R. Schwarz1,2 - K. Reinsch2 - K. Beuermann2 - V. Burwitz3
1 -
Astrophysikalisches Institut
Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
2 -
Universitätssternwarte Göttingen, Geismarlandstraße 11,
37083 Göttingen, Germany
3 - Max-Planck-Institut für Extraterrestrische Physik,
Giessenbachstraße, 85740 Garching, Germany
Received 2 June 2005 / Accepted 4 July 2005
Abstract
Using XMM-Newton we have obtained the first continuous X-ray
observation covering a complete orbit of the longest period
polar, V1309 Ori. The X-ray light curve is dominated by a short, bright
phase interval with EPIC pn count rates reaching up to 15
cts s-1 per 30 s resolution bin. The bright phase
emission is well described by a single blackbody
component with
eV. The absence
of a bremsstrahlung component at photon energies above 1 keV yields a
flux ratio
.
This represents
the most extreme case of a soft X-ray excess yet observed in an
AM Herculis star. The bright, soft X-ray emission is subdivided
into a series of individual flare events supporting the
hypothesis that the soft X-ray excess in V1309 Ori is caused by accretion
of dense blobs carrying the energy into sub-photospheric layers.
On average, the flares have rise and fall times of 10 s.
In addition to the bright phase emission,
a faint, hard X-ray component is visible throughout the
binary orbit with an almost constant count rate of 0.01
cts s-1. Spectral modelling indicates that this emission
originates from a complex multi-temperature plasma. At least
three components of an optically thin plasma with temperatures
kT= 0.065, 0.7, and 2.9 keV are required to fit the observed flux
distribution.
The faint phase emission is occulted during the optical eclipse.
Eclipse ingress lasts about 15-20 min and is
substantially prolonged beyond nominal ingress of the white
dwarf. This and the comparatively low plasma temperature provide
strong evidence that the faint-phase
emission is not thermal bremsstrahlung from a post-shock accretion
column above the white dwarf.
A large fraction of the faint-phase emission is ascribed to the spectral
component with the lowest temperature and could be explained by scattering
of photons from the blackbody component in the infalling material above
the accretion region. The remaining hard X-ray flux could be produced in
the coupling region, so far unseen in other AM Herculis systems.
Key words: accretion, accretion disks - stars: novae, cataclysmic variables - X-rays: binaries - stars: magnetic fields - stars: individual: V1309 Ori
Observationally, the soft X-ray excess has been firmly established for a large number of polars using ROSAT PSPC observations (Beuermann & Burwitz 1995; Ramsay et al. 1994), where also a strong correlation with the magnetic field strength in the accretion region was found. Recently, the status of the energy balance has been critically reconsidered by Ramsay & Cropper (2004) on the basis of a re-analysis of ROSAT PSPC data and a snap-shot survey of 22 polars observed with XMM-Newton in high states of accretion. They question the existence of a soft X-ray excess for the majority of systems and find that high instantaneous accretion rates lead to high soft-to-hard X-ray flux ratios. This is in line with the behaviour of polars observed at very low accretion rates (Ramsay et al. 2004) whose spectra do not show any distinct soft component.
The eclipsing polar V1309 Ori is one of the most peculiar AM Herculis
stars. With an orbital period of 8 h, almost twice as long
as any other system, it is a key object for understanding the
evolution and synchronisation of magnetic CVs (Frank et al. 1995). Another puzzling aspect concerns the dominance of emission from
the accretion stream at infra-red, optical and UV bands
(Shafter et al. 1995)
cyclotron radiation from the accretion shock provides the main
contribution.
Diagnostics of the accretion processes in V1309 Ori are therefore
restricted to X-ray observations.
The most longest pointing with ROSAT (Walter et al. 1995) revealed
that V1309 Ori is a strongly flaring X-ray source with a very
soft spectrum. In this paper we present the first continuous, whole
orbit X-ray observation of V1309 Ori.
V1309 Ori was target of a pointed XMM-Newton observation during revolution
#233 starting on March 18, 2001. Our analysis focuses on data from
the European Photon Counting Cameras (EPIC) which were all operated
with a thin filter. With a total exposure time of 29.1 ks the observations with the two MOS detectors covered just one
complete cycle of V1309 Ori, while the shorter pn exposure (on-time
26.4 ks) missed 9% of the orbit just prior to eclipse
ingress. The counting statistics of the RGS instruments were too low,
and we dismissed those spectra from further analysis. Data were
processed with version 5.3.1 of the XMM-Newton Science Analysis
Software (SAS). Source photons were extracted within
apertures of 45
and 30
radius around the
position of V1309 Ori for the pn and MOS data, respectively. The background
which was at a low and constant level during the pointing was
determined from a source free area close to the target. For spectral
analysis within XSPEC, only single and double events were used
together with the appropriate response files pn_ff20_sdY9 and
m[1,2]1_r7_im_all_2000-11-09. For the pn detector, operated in
full window mode, the fitting procedure was complicated by photon
pile-up for count rates exceeding 5 cts s-1. We tried to
minimise these effects by selecting only photons from either low count
rate intervals or an outer annulus of the PSF.
In order to improve the counting statistics and to provide a compact display, we constructed combined light curves from all EPIC instruments with the count rates of the two MOS CCDs normalised to the level of the pn detector. EPIC photon timings given in the time frame of terrestrial time (TT) have been converted to the barycenter of the Sun. We have adopted the orbital ephemeris of Staude et al. (2001), which uses the mid-eclipse of the white dwarf as zero-point. The corresponding value 24 4 50 339.43503 in the time frame of barycentric Julian ephemeris days (BJED) has been corrected for a typing error and the appropriate number of leap seconds.
Simultaneously to the X-ray observations, V1309 Ori was observed with the
optical monitor (OM) in the UVW2 (1800-2400 Å) band. The OM was
operated in image mode resulting in
s
integrations. We applied aperture photometry to the images provided by
the omichain task to derive background subtracted count
rates of the source. These were converted to flux units using the conversion factor
erg cm-2 s-1 Å-1 cts-1
given in the XMM-Newton documentation.
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Figure 1: a) Combined EPIC pn/ MOS X-ray light curves of V1309 Ori observed with XMM-Newton in March 2001. b) Same as a) but enlarged in count rate space to emphasise the emission components during the faint phase and in the 1-8 keV hard X-ray band (red line). c) 0.1-2.4 keV X-ray light curve of V1309 Ori obtained with the ROSAT satellite at various epochs. d) UV light curve from the optical monitor onboard XMM-Newton taken with the UVW2 filter. Fluxes are given in units of 10-15 erg cm-2 s-1 Å-1 . |
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We also included in our analysis 34 ks of largely unpublished ROSAT data taken between 1991 and 1996 with the PSPC and HRI instruments. These observations were spread into small blocks separated by days or weeks, which cover only small fractions of the orbit at a given epoch. Photon event files have been taken from the ROSAT archive and standard corrections (vignetting, dead-time) have been applied using the EXSAS software package (Zimmermann et al. 1994). For comparison with the PSPC data, HRI count rates have been multiplied with a factor of 6.
In Fig. 1a we show the XMM-Newton EPIC light curve
folded over the orbital ephemeris and binned at a resolution of 30
s. The most prominent feature is a short, bright interval at
= 0.41-0.68 with peak count rates reaching 15 cts/s (or
25 cts/s at a binning of 3 s). At higher temporal resolution
(Fig. 2) most of the bright phase flux can be resolved
into individual flares. The frequency and intensity of the
flares strongly varies with the largest flares seen shortly after
onset of the bright phase and a smoothly declining intensity
thereafter.
Most of the individual flares are well-separated with average rise and
decay times of 10 s. This time-scale corresponds to the
first part of the auto-correlation function which steeply falls
off with an e-folding time of 7 s (Fig. 3). The
correlations remain positive for larger lags up to the
zero-crossing time of
100 s, which is interpreted as the
average time between two consecutive flares. For a dozen well
isolated flares we measured the integrated fluxes as well as the
corresponding peak flux. This sample includes a characteristic set of
faint and bright events ranging between
-
erg cm-2 and
erg cm-2 s-1 and should be representative for the
majority of flares seen in Fig. 2.
Perhaps the greatest surprise are the spectral properties
of the flaring emission best seen in the hard X-ray light curve taken
in the 1-8 keV band (Fig. 1b). While a constant
source of faint, hard X-ray flux (see Sect. 3.2) is
observed at all phases apart from the eclipse, there are no X-ray
photons with energies larger than 1 keV that can be uniquely
attributed to the soft X-ray flares. We derive an upper limit
for the hard X-ray flux from the flares using the counting
statistics of the residual faint flux. In the bright-phase
interval
0.43-0.6, 650 source and background photons
with energies >1 keV have been detected. This corresponds to
a detection limit of 25 photons or 0.0033 cts/s for the
hard X-ray count rate related to the soft X-ray flares. A similar
conclusion is reached if we assume that the 25 photons
from the hard mini-flare observed at
= 0.46 are
associated with a soft flare event. Assuming a bremsstrahlung
spectrum with
= 20 keV the upper limit to
the count rate is translated into a bolometric flux limit
erg cm-2 s-1 . We also supply the
equivalent value for the ROSAT band
erg cm-2 s-1 to aid comparison with the studies of
Beuermann & Burwitz (1995).
A few isolated soft flare events are observed outside the bright-phase interval. For example, there is a series of six mini-flares
seen shortly prior to the eclipse at
= 0.94 for a duration of
400 s. The highly instationary nature of the accretion process
makes it difficult to clearly differentiate between geometrical
aspects and instantaneous variations of the mass transfer rate. The
smooth UV light curve indicates that V1309 Ori was permanently accreting at
a high rate and we conclude that the entire bright interval was not
due to an isolated, long-lasting accretion event. Assuming that
the duration
and the center of the bright phase
reflect the geometrical constraints on the
visibility of the accretion region, would imply an unusual
location of this region at an azimuth
and a
colatitude
(see discussion in
Sect. 5.5).
Additional information on the duration and the center of
the phase interval during which strong flares occur is provided by
various ROSAT light curves (Fig.1c) of V1309 Ori obtained from
pointed observations at different epochs. Compared to the XMM-Newton pointing, flare intervals are distributed over a larger phase
range
0.45-0.87 and shifted towards phase zero. The
brightest episodes are observed around
,
i.e. in the
faint-phase interval of the XMM-Newton observation
indicating a possible migration of the accretion spot. The general
occurrence of the bright intervals is consistent with an accretion
region located at an azimuth
and at a
colatitude
.
All available ROSAT and XMM-Newton
data agree that flares do not occur in the post-eclipse interval,
0.05-0.45.
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Figure 2:
0.18-8 keV EPIC pn count rates during three subsequent bright-phase
intervals (
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During the eclipse by the secondary star
the source of the faint emission is occulted (Fig. 4). The beginning of X-ray ingress coincides with
that of the white dwarf observed at
in the UV
(Staude et al. 2001; Schmidt & Stockman 2001). Surprisingly, the ingress is prolonged to
phases beyond the geometrical occultation of the white dwarf.
In total, a significant excess emission of
photons
has been detected during the eclipse above the background
level of one photon.
A rough estimate of the spectral characteristics of the ingress flux can
be made from the ratio of photons above and below 1 keV which is 0.5.
From spectral analysis (Sect. 4.2) we find a
similar value 0.36 for the
intermediate temperature (0.7 keV) component of the faint emission.
The estimated egress duration of 15-20 min is much longer than
45 s expected for the occultation of an 0.7
white dwarf,
and is difficult to reconcile with a compact emission region. A
similar, corresponding behaviour for eclipse egress is hard to
confirm, due to the low counting statistics and the intrinsic
variability of the source.
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Figure 3: Auto-correlation function of the bright-phase light curve. Time delay bins are 3 s. |
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Figure 4: a) EPIC pn/ MOS X-ray eclipse light curve of V1309 Ori binned at a time resolution of 240 s and b) optical light curve obtained with the 70-cm reflector of the AIP shown for comparison. The vertical lines mark the orbital phases of the white dwarf ingress/egress measured from UV HST light curves (Staude et al. 2001). |
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Figure 5: EPIC pn spectra of V1309 Ori taken during bright and faint phase, together with likely model spectra and their residua. The model of the faint phase spectrum is a 3-temperature mekal fit. The bright phase spectrum shown here includes piled-up photons, while the fit to the data uses fixed parameters derived from the pile-up free spectrum. |
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The UV light curve shown in Fig. 1d is similar to the
optical light curves (Shafter et al. 1995) with a primary maximum around
and a secondary brightening at phase
.
Such asymmetric double-humped shape is interpreted as the result
of the changing projection of the accretion flow. During the rise of
the X-ray bright phase the UV flux is actually decreasing, indicating
that the accretion spot itself contributes only a minor fraction in
the UV range. With the sparse sampling, eclipse ingress and egress
are not resolved. For the single OM exposure centred at
0,
the eclipse is total. The flux levels before and after eclipse are
comparable to that of an HST FOS observation of V1309 Ori taken in
1996 (Staude et al. 2001). We conclude, therefore, that V1309 Ori was in
a normal high state of accretion during the XMM-Newton
observation.
The spectrum of the X-ray emission seen in the flaring
phase can be adequately fitted by a single blackbody and an
extra component representing the unrelated hard X-ray emission
(Fig. 5). For the latter we have adopted the 3-temperature
mekal model determined in Sect. 4.2 with fixed
best-fit parameters. The blackbody temperatures derived from the data of the two MOS CCDs,
eV, and
from the pile-up free pn spectrum,
eV,
agree within the error bars (Table 1). The temperature is comparable with values observed in other polars with a
pronounced blackbody component, which roughly cover the range of
20-60 eV. The hydrogen column density of
(5-
cm-2 towards V1309 Ori is in agreement with previous
ROSAT and BeppoSAX measurements (Beuermann & Burwitz 1995; El Kholy 2004; de Martino et al. 1998)
and slightly lower than the total interstellar galactic column of
1021 cm-2 in this direction. The remaining
uncertainties in
and
imply a corresponding uncertainty in the unabsorbed bolometric
soft X-ray flux
,
which averages
erg cm-2 s-1 for the two MOS CCDs and
erg cm-2 s-1 for the pn. In addition to calibration issues of the low
energy detector response, the pn flux may still be somewhat influenced
by the photon pile-up and we will adopt the MOS flux for later
considerations.
We have searched for possible changes of the blackbody
temperature as a function of count rate. Bright phase X-ray spectra
selected for intervals with count rates lower or higher than 5
cts s-1 have indeed different parameters, but these variations
are only significant at the 3
level and not consistent between
the different detectors. For the MOS1 and the pn CCD, the low
count rate spectra have lower temperatures of 35 and 42 eV,
respectively, compared with
45 eV for the bright flare
measurements with all instruments. The fits of the MOS2 data on
the other hand differ only in the hydrogen column density.
Table 1: Fit results for faint and bright phase X-ray spectra of V1309 Ori.
The faint phase spectrum was extracted for phase ranges
0.03-0.41 and
0.68-0.97 carefully
excluding the eclipse interval as well as a few intermittent
flares. Since the signal-to-noise ratio of the MOS faint phase
spectra was too low to constrain different models only the pn data
were considered further. The spectrum shown in Fig. 5 is
much flatter and harder compared with that of the bright phase,
and can not be fitted by (i) a single blackbody;
(ii) a single temperature optically thin thermal emitter; and
(iii) a combination of both.
The spectral shape rather suggests an optically thin plasma
emitter with a range of comparatively low temperatures. A good fit was
obtained with Mewe models
(mekal , Mewe et al. 1985) with temperatures
,
0.7, and 2.9 keV, yielding
.
The total bolometric flux of the
faint emission is
erg cm-2 s-1 ,
with the largest fraction (
90%) coming from the component with
the lowest temperature. Since the emission measure in the cooling
flow of a bremsstrahlung-dominated plasma shock-heated to
varies approximately as
it is obvious that the
parameter set for a single cooling flow can not match the
observation. Attempts to fit the faint-phase emission with the
multi-temperature model (cmekal , Done & Magdziarz 1998), in which
the emission measure for a given temperature is scaled by the
expression
,
were in fact unsuccessful
(
for any possible combination of
and
.
The residuals for this model are confined to
energies below
0.5 keV, indicating the presence of an independent low temperature component.
Adding a blackbody component to the cmekal model yields an
acceptable fit
,
but also an optically thin
thermal model cannot be excluded. The temperature of the blackbody
is
eV and agrees within the errors with that of the primary soft
bright phase emission.
The bolometric time-averaged bright-phase luminosity of the flare
emission is
erg s-1 calculated for a distance of 625 pc
. With
C=1, this luminosity refers to the emission received from a plane
surface element viewed face-on. If the plane surface
element is located at co-latitude
in a system seen at
inclination i, C=1/cos
at the best visibility of
the element and larger if closer to the limb of the star. With
and
(Staude et al. 2001), a correction by a
factor as large as
7 would be required. There is evidence,
however, from various studies that the blackbody emission arises from
thermally or dynamically elevated mounds of hot photospheric matter
rising up to a couple of percent of the white dwarf radius
(Schwope et al. 2001). In their early study, Heise et al. (1985) noted that
the individual (and transient) mounds are about as high as wide. As a
compromise, we use a factor
,
which corresponds to
isotropic emission into the half sphere. The soft X-ray blackbody
luminosity then is
.
Neglecting, for simplicity, any additional contribution to
the accretion luminosity, e.g., from cyclotron radiation,
corresponds to a time-averaged accretion rate
.
Assuming a standard white dwarf of mass M=0.7
and radius
cm, we obtain
yr-1. For C=2,
yr-1, comparable to the
yr-1 estimated by Beuermann & Burwitz (1995) from the
ROSAT All-Sky-Survey observation of V1309 Ori if one corrects for their
lower (assumed) value of
.
It is also close to the
yr-1 derived by El Kholy (2004)
from the ROSAT pointed observation. Our value of
,
however, falls
substantially below the rates which seem to be typical of dwarf novae
and novalike variables as estimated by Patterson (1984) and implied
by the effective temperatures of accreting white dwarfs if interpreted
in terms of compressional heating (Araujo-Betancor et al. 2005; Townsley & Bildsten 2004). This
seems to support the notion that long-period polars have lower
accretion rates than other CVs (Araujo-Betancor et al. 2005).
Our XMM-Newton observation revealed V1309 Ori as extreme among polars
in its dominance of soft over hard X-ray emission. From the bolometric
flux
of the soft X-ray blackbody and the upper limit to
the bremsstrahlung flux
(Sect. 3.1) we
derive a lower limit to the soft-to-hard bolometric X-ray flux ratio,
.
An even larger value,
,
is obtained if the fluxes are restricted
to the ROSAT band (Beuermann & Burwitz 1995). No other polar
shows such a weak hard X-ray component.
The best explanation for the lack of hard X-ray emission in
polars is the accretion of blobs, or rather field-aligned filaments, of
matter which penetrate to sub-photospheric layers releasing their
energy in shocks which are entirely submerged in the photosphere
(Frank et al. 1988; Beuermann 2004; Kuijpers & Pringle 1982). Shock heating of the infalling
matter can not be avoided in the super-sonic flow, but burying of the
shock and complete thermalization can render the hard X-rays
undetectable.
As noted already by Walter et al. (1995), the soft X-ray emission in V1309 Ori consists of individually identifyable flares. The spectral properties of V1309 Ori suggest a high density of the infalling matter and the temporal structure of the emission suggests the infall of blobs or filaments of matter.The detection of individual flares provides the opportunity to estimate the physical parameters of these filaments. While several causes may be responsible for the high density of the matter, an obvious one is the compression of the filaments perpendicular to the field as r-2.5, with r the radial distance (Beuermann et al. 1987). The surface field strength B in the accretion spot has been measured by Shafter et al. (1995) and Schmidt (2004) from the spacing of the, admittedly weak, cyclotron harmonics, yielding B=61 MG.
The stagnation radius obtained by equating the pressure of the equatorial
magnetic field and the free-fall ram pressure of the accretion stream
expressed in units of the white dwarf radius is
In addition,
the production
of hard X-rays from the low density part of the -distribution
is diminished by the dominance
of cyclotron emission over thermal bremsstrahlung as the primary coolant
in a high field plasma (Woelk & Beuermann 1996), like in V1309 Ori.
Furthermore, the viewing geometry towards moderately buried shocks may
play a role.
Reprocessing of hard X-rays occurs already for moderate suppression of
the shock if the accretion spot is located sufficiently close to the
limb of the star. For a geometry as in V1309 Ori with a minimum viewing
angle
the actual path length through
atmospheric matter will exceed that for a face-on view by a large
factor and shield shocks that would be directly visible in a face-on
view (Frank et al. 1988; Beuermann 2004).
Another constraining parameter is the number of blobs simultaneously
impinging on the white dwarf. The visibility of shocks will also be
influenced by the interaction of the individual impact spots and the
splashes caused by them, an effect which increases with the number N
of such impact regions present simultaneously. The structured X-ray
light curve of V1309 Ori suggests that this number is small, of the order
of unity, and certainly less than estimated for AM Her by
Hameury & King (1988).
Several complications may disturb the simple picture presented above. E.g., the density in the threading region at the start of the quasi-free fall may vary between different objects because it depends on the balance between cooling and heating experienced by the matter in the accretion flow, processes which are difficult to assess observationally and theoretically. Break-up of the stream into separate blobs may occur already near the inner Lagrangian point or be effected by instabilities in the magnetosphere (Hameury et al. 1986). As a tendency, we presume that the extended path lengths in the long-period system V1309 Ori favour cooling and higher densities.
While the ratio
tends to increase with the
magnetic field strength B (Ramsay et al. 1994; Beuermann & Schwope 1994), there is no
simple one-to-one relationship. In the polar sample studied during
the ROSAT All-Sky-Survey, the highest ratios
are observed for systems with a field strength similar to
that of V1309 Ori (Beuermann & Burwitz 1995). There are some exceptions,
however, suggesting that compression in the magnetic field is not
sufficient to suppress all hard X-ray emission. The complex
observational relation between
and B is most
easily explained if there are sources of keV X-rays which are not
associated with buried shocks. Such source is present also in V1309 Ori and
is addressed in the Sect. 5.4.
In summary, we consider it plausible that a number of reasons combine to cause the observed dominance of soft X-rays observed in V1309 Ori.
In contrast to many other polars, the number of simultaneously
impacting filaments is small and a large fraction of the soft X-ray
flares can be individually resolved, thus providing imminent clues on
the temporal behaviour and the impact energetics. On average, the
flare profiles are symmetric with rise and decay times of about
10 s. The lack of obvious temperature changes throughout individual
bursts suggests that the intensity profile of the flare is determined
by a corresponding profile of the local mass flow rate per unit area
and instantaneous thermalization. A finite cooling time of the heated
surface elements is not discernible in the data. The individual flares
in V1309 Ori release energies in the range of (1-
erg,
which we estimate using a distance of 625 pc and a geometry factor of
.
For a white dwarf of 0.7
,
this range of energies
corresponds to a range of blob masses
-
g.
At flare maximum, the blackbody emitting areas
computed
from
Besides the blackbody-like soft X-ray component visible only
during the flares, significant excess emission is detected at energies
up to 3 keV during almost all orbital phases. The spectral and temporal
behaviour of this faint-phase emission is complex and atypical of the
hard X-ray emission which is seen from other magnetic CVs and usually
attributed to bremsstrahlung from the accretion shock. The shock
temperature is expected to be (Aizu 1973)
Modelling of low level variations of the optical circular polarisation
indicates a two-pole geometry (Katajainen et al. 2003; Buckley & Shafter 1995) with the two
poles located at
and
.
Because of the strong
stream emission, however, the polarised emission is faint and conclusions
on the accretion geometry should be considered with some caution.
Our X-ray observations provide some additional insight. The flaring
soft X-ray emission seen between phases 0.4 and 0.95 in the ROSAT and
XMM data are in general agreement with the azimuth of the
second optical spot.
The large variation of the phase intervals during which
soft X-ray emission is seen at different epochs
indicates either real changes of the accretion geometry or
fluctuations in .
The latter possibility is supported by
small scale flaring which continues between phases 0.7 and 0.95
during the XMM-Newton pointing.
If
on the other hand, the XMM-Newton bright phase between
0.41-0.68
indicates
the location of the X-ray emitting region
then its position would be quite peculiar.
The centre of this bright interval would be
consistent with a spot at an azimuth
which is
almost antipodal to the direction towards the secondary star
(
).
Only the polar HY Eri (Burwitz et al. 1999) has a main accretion region
at such extreme position, whereas the primary accretion pole seems to
be generally confined in the range 0-60
(Cropper 1988).
Because of the pronounced time variability a
final conclusion cannot be drawn. The second optically derived accretion
spot,
,
is visible after the eclipse during orbital
phases 0.05-0.39. In principle, this pole could be responsible for the
post-eclipse faint-phase X-ray emission. The spectral properties of
this component do not correspond to the known characteristics of shock
emission, however. We consider, therefore, the scattering scenario
described above as more likely and conclude that the second optical pole
is not seen in X-rays.
Our XMM-Newton and ROSAT observations of V1309 Ori have revealed the most extreme example of a soft X-ray excess yet found in any magnetic CV. Our density estimate and constraints from the observation itself show that the bulk of the mass flow will be buried to sufficiently deep sub-photospheric layers. Other mechanisms (cyclotron cooling, geometry) may be responsible for the quenching of additional hard X-ray emission from any residual low density material.
The soft X-ray emission is seen between orbital phases 0.4 and 0.95 consistent with an origin from one of the polarimetrically defined accretion spots. Further observations that will provide a much better definition of the mean orbital light curve are required to clarify the yet uncertain accretion geometry of V1309 Ori.
The softness of the X-ray emission of the primary accretion region and its favourable geometry allowed us to disentangle two other emission components which have not been resolved in any other polar so far. The softer of these components has been interpreted as scattering or reflection of primary soft X-ray photons in the column closely above the impact region.
The prolonged eclipse on the other hand, indicates the presence of a much more extended emitting source for the harder faint phase emission, most likely the accretion stream. A possible mechanism for this component is shock heating at the stagnation region in the magnetosphere. It appears, however, that further progress and a definite conclusion on this interesting possibility require a much better definition of the mean orbital light curve and an improved statistical definition of the decreasing X-ray emission during the eclipse.
Acknowledgements
R.S. was and is supported by the Deutsches Zentrum für Luft- und Raumfahrt (DLR) GmbH under contracts No. FKZ 50 OR 0206 and 50 OR 0404.