g.
mag.) in the optical.
This stability is also evident in the observations presented
in Table 3 - obtained by us on 1996 October 3-5 with the 1.9 m
telescope of the South African Astronomical Observatory (SAAO)
and 2000 August 17-24 with the 0.7 m telescope of the Crimean Astrophysical
Observatory (CrAO). With the exception of the single V band datum from Kilkenny et al. 1993
these observations are consistent with no variability.
However JHK band observations made with the Telescope Carlos Sanchez
(TCS) between 1995-2000 (Table 4) appear to show significant variability
( mag) in H and K bands respectively. The same standard star (BS 7236)
and reduction procedures were adopted for all observations, and given the
proximity of BS 7236 to LS 5039 the possibility of differential extinction
due to dust close to the horizon can also be eliminated as a cause
of such variability. The source of such variability is not clear. The wind
from the O6.5V((f)) primary is not expected to contribute significantly at
these wavelengths (Runacres, priv. comm.), while extrapolation of the radio
flux to near-IR wavelengths yields a flux 3 orders of magnitude too small to
explain the observed variability.
Observation | B | V | R | I | J | H | K | L |
K93, SJ93 | - |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
- |
1996 Oct. |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
1998 June 7 | - |
![]() |
![]() |
![]() |
- | - | - | - |
1998 June 8 | - |
![]() |
![]() |
![]() |
- | - | - | - |
2000 Sept. |
![]() |
![]() |
![]() |
![]() |
- | - | - | - |
Date | J | H | K |
14/10/95 |
![]() |
![]() |
![]() |
31/7/96 |
![]() |
![]() |
![]() |
19/7/97 |
![]() |
![]() |
![]() |
21/7/97 |
![]() |
![]() |
![]() |
26/7/99 |
![]() |
![]() |
![]() |
28/7/99 |
![]() |
![]() |
![]() |
31/7/99 |
![]() |
![]() |
![]() |
2/10/99 |
![]() |
![]() |
![]() |
7/10/99 |
![]() |
![]() |
![]() |
6/7/00 |
![]() |
![]() |
![]() |
17/10/00 |
![]() |
![]() |
![]() |
18/10/00 |
![]() |
![]() |
![]() |
In order to search for medium term, day-to-day variability - possibly
indicative of orbital modulation - photometric observations of LS 5039
were made using the 1.3 m telescope at
the Skinakas Observatory (Crete, Greece). Standard Johnson B and V filters
were used in combination with a tektronix
CCD with
24
m pixels. The source was observed on 21 nights during the period
May 12, 2000 to June 24, 2000.
The seeing during the observation run varied
between
and
.
Standard image processing (bias
subtraction and flat-fielding) was applied to all images.
To improve the accuracy of the results we performed relative photometry of
the source with respect to three reference stars in the CCD frame. We
checked the constancy of these three stars by obtaining the difference
between one of them and the average of the other two. The standard
deviation of the individual light curves throughout the period was
.
An idea of the accuracy of our relative photometry can be
obtained by taking the standard deviation of the global light curve of the
three reference stars:
and
for the B and
V bands, respectively.
LS 5039 shows variations within a night with an amplitude of a few hundreths of magnitude. These low amplitude variations also occur from one night to the next but on a longer time scale no particular periodic or quasiperiodic trend is apparent. From these observations we can exclude the presence of (periodic) orbital modulation between 3-15 days, noting that if periodic variability were present at the level observed in HDE 226868 (Brocksopp et al. 2000) - the mass donor in Cygnus X-1 - our observations would have detected it. Aperiodic photometric variability at this level is not unknown in O type stars. Balona (1992) surveyed 16 bright O stars and found variability in all the supergiants observed, and a subset of the main sequence stars, including HD 101190, an O6V((f)) star (for which no periodicity was identifiable). Balona (1992) suggests that either variations in mass loss rates or non-radial pulsations could cause the observed variability but is unable to discriminate between the two possibilities.
We have estimated the interstellar reddening to the source by using the
correlation between the E(B-V) colour excess and the equivalent width of
diffuse absorption features of interstellar origin (Herbig 1975). We used
the lines 4430,
6376/79 and
6613 only since a
clean measurement of their equivalent width can be carried out. Due to the
relative low resolution of our spectra some of the interstellar features
appear blended with other interstellar or atmospheric lines. We find the
mean colour excess to be
,
in agreement with an earlier
measurement by M97 of
using the same spectroscopic
method. However we find that this estimate suggests a significant
colour excess when applied to the photometry from 1996
October (Table 3). By comparison we find that applying the photometrically
derived reddening estimate from our data (
)
results in no excess emission in the near IR. Additionally, the X-ray data
support a higher value for the colour excess. From a
BeppoSAX observation a hydrogen column density of
cm-2 is found (Reig, private
communication), implying
(Predehl & Schmitt 1995) - not
statistically different from the value adopted from the photometric data.
We therefore choose to adopt
for LS 5039 given that it is consistent with both the
near-IR colours of, and hydrogen column density to, LS 5039.
The results of GBI monitoring of LS 5039 have been presented by both
Paredes et al. (2000) and R99. R99 analysed
days worth
of data but, while claiming moderate variability, could find no
evidence of periodicity. Here we reanalyse the complete dataset (284 data points
spanning 340 days between 1998 September 16-1999 August 22)
taking into account the detection threshold at both wavelengths.
The 2.25 GHz data is simple to analyse as it lies far above the significant
detection limit of 10 mJy. The 2.25 GHz light curve has a mean value of
31.3 mJy and a standard deviation of 6.2 mJy; given that the noise level
for this frequency is 4 mJy, the source is detected at the
level on average. The instrumental noise corrected source variance
is 22.1 mJy2. Therefore, the rms source variability amplitude at
the 2.25 GHz band is
.
This result is significant at a
95% confidence level (i.e. it is very likely that there is variability
above the expected noise level).
The situation is different in the 8.3 GHz band. The instrumental noise
level (i.e.
is 6 mJy and several data points in the light
curve lie at or below this limit. In order to estimate the mean and variance of
the light curve we assume that the signal is drawn from a Gaussian
distribution, truncated at 10 mJy (which is
)
and minimise the likelihood function. Our best mean and standard
deviation estimates are 13.7 and 6.7 mJy respectively (significantly
different from the results if the truncation at 10 mJy is not
considered). Hence, the instrumental noise corrected source standard
deviation is formally 2.3 mJy (and the rms source variability amplitude
)
although this result is not statistically significant. We
conclude that the rms variability amplitude of the source is similar in
the two radio bands, although the result about the 8.3 GHz band should
be treated with caution as the source at that band is detected at only
the
level on average.
Examination of the power spectra of the two sets of data show that there
are no significant peaks. To determine what periodic signals could be
hidden within the noise a series of Monte Carlo simulations were
construced of a fake 2.25 GHz data set drawing noise randomly
from a gaussian with mJy and adding a periodic signal of
amplitude 4.7 mJy (or less). Variability would be clear and unambiguous
on periods of 2 day (limited by the sampling interval) to 340 days (the total data
set) for a signal of > 1.2 mJy amplitude, that no such signal is seen
is very strong evidence that none exists. The 8.3 GHz data is
impossible to analyse in any significant way as the periodic signal
(if there is one) is totally dominated by noise.
For this reason it is impossible to determine any information about the spectral index of the source. Monte Carlo simulations were used to construct fake data sets where the spectral index was known and then noise was added in the hope that there would be a difference between these fake data sets that would give an indication of the behaviour of the real source. Unfortunately the addition of the noise swamps any underlying behaviour making them indistinguishable. The binning of the data in order to (hopefully) minimise the noise does not help. Higher signal-to-noise observations are clearly required to make further progress in understanding the radio behaviour of LS 5039.
Currently radio emission is observed from per cent of all X-ray
binaries, where it is thought to arise via the synchrotron mechanism in
collimated jets. The number of confirmed radio emitting HMXB is still
small, with only 5 other members of this rather heterogeneous
group known in addition to LS 5039 (see Table 5), therefore it is of interest
to compare the properties of LS 5039 to other members of this grouping.
Source | Primary + |
Secondary | |
Cygnus X-3 | Wolf Rayet +? |
Cygnus X-1 | O9.7Iab +black hole |
LSI+61 303 | Be +neutron star |
SS 433 | OBe? +? |
CI Cam | sgB[e] +? |
LS 5039 | O6.5V((f)) +? |
Of these systems CI Cam has been observed to undergo only one radio outburst and the detection of apparently bipolar jets at this time has subsequently been questioned. Photometric monitoring of the post-outburst behaviour of CI Cam revealed a subsequent episode of dust formation (Clark et al. 2000) - which has not been observed in the other radio emitting HMXB - which raises the question of whether the radio emission arises in a jet or a less collimated outflow, such as is seen for dusty novae. Both Cyg X-3 and SS 433 exhibit episodes of aperiodic flaring which are not apparent in the radio lightcurves of LS 5039, most likely due to instabilities or structure in the wind of the primaries leading to variable accretion rates. LSI+61 303 shows periodic radio flaring at its orbital period - once again this appears to be related to the interaction of the accretor with the highly aspherical wind of the Be star primary. Quasi-periodic X-ray outbursts are a common feature of Be/X-ray binaries and are attributed to the interaction of the compact object with the dense circumstellar disc of the Be star primary around periastron. These observations suggest the wind geometry of the mass donors in HMXB plays a significant role in their X-ray and radio properties.
Since both LS 5039 and HDE 226868 (O9.7 Iab) - the mass donor in Cyg X-1 -
are expected to have powerful quasi-spherical winds we might expect
both systems to share similar properties.
However, while both systems are persistent radio sources,
Pooley et al. (1999) find periodic modulations at
5.6 days (orbital period) and days in the radio emission of Cyg X-1,
which, if present at the same amplitude would have been detectable in
LS 5039 (Sect. 5). Additionally, periodic modulation at the orbital period
is also observed in the X-rays and UBVJHK broad band photometry of
Cyg X-1 (Brocksopp et al. 2000) - again absent in LS 5039.
We suggest that the differences in
properties of the 2 systems are a product of differences in
the stellar winds of both primaries in conjunction with a possible larger
orbital separation for LS 5039 (see below).
Although the properties of the stellar wind of LS 5039 have yet to be
determined (our optical spectra are not of sufficient resolution for detailed
modeling), the terminal velocity,
,
and mass loss rate,
,
for the slightly later O7V star HD 47839 are presented by Lamers
& Leitherer (1993). While
km s-1
is within a factor of 2 of that of HDE 226868
(
km s-1, Gies & Bolton 1986),
yr-1
is potentially an order of magnitude lower than that of HDE 226868
(
yr-1; Gies & Bolton 1986). Brocksopp et al.
(2000) attribute the orbital modulation of the radio flux of Cyg X-1 to
variable absorption by the dense stellar wind of HDE 226868 due to orbital
motion of the compact companion. Using Eq. (4) of Leitherer et al.
(1995) we find that the radio photosphere of LS 5039 is a factor of
4 smaller than that of HDE 226868 (due to the
dependance of the radio photosphere
radius) implying that modulation due to wind opacity is
likely to be less important in LS 5039, resulting in weaker
orbital modulation in the radio lightcurve (we also note that the linear size of the radio jets is
larger than the radio photosphere of LS 5039; Paredes et al. 2000).
Based on the rate of the energy loss of electrons to Inverse Compton
scattering in the UV radiation field of the star
Paredes et al. (2000) suggest an orbital radius of
cm for the compact object in LS 5039.
The canonical mass and radius of an O6.5V((f)) star are rather
uncertain - here we adopt
and
(Puls et al. 1996). Then assuming a neutron star companion and applying Keplers
Third Law we find a tentative orbital period of
days and
for LS 5039 compared to only
for
Cyg X-1. With such a small orbital separation in Cyg X-1
it is expected that both the stellar wind and star itself will be
distorted by the presence of the
black hole. Tidal
distortion of the stellar photosphere is thought to be responsible for the
(elipsoidal) modulation of the photometric lightcurves on the orbital period
(Brocksopp et al. 2000). Distortion of the wind due to the gravitational
potential of the black hole is also
thought to increase the mass loss rate through
the L1 point; the higher wind density (and hence emission measure)
leading to the emission components visible in the H
and He II
lines. The lack of medium term photometric variability
found for LS 5039 (Sect. 3.2) is thus consistent with a larger orbital radius
proposed for the compact companion in this system by Paredes et al. (2000), which would naturally
imply substantially reduced, or no, tidal
interaction between both components of the binary (numerical simulations by
Blondin et al. 1991, demonstrate that the
distortion of the stellar wind and surface drop off rapidly with increasing
orbital separation).
A large orbital separation also naturally explains
the lower X-ray flux in LS 5039 compared to Cyg X-1
( orders of magnitude; R99).
Geometrical dilution will reduce the wind density at the proposed orbital
separation of LS 5039 by
orders of magnitude compared to that at
the orbital radius of Cyg X-1, with a possible further reduction depending on
the base density of the wind. Although the limited temporal sampling of both the photometric and radio lightcurves
precludes the search for periodicities of
days we believe that such a short orbital period is unlikely. In addition to the argument for a large orbital period of Paredes et al. (2000)
accretion onto a possible White Dwarf companion in a close
(
days) orbit can be excluded - from Eq. (16) of Waters (1988)
we find that the expected X-ray luminosity would be an order of magnitude smaller
than that observed. We also note that such a short orbital period for a possible neutron star companion
can also be excluded since the O6.5V((f)) star would be larger than the Roche Lobe
(Eggleton 1983) leading to Roche Lobe overflow mass loss and a much larger X-ray luminosity
than observed.
The lack of periodic
variability in the X-ray lightcurve also suggests a low (e<0.1) orbital
eccentricity for LS 5039 given that accretion from a stellar wind with a
1/r2 density distribution results in an
accretion rate ranging from
to
(assuming on Bondi-Hoyle accretion theory). Given the long circularisation
timescale for such a wide orbit (longer than the main sequence lifetime of an
O6.5 star), this suggests that the system was left with such a low
eccentricity after the supernova which produced the compact object.
Therefore, the low level aperiodic variability observed in the radio and X-ray
wavebands is most likely due to inhomogineities or clumping in the
outer stellar wind
leading to changes in the accretion rate onto the compact companion
(O star winds are known to be highly structured close
to the star (
)
although their geometry at large radii are at
present uncertain). We note however, that if such a large orbital radius is
adopted then the relative velocity of the wind past the compact object - effectively
- should preclude the formation of an accretion disc around it according
to theories of standard direct wind fed accretion. Since the
presence of an accretion disc is thought to be a prerequisite for the
production of a collimated jet in X-ray binaries this is clearly problematic
for such a large adopted orbital radius.
Finally, although apparently a direct wind fed system, LS 5039 clearly does not occupy either of the conventional Be or supergiant subclasses of HMXB. Negueruela & Reig (2001) have remarked that there seem to be several such systems which either show hybrid properties of both subclasses or simply do not fit into either group.
We have presented optical and near-IR spectroscopy that confirm that the mass
donor in LS 5039 is an early O6.5V((f)) star. Long term observations show that
there are no long term trends in the optical properties of the O6.5V((f))
star. Likewise, while there are day to day variations of a few hundredths of
a magnitude in the optical lightcurve we find no coherent periodicities or long
term trends that might be the result of orbital modulation; the most likely
explanations for the low level optical variability are changes in the mass loss rate or
non radial pulsations in the system primary. Variations of mag are
apparent in the H and K band photometry between 1995-2000; at present there are
no obvious mechanisms for such variability within the system.
Variability is also present in both X-ray and radio wavebands. However
there is no evidence for significant flaring or periodic modulation;
we note that if orbital modulation of the radio lightcurve was
present above
per cent of the mean flux level at 2.25 GHz
(as is seen in Cyg X-1) it would have been detectable.
Although we cannot confirm the orbital separation suggested by Paredes et al. the
lack of orbital modulation in the radio, X-ray and optical lightcurves is
consistent with a rather large orbital separation and hence long orbital period.
We therefore attribute the differences in the properties of
the LS 5039 and the similar system Cyg X-1
to a combination of the weaker stellar wind and larger
orbital separation of LS 5039.
Acknowledgements
This paper is partially based on observations collected at the European Southern Observatory, Chile. The United Kingdom Infrared Telescope is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. Skinakas Observatory is a collaborative project of the University of Crete, the Foundation for Research and Technology-Hellas and the Max-Planck-Institut für Extraterrestrische Physik. P. Reig acknowledges support via the European Union Training and Mobility of Researchers Network Grant ERBFMRX/CT98/0195.