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arXiv:1010.5703v1 [astro-ph.HE] 27 Oct 2010
Astronomy & Astrophysics
manuscript no. aa˙xmm˙SMC3 c
ESO 2010
October 28, 2010
The XMM-Newton survey of the Small Magellanic Cloud:
A new X-ray view of the symbiotic binary SMC3
R. Sturm1, F. Haberl1, J. Greiner1, W. Pietsch1, N. LaPalombara2, M. Ehle3, M. Gilfanov4, A. Udalski5, S.
Mereghetti2, and M. Filipovi´c6
1Max-Planck-Institut f¨ur extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
2INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
3XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 50727, 28080 Madrid, Spain
4Max-Planck-Institut f¨ur Astrophysik, Karl-Schwarzschild-Str.1, 85741 Garching, Germany;
Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
5Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland
6University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW1797, Australia
Received 21 October 2010
ABSTRACT
Context.
The XMM-Newton survey of the Small Magellanic Cloud (SMC) was performed to study the population of X-ray sources
in this neighbouring galaxy. During one of the observations, the symbiotic binary SMC3 was found at its highest X-ray luminosity
observed until now.
Aims.
In SMC3 wind accretion from a giant donor star onto a white dwarf is believed to cause steady hydrogen burning on the white
dwarf surface, making such systems candidates for supernova type Ia progenitors. It was suggested that the X-ray source is eclipsed
every ∼4.5 years by the companion star and its stellar wind to explain the large X-ray variability seen in ROSAT data. We use the
available X-ray data to test this scenario.
Methods.
We present the ∼20 year X-ray light curve of SMC 3 and study the spectral evolution as seen with XMM-Newton/EPIC-pn
to investigate possible scenarios which can reproduce the high X-ray variability.
Results.
We did not find significant variations in the photo-electric absorption, as it would be expected during eclipse ingress and
egress. Instead, the X-ray spectra from different intensity levels, when modelled by black-body emission, can be better explained by
variations either in normalisation (by a factor of ∼50) or in temperature (kT between 24 eV and 34 eV). The light curve shows maxima
and minima with slow transitions between them.
Conclusions.
To explain the gradual variations in the X-ray light curve and to avoid changes in absorption by neutral gas, a predom-
inant part of the stellar wind must be ionised by the X-ray source. Compton scattering with variable electron column density (of the
order of 5×1024 cm−2) along the line of sight could then be responsible for the intensity changes. The X-ray variability of SMC3 could
also be caused by temperature changes in the hydrogen burning envelope of the white dwarf, an effect which could even dominate if
the stellar wind density is not sufficiently high.
Key words. stars: individual: SMC3 – (stars:) binaries: symbiotic – X-rays: binaries – (stars:) white dwarfs – galaxies: individual:
Small Magellanic Cloud
1. Introduction
The symbiotic star SMC3 (Morgan 1992) in the Small
Magellanic Cloud (SMC) was discovered as super-soft source
(SSS, Hasinger 1994) in X-rays during the ROSAT all sky sur-
vey (Kahabka & Pietsch 1993). It is thought to be an interacting
binary system, consisting of a cool M0 giant and a hot white
dwarf (WD) in a wide orbit. In this model accretion from the
stellar wind of the giant donor onto the WD leads to steady hy-
drogen burning on the WD surface which powers the high X-ray
luminosity (Kahabka & van den Heuvel 1997).
A series of ROSAT observations covering ∼6 years (from
Oct. 1990 to Nov. 1993 with the PSPC and from Apr. 1994 to
Nov. 1996 with the HRI), revealed high X-ray variability (a fac-
tor of ≥80 in ROSAT PSPC count rate), which was explained by
an eclipse of the WD by the donor star (Kahabka 2004). This
scenario needs obscuration of the X-ray emission region by the
dense stellar wind close to the giant to account for the shape and
the long duration of several months of the eclipse ingress and
egress.
An optical outburst between December 1980 and November
1981 of up to 3 mag in the U band was reported by Morgan
(1992). During this outburst no changes were detected in the I
band and therefore, its origin was assigned to the hot stellar com-
ponent. It is not clear, if the non-detection of SMC 3 with the
Einstein satellite was due to X-ray inactivity before the optical
outburst or to insufficient sensitivity (Kahabka 2004). The en-
richment of nitrogen also suggests evidence for a thermonuclear
event (Vogel & Morgan 1994). Results from modelling multi-
wavelength data of SMC 3 with non-LTE models under the as-
sumption of a constant X-ray source were presented in Orio et al.
(2007) and Jordan et al. (1996). Orio et al. (2007) found that the
variability cannot be caused by photo-electric absorption and
suggested a ”real” eclipse by the red giant.
SMC3 was in a very luminous state during observation of
field number 13 of the XMM-Newton (Jansen et al. 2001) large
program SMC survey (Haberl & Pietsch 2008). This enables
spectral analysis of the EPIC-pn (Str ¨uder et al. 2001) data with
unprecedented statistical quality. Four spectra from different in-
2 Sturm et al.: A new X-ray view of the symbiotic binary SMC3
tensity states allow us now to study the spectral evolution of the
hot component of SMC 3. We present the light curve starting
with the first ROSAT detection in 1990, to investigate the nature
of the variability seen from this system.
2. Observations and data reduction
SMC3 was serendipitously observed four times with XMM-
Newton at off-axis angles between 8′and 14′. Table 1 lists some
details of the observations with the EPIC instruments operated
in full-frame mode. In addition to the observation in Oct. 2009
from the SMC large survey program, we analysed three obser-
vations from 2006 and 2007 available in the archive. The first
observation in March 2006 revealed SMC 3 in a high intensity
state, but suffered from very high background. These data were
used in the study of Orio et al. (2007). The two observations
in April and October 2007 showed the source at low intensity.
The detection of the source in the later observation was noted by
Zezas & Orio (2008).
To process the data, we used XMM-Newton SAS 10.0.01
with calibration files available until 17 June 2010, including the
latest refinement of the EPIC-pn energy redistribution. As a stan-
dard, we selected good time intervals (GTIs) with an EPIC-pn
background rate below 8 cts ks−1arcmin−2(single- and double-
pixel events, 7−15 keV). However, for the observation in 2006
the background was between 500 and 2500 cts ks−1arcmin−2
and in order to retain any data, no background screening was
applied. Since soft proton flares usually show a rather hard spec-
trum, the contribution to the super-soft spectrum of SMC3 was
still acceptable (cf. Table 1). The SAS task eregionanalyse
was used to determine circular extraction regions by optimising
the signal to noise ratio, as shown in Fig. 1 and listed in Table 1.
We ensured that the source extraction region had a distance of
>10′′ to other detected sources. For the background extraction
region, we chose a circle on a point source freearea on the same
CCD as the source. Since the MOS-spectra have lower statisti-
cal quality by a factor of 10 for such soft spectra and to avoid
cross calibration effects between the EPIC instruments2, we are
concentrating on the EPIC-pn spectra in this study. For the ex-
traction of EPIC-pn spectra, we selected single-pixel events with
FLAG = 0. We binned the spectra to a minimum signal-to-noise
ratio of 5 for each bin using the task specgroup.
3. Spectral analysis of the EPIC-pn data
We used xspec (Arnaud 1996) version 12.5.0x for spectral fit-
ting. For all models, the Galactic photo-electric absorption was
fixed at a column density of NH,gal =6×1020 cm−2and elemental
abundances according to Wilms et al. (2000), whereas the SMC
column density was a free parameter with abundances at 0.2 for
elements heavier than Helium. At first, we investigated the recent
EPIC-pn spectrum of 2009, which has unprecedented statistics
compared to previous X-ray observations. For black-body emis-
sion, we obtain a best-fit with χ2/dof =213/74 with the best-fit
parameters: NH,SMC =7.84+0.44
−0.24 ×1020 cm−2,kT =33.9±0.5
eV, and a bolometric luminosity of Lbol =6.25+1.08
−0.86 ×1038 erg
s−1. This luminosity is super-Eddington (and leading to a WD
radius ∼5−10 times larger than expected for a 1 M⊙WD),
which is often caused by the black-body approximation (see
1Science Analysis Software (SAS), http://xmm.esac.esa.int/sas/
2EPIC Calibration Status Document,
http://xmm2.esac.esa.int/external/xmm sw cal/calib/index.shtml
29:00.0
-73:30:00.0
31:00.0
32:00.0
33:00.0
34:00.0
35:00.0
Declination
0301170501
Bg
SMC3
0404680301
Bg
SMC3
49:00.0 30.0 0:48:00.0 47:30.0
29:00.0
-73:30:00.0
31:00.0
32:00.0
33:00.0
34:00.0
35:00.0
Right ascension
Declination
0503000201
Bg
SMC3
49:00.0 30.0 0:48:00.0 47:30.0
Right ascension
0601211301
Bg
SMC3
Fig.1. Combined EPIC colour images of SMC 3 from the four
XMM-Newton observations. Red, green, and blue colours de-
note X-ray intensities in the 0.2−1.0, 1.0−2.0 and 2.0−4.5 keV
bands. Circles indicate the extraction regions. Note the high
background during the observation shown in the upper left which
strongly contributes to higher energies.
e.g. Greiner et al. 1991; Kahabka & van den Heuvel 1997, and
references therein). The residuals around 500 eV (cf. Fig. 2)
suggest contribution of nitrogen line emission, as observed in
post-nova X-ray spectra of SSS (e.g. Rohrbach et al. 2009). The
quality of the fit was improved to χ2/dof =119/72 by includ-
ing two Gaussian lines with fixed energy at 431 eV (Nvi) and
500 eV (N vii) and line widths fixed to 0. The equivalent widths
of 19 eV and 32 eV for the Nvi and N vii lines, respectively,
are physically plausible, but the residuals could at least partially
be also caused by calibration uncertainties. Alternatively, allow-
ing the oxygen abundance in the SMC absorption component
as free parameter also improved the fit (χ2/dof =123/73), re-
sulting in an oxygen abundance of 9.9+4.5
−2.7times solar with an
NH,SMC =4.39+0.66
−0.79 ×1020 cm−2. A hard spectral component,
which could possibly be caused by the wind-nebula, is not seen
in the spectrum. Adding an apec plasma emission component to
the black-body emission, with fixed temperature of kT =500
eV and SMC-abundances yields an upper limit for the emission
measure of EM =8.6×1057 cm−3.
The derived values for the absorption NH,SMC are well below
the total SMC absorption in the direction of SMC 3 (∼5×1021
cm−2; see Stanimirovic et al. 1999). This suggests that the sym-
biotic system is located on the near side of the large amount of
H I present in the SMC Bar.
We also tested non-local thermal equilibrium models pro-
vided by Thomas Rauch3(Rauch & Werner 2010). We found the
best-fit for a pure He atmosphere with WD surface gravity log
g=5 (χ2/dof =220/74). For a pure hydrogen atmosphere with
log g =9 we found a best-fit at χ2/dof =261/74. If the spec-
trum in fact contains emission lines, this might be the reason
of the inferior fit of the non-LTE models which produce spectra
which are dominated by absorption lines from the white dwarf
3http://astro.uni-tuebingen.de/∼rauch/
Sturm et al.: A new X-ray view of the symbiotic binary SMC3 3
Table 1. XMM-Newton EPIC-pn observations of SMC 3
ObsID Satellite Date Time Filter Net Exp Net cts. Bg(a)Net cts. Bg(a)R(b)
sc R(b)
bg
Revolution (UT) [s] (0.2−10.0 keV) (0.2−1.0 keV) [′′] [′′ ]
0301170501 1149 2006 Mar 19 14:45-20:17 medium 10446(c)8287 12% 8246 2% 19 30
0404680301 1344 2007 Apr 11-12 20:00-02:15 thin 13986 170 13% 137 5% 16 50
0503000201 1444 2007 Oct 28 06:11-11:52 medium 16607 451 9% 407 4% 21 40
0601211301 1798 2009 Oct 3 05:31-14:12 thin 26535 39456 2% 39338 1% 69 55
(a)ratio of background count rate to source count rate in the same energy band.
(b)radius of the source and background extraction region.
(c)no GTI screening was applied.
atmosphere. Since the black-body spectrum resulted in a better
fit, we decided to use this model in our further investigations.
To study the spectral evolution of SMC 3, we fitted the EPIC-
pn spectra of all four epochs simultaneously with a set of models
based on absorbed black-body emission. In each model only one
individual parameter for each spectrum and two common param-
eters for all spectra were allowed to vary (see below). Since the
statistical quality of the high-flux spectrum is far better than for
the other spectra, it also dominates the resulting χ2. This leads
to relatively bad fits for the simple black-body models. Adding
emission lines would improve the fits (see above). However,
given the limited spectral resolution of the EPIC-pn instrument it
is not clear if the lines have any physical meaning. Because we
were mainly interested in the evolution of spectral parameters,
we decided to use the simplest model.
Our first model (model 1 in Table 2) assumes temperature
and luminosity not varying with time, while the absorbing col-
umn density can change with time. This corresponds to the
eclipse model with varying absorption by the dense donor wind,
as suggested by Kahabka (2004). This model gives an insuffi-
cient fit to the data (see Table 2). Although, the fit is statistically
dominated by the two high-flux spectra, the spectral shape of the
low-flux spectra cannot be reproduced by a high column density
which predicts much less flux at lowest energies.
For model 2, we fixed the spectral shape (same temperature
and absorption) and allowed the luminosity to change (i.e. fitting
individual normalisations which corresponds to a variable size of
the emission area). This fits the data much better (Table 2).
An even better fit was achieved by our third model with
varying source temperature. The corresponding black-body lu-
minosities were related to the temperature (Lbol ∝T4). The re-
sults for this model are again described in Table 2 and the indi-
vidual spectra with the model fit are plotted in Fig. 2.
4. The X-ray light curve of SMC 3
To analyse the temporal behaviour of the system, we reconciled
the X-ray light curve of SMC 3 starting from the first detection
by ROSAT in 1990. To convert the ROSAT count rates, provided
by Kahabka (2004), into fluxes, we simulated a ROSAT PSPC
spectrum based on the spectral model derived from the simul-
taneous fit with variable normalisation and the PSPC detector
response. We obtained a conversion factor of 1.54 ×10−11 erg
cm−2cts−1. All fluxes are computed for the 0.2−1.0 keV band.
Because of the dominant statistics of the XMM-Newton high-
flux spectrum in the model fit, this factor rather corresponds to
the high-flux state. Using the model with variable temperature,
the conversion factor for the low flux can be lower by a factor
0.1
1
Counts s−1 keV−1
0.2 0.5
−4
−2
0
2
χ
Channel energy (keV)
2006 Mar
2007 Apr
2007 Oct
2009 Oct
Fig.2. EPIC-pn spectra of SMC 3 together with the best-fit
black-body model 3 with variable temperature. No significant
emission is seen above 0.7 keV.
of .2. Analogous, for the ROSAT HRI, this simulation yields a
conversion factor of 4.63 ×10−11 erg cm−2cts−1.
We searched the Swift archive for observations covering
SMC3. The source was detected in observation 00037787001
with ∼3 ks exposure on 18 August 2008. The source and back-
ground spectrum was extracted with xselect and the effective
area file was created using xrtmkarf. The resulting spectrum
contains 135 net counts, which is insufficient to distinguish be-
tween the above described models. Thus we fitted only the nor-
malisation and assumed the spectral shape according to the si-
multaneous fit to the EPIC-pn spectra with variable normalisa-
tion. This fit yields a flux of 2.61+0.26
−0.15 ×10−12 erg cm−2s−1.
To derive XMM-Newton fluxes, we integrated the best-fit
model with variable temperature as described above. The source
flux during a Chandra observation in February 2003 was de-
duced from the parameters of the best-fit black-body model re-
ported by Orio et al. (2007).
Figure 3 shows the X-ray light curve of SMC 3 since the first
detection by ROSAT in 1990. By modelling the light curve with
several eclipses, we realised, that (i) the transition from high to
low intensity occurs over a long time period and that (ii) the time
scales of the duration of the high and low intensity intervals are
of the same order. Thus, instead of eclipses the light curve may
also be interpreted by several periodic outbursts.For demonstra-
tion, the dashed line in Fig. 3 shows a fit with a sine function.
To account for uncertainties in the flux conversion and cross cal-
4 Sturm et al.: A new X-ray view of the symbiotic binary SMC3
Table 2. Results from the simultaneous black-body fit to the EPIC-pn spectra.
Model 1 Model 2 Model 3
Tand Lbol constant with time NHand Tconstant with time NHconstant with time
NHvariable Lbol variable Tvariable, Lbol,i=Lbol,1(Ti/T1)4
kT =(32.5±0.5) eV NH=(0.77 ±0.04) ×1021 cm−2NH=(0.77 ±0.02) ×1021 cm−2
Lbol =(8.8±1.6) ×1038 erg s−1kT =(33.7±0.5) eV Lbol,1=(5.5±0.3) ×1038 erg s−1
NH,1=(1.08 ±0.05) ×1021 cm−2Lbol,1=(4.43 ±0.78) ×1038 erg s−1kT1=(32.7±0.2) eV
NH,2=(6.75 ±0.48) ×1021 cm−2Lbol,2=(0.12 ±0.02) ×1038 erg s−1kT2=(24.3±0.3) eV
NH,3=(6.91 ±0.23) ×1021 cm−2Lbol,3=(0.18 ±0.03) ×1038 erg s−1kT3=(25.3±0.2) eV
NH,4=(0.81 ±0.05) ×1021 cm−2Lbol,4=(6.47 ±1.15) ×1038 erg s−1kT4=(33.8±0.4) eV
χ2/dof =940/154 =6.10 χ2/dof =383/154 =2.49 χ2/dof =365/154 =2.37
ibration between the different instruments we included a 20%
systematic error on the flux. As best-fit ephemeris for the X-ray
minimum we then obtain (90% confidence errors):
MJDmin,x=(49382 ±10) +N×(1634 ±7) days.
The relatively high flux measured in the last XMM-Newton ob-
servation might suggest possible changes in the amplitude of the
modulation.
5. MACHO and OGLE data
The OGLE-II (Udalski et al. 1997) and OGLE-III (Udalski et al.
2008) I-band as well as the MACHO B-band light curves are
shown in the lower two panels of Fig. 3. The calibrated MACHO
light curve was shifted in magnitude to match its average B mag-
nitude with that measured by Zaritsky et al. (2002). In the I-band
of OGLE-II and the B-band of MACHO, Kahabka (2004) found
correlating quasi-periodic oscillations with periods around 110
days which might be related to pulsations of the red giant star.
These short variations are also present in the OGLE-III data.
With the OGLE-III data, which cover a much longer time in-
terval, we can now rule out a significant variation with the 1630
day cycle as suggested by the X-ray light curve. As noted by
Kahabka (2004), the MACHO light curve shows a quasi si-
nusoidal modulation with a period of ∼4 years, in addition to
shorter variations. A fit of a sine function to the MACHO B-
band data (solid line in Fig. 3), results in an ephemeris for the
optical minimum of
MJDmin,B=(49242 ±9) +N×(1647 ±24) days.
To account for the short-term variations we added a systematic
error of 0.05 mag to the B-band magnitudes. Formally, the fits to
X-ray and MACHO light curves indicate a phase shift of (140 ±
14) days (optical preceding the X-rays) while the periods agree
within the errors. However,it should be noted that the MACHO
light curve covers only ∼1.5 cycles and is superimposed by the
short term variations which may influence the results.
6. Discussion
SMC 3 was observed with XMM-Newton at four epochs, cover-
ing the super-soft X-ray source twice at high and twice at low
intensity. A strong variationin the X-ray flux by a factor of more
than ∼50 between minimum and maximum intensity is found
in the 0.2−1.0 keV band. We showed, that the light curve can
qualitatively be described by a sine function with a period of
0
2
4
6
F0.2−1.0 keV (10−12 ergs cm−2s−1)
13.3
13.2
I mag
4.8×1045×1045.2×1045.4×1045.6×104
16.6
16.4
16.2
~ B mag
MJD
ROSAT PSPC
ROSAT HRI Chandra XMM−Newton Swift
OGLE II + III
MACHO
Fig.3. The 0.2−1.0 keV X-ray light curve of SMC3 (upper
panel), the light curves in the I-band (OGLE, middle panel) and
the approximate B-band (MACHO, lower panel). The dashed
line shows the best-fit sine function to the X-ray light curve and
the solid line the best-fit sine function to the MACHO light curve
(see text).
1633 days. This simple model can only be a crude approxima-
tion of the light curve, but the ∼20 year coverage indicates a
high regularity of the period with similar duration of high and
low intensity intervals. The regularity of the X-ray light curve,
with meanwhile four observed minima, strongly supports the in-
terpretation of the 4.5 year period as the orbital period of the
binary system. Assuming masses of 15 M⊙and 1 M⊙for the M-
giant and the white dwarf, respectively, the orbital period implies
a semi-major axis of the binary system of 6.83 AU.
We analysed the spectral evolution and found, that the vari-
ability of the X-ray flux cannot be explained by photo-electric
absorption by neutral gas with varying column density. To
avoid the strongly energy-dependent attenuation of soft X-rays,
Kahabka (2004) discussed absorption due to highly ionised gas.
In this picture the strong X-ray source ionises the stellar wind
around it. Compton scattering on free electrons would then re-
duce the X-ray flux along the line of sight most efficient when
looking through the dense innermost regions near the M-star.
This mimics variable intensity with little energy dependence (no
significant change of spectral shape). Using a Compton scatter-
ing model (cabs in xspec), instead of variable normalisation,
would require a column density of >4.8 ×1024 cm−2(completely
Sturm et al.: A new X-ray view of the symbiotic binary SMC3 5
ionised absorber) to reduce the X-ray intensity from maximum
to minimum. In this picture scattering of X-rays into the line of
sight is neglected or at least assumed not to change significantly
between the two states. Using the estimated mass, size and den-
sity of the ionised wind region as given by Orio et al. (2007)
yields a column density to its centre of 5.8 ×1023 cm−2. This is
a factor of ∼8 lower than our estimate from Compton scattering
and may be explained by the simplified assumption of a constant
wind density while the line of sight during the low intensity ob-
servations should pass through the denser wind regions near the
giant star. In this picture most of the stellar wind must be ionised,
consistent with the fact that we do not see a variablecontribution
of photo-electric absorption by neutral gas. In this model, the
low intensity can still be explained by an eclipse of the X-ray
source by the giant star and its stellar wind. An eclipse by the
star only would be short (∼3% of the orbital period) with sharp
ingress and egress while the dense inner wind regions cause a
long gradual eclipse ingress (and egress) by increasing (decreas-
ing) Compton scattering along the line of sight. The shape of the
light curve should then depend on the geometry of the binary
system and the distribution of free electrons in the stellar wind.
Our spectral analysis shows, that the X-ray variability can
alternatively be dominated by temperature changes, varying be-
tween 24 and 34 eV. Assuming a constant size of the emitting
area, this corresponds to a variation in Lbol (for spherically sym-
metric emission) by a factor of 4.3. The larger variation in ob-
served instrumental count rates would then be caused by shifting
the spectra with lower temperature out of the sensitive energy
band of EPIC. A possible scenario might be an elliptical orbit
of the white dwarf around the M-giant (or equatorial mass ejec-
tion with inclined WD orbit), causing accretion at different rates.
Variable accretion, even at low level, can lead to large temper-
ature changes in the burning layer (Paczynski & Rudak 1980).
Similar scenarios were used to explain X-ray variability in other
SSS (e.g. AG Dra, Greiner et al. 1996). However, we note, that
in those cases usually an anti-correlation of X-ray and optical
luminosity is observed, whereas in the case of SMC 3 these two
are clearly correlated.
Assuming the same temperature and absorption (and no
change in Compton scattering) for the low and high intensity
spectra, the inferred radii would be different by a factor of ∼8
to account for the factor of 60 difference in Lbol. In general for
stable shell burning on the WD surface, an increase of the hydro-
gen burning envelope (e.g. due to a higher accretion rate) leads
to an increase of both, temperature and radius (Fujimoto 1982).
Increasing temperature and declining Compton scattering both
lead to an increasing X-ray luminosity. It depends on the orien-
tation of the orbit with respect to the observer, how much the
two effects act in phase. Additional temperature variations may
therefore reduce or increase the amount of Compton scattering
required to explain the X-ray luminosity variations.
Superimposed on the general long term variation in the X-
ray light curve, we probably see effects imposed by the donor
star. The ∼110 days brightness variations seen in the I-band sug-
gests changes in the stellar wind which can lead to variations
in the mass accretion rate onto the white dwarf. Since the 1630
day X-ray period is not visible in the I-band, it is unlikely that
this period is caused by the cool stellar component, which seems
to remain rather unaffected by the process producing this varia-
tion. In the B-band, both modulations are seen, the 1630 day pe-
riod derived from the X-rays and the ∼110 day variations which
correlate with the I mag (as already pointed out by Kahabka
2004). Therefore, the cool companion star and the region where
the X-ray emission is produced most likely both contribute to
the B-band. If viewing effects produce the variation in B in a
similar way as in the X-rays (by changing extinction) or if heat-
ing of the cool star by the X-ray source is causing this varia-
tion remains unclear: While Kahabka (2004) finds X-ray heat-
ing insufficient to account for the observed B-band modulation,
Orio et al. (2007) discuss irradiation effects influencing the mass
outflow as very important.
7. Conclusions
The new X-ray data of SMC 3 show that two scenarios can qual-
itatively explain the spectral evolution and the shape of the light
curve. The evolution of the X-ray spectra is incompatible with
changing photo-electric absorption by neutral gas, but is consis-
tent with energy-independent intensity and/or with temperature
variations. As suggested before by Kahabka (2004), Compton
scattering in a predominantly ionised stellar wind could lead to
the observed intensity variations if the stellar wind density (mass
loss rate) is high enough. Additional temperature changes in the
burning layer of the WD which are caused by variable accretion,
can reduce the required wind densities.
Acknowledgements. This publication is partly based on observations with
XMM-Newton, an ESA Science Mission with instruments and contributions
directly funded by ESA Member states and the USA (NASA). The XMM-
Newton project is supported by the Bundesministerium f¨ur Wirtschaft und
Technologie/Deutsches Zentrum f¨ur Luft- und Raumfahrt (BMWI/DLR, FKZ
50 OX 0001) and the Max-Planck Society. R.S. acknowledges support from the
BMWI/DLR grant FKZ 50 OR0907. This paper utilises public domain data ob-
tained by the MACHO Project, jointly funded by the US Department of Energy
through the University of California, Lawrence Livermore National Laboratory
under contract No. W-7405-Eng-48, by the National Science Foundation through
the Center for Particle Astrophysics ofthe University of California under coop-
erative agreement AST-8809616, and by the Mount Stromlo and Siding Spring
Observatory, part of the Australian National University.
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