A&A 394, 205-211 (2002)
DOI: 10.1051/0004-6361:20021089
L. Boirin1, 2 - A. N. Parmar1 - T. Oosterbroek1 - D. Lumb1 - M. Orlandini3 - N. Schartel4
1 - Astrophysics Mission Division, Research and Scientific
Support Department of ESA, ESTEC, Postbus 299,
2200
AG Noordwijk, The Netherlands
2 -
Centre d'Étude Spatiale des Rayonnements, CNRS/UPS,
9 Av. du Colonel Roche, 31028 Toulouse Cedex 4, France
3 -
Istituto Tecnologie e Studio Radiazioni Extraterrestri,
CNR, via Gobetti 101, 40129 Bologna, Italy
4 -
XMM-Newton Science Operation Center, ESA, Vilspa,
Apartado 50727, 28080 Madrid, Spain
Received 24 April 2002 / Accepted 24 July 2002
Abstract
We have observed the X-ray transient XTE J0421+56 in quiescence
with XMM-Newton. The observed spectrum is highly unusual being
dominated by an emission feature at 6.5 keV. The spectrum can
be fit using a partially covered power-law and Gaussian line model, in
which the emission is almost completely covered (covering fraction of
0.98 -0.06+0.02) by neutral material and is strongly absorbed
with an
of (
5 -2+3)
1023 atom cm-2. This absorption is
local and not interstellar. The Gaussian has a centroid energy of
keV, a width
keV and an equivalent width
of
940 +650-460 eV. It can be interpreted as fluorescent
emission line from iron. Using this model and assuming XTE J0421+56 is at a
distance of 5 kpc, its 0.5-10 keV luminosity is
erg s-1. The Optical Monitor onboard XMM-Newton indicates a Vmagnitude of
.
The spectra of X-ray transients in
quiescence are normally modeled using advection dominated accretion
flows, power-laws, or by the thermal emission from a neutron star
surface. The strongly locally absorbed X-ray emission from
XTE J0421+56 is therefore highly unusual and could result from the compact
object being embedded within a dense circumstellar wind emitted from
the supergiant B[e] companion star. The uncovered and unabsorbed
component observed below 5 keV could be due either to X-ray emission
from the supergiant B[e] star itself, or to the scattering of
high-energy X-ray photons in a wind or ionized corona, such as
observed in some low-mass X-ray binary systems.
Key words: accretion, accretion disks - stars: individual: XTE J0421+56 - X-rays: general
XTE J0421+56 was discovered by the All-Sky Monitor onboard RXTE as a soft
X-ray transient during an outburst in 1998 March 31
(Smith et al. 1998). This outburst was observed by CGRO
(Paciesas & Fishman 1998), RXTE
(Revnivtsev et al. 1999; Belloni et al. 1999), ASCA
(Ueda et al. 1998) and BeppoSAX (Frontera et al. 1998; Orr et al. 1998). The source brightened rapidly,
reaching an intensity of 2 Crab after a few hours, then quickly
decayed with an initial e-folding time of only 0.6 days before
reaching quiescence in less than 2 weeks. This was the fastest rise
and decay of any outburst from a soft X-ray transient (see
e.g., Chen et al. 1997). The outburst X-ray spectra from XTE J0421+56 are complex
and can not be fit by any of the models usually applied to soft X-ray
transients. The ASCA outburst spectrum was fit with a two
temperature optically thin thermal model and an additional broad Fe-K
emission line at 6.4 keV. Both BeppoSAX outburst spectra were described
using a two temperature bremsstrahlung model and narrow emission line
features identified with O, Ne/Fe-L, Si, S, Ca and Fe-K
(Orr et al. 1998). Emission lines at energies of
6.5 keV and
8 keV are detected in the RXTE outburst spectra.
Optical and radio observations allowed XTE J0421+56 to be rapidly identified
with CI Cam, also known as MWC 84
(Wagner & Starrfield 1998; Hjellming & Mioduszewski 1998; Robinson et al. 1998). It
is a frequently observed source in ultra-violet (UV), optical and
infra-red (IR) wavelengths. Its V magnitude over long term
observations shows a 0.4 mag amplitude variability and has
a mean value of 11.6, both before (1989-1994) and after (1998-1999)
the 1998 X-ray outburst (Bergner et al. 1995; Clark et al. 2000). CI Cam is a supergiant B[e] star (Clark et al. 1999; Robinson et al. 2002), or a
sgB[e] star, following the notation of Lamers et al. (1998), i.e. a
supergiant showing the B[e] phenomenon. The B[e] phenomenon concerns
many objects of different masses and evolutionary phases (see
e.g., Lamers et al. 1998). One of the common properties of stars exhibiting
the B[e] phenomenon is the presence of forbidden emission lines in
their optical spectra (the notation "[e]'' refers to the one used for
forbidden lines). Another common property is to show a strong IR
excess attributed to hot circumstellar dust. In these respects, stars
with the B[e] phenomenon clearly differ from the ordinary Be stars
which are rapidly rotating stars near the main sequence losing mass in
an equatorial wind. In practice, the spectroscopic and photometric
properties of stars with the B[e] phenomenon are also easily
distinguished from those of ordinary Be stars. CI Cam/XTE J0421+56 is the
first high-mass X-ray binary (HMXB) with a sgB[e] mass donor
companion. Another source suspected to be a HMXB with a mass donor
showing the B[e] phenomenon is the optical/X-ray source
HD 34921/1H 0521+37 (Clark et al. 1999). Adopting the
classification criteria and notation of Lamers et al. (1998),
Clark et al. (1999) identify the companion star in this system as an
"unclB[e] star'' (unclassified B[e] star).
Optical high-dispersion spectroscopy of CI Cam led
Robinson et al. (2002) to the conclusion that the sgB[e] star
emits a two component wind. One component is a hot, high-velocity
wind. The other component is a cool, low-velocity and very dense
(electron number density log > 9.5) wind. The wind is roughly
spherical and continuously replenished. The mass-loss rate due to the
wind is very high:
> 10-6
yr-1. This wind fills the
space around the sgB[e] star and, from the size of the IR-emitting
dust shell, extends to a radius between 13 and 50 AU. Thus, the
circumstellar material around CI Cam is much denser, far more
extended, and much less confined to the equatorial plane than the
circumstellar material around a Be star. The passage of the compact
X-ray source through such a complex and dense environment is likely
to strongly affect the X-ray properties of the source.
Within this picture, Robinson et al. (2002) suggest that the 1998 outburst was caused by the same disk instability mechanism responsible for the outbursts in X-ray novae, i.e. by an instability in the accretion disk around the compact object (see e.g., Lasota 2001). It would thus differ from the outbursts observed in ordinary Be HMXB that recur at multiples of the orbital period, when the compact object comes close to the Be star at periastron and plunges into its equatorial wind.
The distance to XTE J0421+56 is uncertain. Based on optical spectroscopic
properties, on radial velocity measurements of CI Cam and on
considerations about the structure of the Galaxy,
Robinson et al. (2002, Sect. 2.3) estimate that the distance to
the source is much larger than the 2 kpc previously
considered. Robinson et al. (2002) use a distance of 5 kpc and
note that it is likely to be a lower limit to the true distance which
could be up to 10 kpc. In this paper, we assume a distance of 5 kpc.
This distance makes XTE J0421+56 among the most luminous transients. The
2-25 keV luminosity at the peak of the outburst was
erg s-1, assuming the revised distance of 5 kpc
(Orlandini et al. 2000; Robinson et al. 2002).
The unusual nature of CI Cam makes the interstellar absorption
towards the star difficult to estimate. From an UV spectrogram,
Robinson et al. (2002) derive a differential extinction E(B-V) of
,
but do not attempt to separate circumstellar from
interstellar extinction. From an analysis of diffuse interstellar
bands in the optical spectrum of CI Cam, Clark et al. (2000) derive
an interstellar E(B-V) of
and an
of
,
which implies an interstellar X-ray absorption,
,
of
1021 atom cm-2 (Parmar et al. 2000, Sect. 3). Extinction
at soft X-ray wavelengths yielded an
of
1022 atom cm-2 near the peak of the outburst, and the
decreased to
2.2
1021 atom cm-2 as XTE J0421+56 approached quiescence
(Belloni et al. 1999). This rapid change in the X-ray extinction,
as well as the change in the IR flux after the outburst
(Clark et al. 2000), indicate that much of the extinction to
CI Cam is local, not interstellar (Robinson et al. 2002).
No bursts, pulsations or quasi-periodic oscillations have been detected from XTE J0421+56 (Belloni et al. 1999). The large ratio of peak to quiescent luminosity is taken as evidence for the compact object being a black hole (Robinson et al. 2002). Furthermore, CI Cam has been detected as a relatively bright radio source (Hjellming & Mioduszewski 1998) which is more typical of black hole candidates than neutron star systems (see Belloni et al. 1999, Sect. 5).
XTE J0421+56 was observed in quiescence by BeppoSAX on 1998 September 3, 1999
September 23 and 2000 February 20
(Orlandini et al. 2000; Parmar et al. 2000). In 1998, the source was
soft (power-law photon index, ,
of
4.0 +1.9 -0.9) with a
low
of
(1 +5 -1)
1021 atom cm-2. In 1999, the source had
hardened (
= 1.86 +0.27 -0.32) and brightened and became
strongly absorbed with an
of
1023 atom cm-2. There is
evidence for a narrow emission line in both spectra at
7 keV. In
2000, the source was not detected. At 5 kpc, the 1-10 keV
luminosities were
,
,
and <
erg s-1, in 1998, 1999, and 2000, respectively.
These results are summarized in Table 1.
Obs. | Year | ![]() |
![]() |
Ref. |
(atom cm-2) | (erg s-1) | |||
SAX | 1998 |
![]() |
![]() |
[1], [2] |
SAX | 1999 |
![]() |
![]() |
[2] |
SAX | 2000 | <
![]() |
[2] | |
XMM | 2001 |
![]() |
![]() |
[3] |
Here, we report on the XMM-Newton observation of XTE J0421+56 in quiescence performed on 2001 August 19. We present and discuss the nature of the X-ray spectrum and derive the V magnitude of the source using the Optical Monitor.
The XMM-Newton Observatory (Jansen et al. 2001) includes three 1500 cm2 X-ray telescopes each with an European Photon Imaging Camera (EPIC, 0.1-15 keV) at the focus. Two of the EPIC imaging spectrometers use MOS CCDs (Turner et al. 2001) and one uses pn CCDs (Strüder et al. 2001). Reflection Grating Spectrometers (RGS, 0.35-2.5 keV, den Herder et al. 2001) are located behind two of the telescopes. In addition, a coaligned optical/UV Monitor (OM, 160-600 nm, Mason et al. 2001) is included as part of the payload.
The region of sky containing XTE J0421+56 was observed by XMM-Newton on 2001 August 19 between 07:05 and 16:16 UTC.
The X-ray cameras were operating in their Prime Full Window mode with
Medium thickness filters. We used the X-ray data products generated
by the Pipeline Processing Subsystem in September 2001. We
further filtered these products using the Science Analysis Software
(SAS) version 5.1.0 and especially the tasks evselect and xmmselect. Electronic noise and hot or flickering pixels were
rejected. The observation is contaminated by high backgrounds
intervals due to solar activity. We excluded these times by selecting
intervals where the overall >1 keV pn count rate was <
.
Source counts were extracted in a 30
radius
circular region centered on XTE J0421+56. The source is close to a chip
border in the pn detector. Background counts were obtained from a
circular region of 90
radius offset from the source position.
For the pn, single and double pixel events were selected
(patterns 0 to 4). For the MOS, events corresponding to patterns from
0 to 12 were selected. The exposure after applying these filtering
criteria is 12 ks. This is substantially below the expected
exposure due to the removal of intervals of high solar
activity. Figure 1 shows the EPIC pn image of the
region of sky containing XTE J0421+56. The source, although faint, is
clearly detected.
We next extracted spectra. For the pn, subtracting the source and
background count rates gives a net count rate of
s-1 in the 0.1-12 keV energy range, where the
source is detected. This corresponds to
280 net counts detected.
We used the standard pn response matrix file
epn_ff20_sY9_medium.rmf. Approximately 60 net counts are detected
in each MOS camera. We used the response matrix files
m1_medv9q19t5r5_all_15.rsp and m2_medv9q19t5r5_all_15.rsp for
MOS 1 and MOS 2, respectively. In order to ensure applicability of
the
statistic with so few counts, we rebinned the MOS and pn
spectra such that at least 25 net counts per bin were present. The
resulting MOS spectra have 2 bins and the pn spectum has 11 bins,
allowing only simple models to be tested. XTE J0421+56 is not detected in the
RGS. Modeling of the spectra was carried out using XSPEC version
11.1.0. All spectral uncertainties are given at 90% confidence and
upper limits at 95% confidence.
![]() |
Figure 1:
An EPIC pn image of the region of sky containing XTE J0421+56. The
radius of the circle showing the source position is 30
![]() |
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The OM was operating in its default Imaging Mode. Five consecutive
exposures of 1000 s were obtained through the V filter (510-580 nm),
followed by five consecutive exposures of 4540 s through the UVW1
filter (245-320 nm). We have analyzed 2
2
high
resolution images centered on XTE J0421+56.
Instrumental magnitudes of the source were extracted for each image using DAOPHOT. We computed the average instrumental magnitudes for both filters and used these values to correct the average Vinstrumental magnitude into a standard V magnitude of the Johnson's UBV system. We used the color transformation relation shown e.g., in Antokhin et al. (2002) and the coefficients from the Current Calibration File OM_COLORTRANS_0006.CCF.
The combined MOS 1, MOS 2 and pn spectrum of XTE J0421+56 is shown in
Fig. 2. Its shape is highly unusual. It is
clearly dominated by an emission feature peaking at
6.5 keV
.
We first tried to fit the spectrum with simple descriptive models. A
model consisting of a power-law with a photon index,
,
to account for the low-energy feature of the spectrum
(
5 keV) and a Gaussian to account for the high-energy
feature (above
5 keV) does not provide an acceptable fit to
the data giving a reduced
(
)
of 2.07 for 10
degrees of freedom (d.o.f.). A similarly poor result is
obtained when including photo-electric absorption (phabs within
XSPEC) to this model in order to account for an absorption of the
X-rays. Both a broad and a narrow component seem to be needed to
account for the high-energy feature. A relatively good (
of 1.56 for 8 d.o.f.) description of the spectrum can be obtained
using a model consisting of a power-law (
1.9) to
account for the low-energy feature plus another power-law
(
)
to account for the broad high-energy feature and
a Gaussian at
6.4 keV to account for the narrow high-energy
feature.
We then adopted a different fitting approach by modeling the broad
high-energy feature as a highly absorbed continuum component. We
tried a model consisting of a power-law for the low-energy feature,
plus a power-law and a Gaussian both modified by absorption from
neutral material for the high-energy feature (model powerlaw+phabs(powerlaw+Gaussian) within XSPEC). This model fits the
spectrum relatively well, with a
of 1.04 for 7 d.o.f.
The best-fit parameters for this model are given in Table 2.
We note that, without the Gaussian in the
model, the fit is poor, with a
of 2.19 for 10 d.o.f. An
F-test indicates that the probability,
,
of rejecting the
hypothesis that the fit is better including the Gaussian is
4.3%. This indicates that the Gaussian is significant at
95.7% confidence. In order to account for potential absorption of
the first power-law emission by the line of sight interstellar medium,
we included an additional photo-electric absorption component (phabs within XSPEC) to the previous model. The resulting model (phabs(powerlaw+phabs(powerlaw+Gaussian)) fits the spectrum relatively
well, with a
of 1.22 for 6 d.o.f., although an F-test
indicates that the addition of the absorption component does not
improve the fit (
of 99.9%). The best-fit parameters
obtained using this model are also given in Table 2.
The
of the additional absorption component
is <1.8
1021 atom cm-2. The other parameter values are consistent with those
obtained previously without the additional phabs component.
We then tried to fit the spectrum using a partially covered
power-law and Gaussian model, the pcfabs(powerlaw+Gaussian)
model within XSPEC. In this model, some absorbing material covers a
fraction (from 0 to 1) of the power-law and Gaussian
emission. The absorption is by neutral material with solar
abundances and uses cross-sections from Balucinska-Church & McCammon (1992).
This model fits the spectrum very well with a
of 1.04 for 8
d.o.f. The best-fit parameters are given in
Table 2. Without the Gaussian in the model, the
fit is poor, with a
of 2.47 for 11 d.o.f. An F-test indicates
that the probability,
,
of rejecting the hypothesis that the fit
is better including the Gaussian is 1.8%. This indicates that the
Gaussian is significant at 98.2% confidence. In order to account for
potential interstellar absorption of the uncovered component of the
emission, we included an additional photo-electric absorption
component to the previous model. The resulting model (phabs(pcfabs(powerlaw+Gaussian)) fits the spectrum relatively well,
with a
of 1.18 for 7 d.o.f., although an F-test indicates
that the addition of the absorption component does not improve the
fit, with
of 99.7%. The best-fit parameters obtained using
this model are given in Table 2. The
of the
additional absorption component is <2.0
1021 atom cm-2. The other parameter
values are consistent with those obtained previously, without the
additional phabs component.
![]() |
Figure 2: The MOS 1 (squares), MOS 2 (circles) and pn (thick crosses) count spectra of XTE J0421+56 in quiescence. |
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MOS 1 + MOS 2 + pn spectrum | ||
Model | pl+phabs(pl+Gauss) | phabs(pl+phabs(pl+Gauss)) |
![]() |
1.04 (7) | 1.22 (6) |
![]() |
... | <
![]() |
![]() |
1.2+0.6-0.7 | 1.2+1.1-0.8 |
![]() |
(![]() ![]() |
(
5+3-2)
![]() |
![]() |
![]() |
1.0+1.2-0.9 |
Gaussian energy |
![]() |
![]() |
Gaussian width ![]() |
<0.29 keV | <0.29 keV |
Gaussian equivalent width | 1010+720-480 eV | 1010+720-480 eV |
Model | pcfabs(pl+Gauss) | phabs(pcfabs(pl+Gauss)) |
![]() |
1.04 (8) | 1.18 (7) |
![]() |
... | <
![]() |
![]() |
(
5 -2+3)
![]() |
(
5+3-2)
![]() |
CvrFract | 0.98 -0.06+0.02 | 0.98+0.02-0.06 |
![]() |
![]() |
1.2+1.1-0.7 |
Gaussian energy | 6.43+0.10-0.09 keV |
![]() |
Gaussian width ![]() |
<0.28 keV | <0.28 keV |
Gaussian equivalent width | 940+650-460 eV | 940+650-460 |
Summarizing, we have obtained acceptable fits to the combined pn and MOS spectra of XTE J0421+560 in quiescence using two different two component models. One component is strongly absorbed and the other may be unabsorbed, or only slightly absorbed (Table 2). In both models, the shape of the high-energy continuum is dominated by absorption resulting in a strong cutoff above 7.1 keV due to neutral iron absorption edge. This produces a very sharply peaked spectral feature which resembles a broad emission line. In addition, there is some evidence for the presence of an additional narrow emission feature at an energy of 6.4 keV. (This feature is required at 98.2% confidence in the case of the partially covered power-law and Gausssian model). The two models presented in Table 2 differ by how the low-energy component is modeled. This is either by partial covering, in which case the low-energy component is constrained to have the same underlying spectral shape as the absorbed high-energy component, or by a power-law with an unconstrained slope. For completeness, Table 2 lists the results if additional low-energy absorption is included to account for interstellar absorption. However, the fits do not require such absorption, and the upper-limits are consistent with that expected from the likely interstellar values.
The partially covered power-law and Gaussian model and the
power-law plus absorbed power-law and Gaussian model give equally
good fits with a
of 1.04 for 8 d.o.f. Figure 3
shows the former model fit to the pn
spectrum only for clarity. In this model, the power-law and
Gaussian emissions are almost completely covered (with a covering
fraction of 0.98) by neutral material and strongly absorbed with an
of (
5 -2+3)
1023 atom cm-2. The equivalent width of the
Gaussian line feature is
940 +650-460 eV. This model is
equivalent to the sum of an absorbed (and covered) plus an unabsorbed
(and uncovered) component with the same spectral shape. The
contributions of both these components are shown separately in the
right panel of Fig. 3. The unabsorbed omponent
(dashed line) dominates the emission
5 keV. We refer to
this component as the low-energy component. On the contrary, the
absorbed component (dotted line) clearly dominates the emission
5 keV. We refer to this component as the high-energy
component. Using the partially covered power-law and Gaussian
model, we derive a 0.5-10 keV absorbed flux of
erg cm-2 s-1 and a unabsorbed flux of
erg cm-2 s-1 by setting
to 0. This corresponds to an
unabsorbed luminosity of
2 erg s-1 at a distance of
given in kpc, and to a unabsorbed luminosity
of
erg s-1, at a distance of 5 kpc. The
low-energy component contributes 9.8% to the total absorbed
flux in the 0.5-10 keV range. The 1-10 keV luminosity
is given in Table 1 for comparison with previous
BeppoSAX observations of XTE J0421+56 in quiescence.
![]() |
Figure 3: The spectrum of XTE J0421+56 in quiescence (only the pn is shown for clarity). The left panels show the count spectrum (data and folded model). The solid line is the best-fit using the partially covered power-law and Gaussian model. The lower left panel shows the residuals from the fit in terms of standard deviations. The right panel shows the unfolded spectrum. The dashed line shows the contribution of the unabsorbed component (the low-energy component) to the total model. The dotted line shows the contribution of the absorbed component (the high-energy component). The total model is not shown in this panel for clarity. |
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The average UVW1 instrumental magnitude of XTE J0421+56 during the XMM-Newton
observation is
.
The average standard V magnitude is
.
Optical observations of XTE J0421+56 (CI Cam) exist prior to the 1998 outburst. A V magnitude of 11.4 is attributed to CI Cam during observations made in the 1970's (Allen & Swings 1976).
Long term observations of the source were carried out between 1989 and
1994. CI Cam covered a range of V magnitudes between
and
(47 data points), thus showing a
0.4 mag amplitude variability on a day-to-day timescale
(Bergner et al. 1995). The mean V magnitude of CI Cam during this
pre-outburst interval was
.
Performing a Fourier
analysis on these data, Miroshnichenko (1995) found
evidence for a 11.7 days quasi-period which he interpreted as a
possible orbital period.
Clark et al. (2000) report on observations of CI Cam performed
after the outburst, between 1998 August and 1999 March. The Vmagnitude of CI Cam during this post-outburst interval ranges between
and
(18 data points) and shows the
same
0.4 mag variability with the same mean value as
during the pre-outburst period.
Thus, the V magnitude of
during the XMM-Newton
observations is outside the range of magnitudes reported previously,
indicating that the source had become fainter. However, this
magnitude represents only one data point. So, it is difficult to
determine if this higher value is due to variability on a day-to-day
timescale, or if the source has become fainter for an extended
interval.
The quiescent XMM-Newton spectrum of XTE J0421+56 can be fit by a
power-law plus absorbed power-law and Gaussian model or alternatively
by a partially covered power-law and Gaussian model. In both models,
the high-energy component corresponds to a strongly absorbed continuum
(
of
5 -2+3
1023 atom cm-2). This fitting approach
is supported by the fact that a similarly strongly absorbed quiescent
emission was also reported from the source during the 1999 BeppoSAX observation. Parmar et al. (2000) fit the BeppoSAX 2-10 keV spectrum
with an absorbed (
of
1023 atom cm-2) power-law
(
= 1.86 +0.27-0.32) together with a Gaussian emission
feature with an energy of 7.3
0.2 keV and an equivalent width of
620
350 eV. Since an unabsorbed component is unambiguously
detected at low-energy in the XMM-Newton quiescent spectrum of
XTE J0421+56, we re-examined the BeppoSAX spectrum reported in
Parmar et al. (2000) to see if there is evidence for the presence
of a similar component. The 0.2-2 keV LECS count rate of
s-1 suggests that such a
component may be present. In order to investigate this further, we fit
the partially covered XMM-Newton model discussed above to the
0.5-10 keV BeppoSAX spectrum allowing the spectral parameters to
vary. The line energy and width which were poorly constrained were
set to the best-fit XMM-Newton values. This gives a
of 1.43
for 38 d.o.f. The uncovered component contributes (
)% of the
total 1-10 keV absorbed flux, consistent with the ratio observed with
XMM-Newton. Thus, the presence in the BeppoSAX 1999 spectrum of an
unabsorbed component, similar to that detected in the XMM-Newton
spectrum cannot be excluded.
When absorption is added to either the power-law plus absorbed
power-law and Gaussian model or the partially covered power-law and
Gaussian model used to fit the XMM-Newton spectrum, the
of this
component is <2.0
1021 atom cm-2 (see Table 2).
This value is consistent with that obtained when XTE J0421+56 approached
quiescence after the outburst (Belloni et al. 1999). These low
values of
confirm that the interstellar column towards
XTE J0421+56 is not high. On the other hand, X-ray results show that the
column density intrinsic to the system can be very high, and as
inferred from the large range of absorption obtained (from roughly 0.2
to 50
1022 atom cm-2), very variable. Thus, this confirms the picture that most
of the absorption towards XTE J0421+56 is local and not interstellar.
The Gaussian feature observed at 6.4 keV can be interpreted as a
fluorescent emission from iron. Such an interpretation is consistent
with the modeling of the spectrum using partial covering since it
suggests the presence of significant cold absorbing material in the
system. The large equivalent width observed can be explained if cold
material surrounds the X-ray emitter with a large column density,
which is also consistent with our modeling and with the picture, drawn
from optical spectroscopy, of a compact object embedded in a very
dense wind (Robinson et al. 2002). However, the spectral quality
is too low to test for the presence of associated signatures of cold
material such as absorption edges. A reflection component due to the
presence of cold material could be expected as well, but such a
component usually peaks above the energy range covered here (between
10 and 100 keV). Gaussian emission features were detected
during the 1998 and 1999 BeppoSAX observations of XTE J0421+56 in quiescence at
energies of
7.0+1.6-0.2 and 7.3
0.2 keV
respectively. Their different energies as compared to the 6.4 keV
feature detected with XMM-Newton may indicate a different origin, or
different physical conditions in the emission region. Emission
features at
6.4 keV were also detected in outburst spectra from
XTE J0421+56 and mostly interpreted as emission lines produced by an
optically thin plasma (see
e.g., Ueda et al. 1998; Revnivtsev et al. 1999). Such a mechanism can
not be excluded in the case of the XMM-Newton observation of XTE J0421+56,
although the geometry and emission processes involved during
quiescence are likely to be very different from those involved during
the outburst.
The partially covered power-law and Gaussian model as well as the
powerlaw plus absorbed power-law and Gaussian model suggest the
presence of two components: an unabsorbed component mainly observed
5 keV (the uncovered or low-energy component), and second, a
strongly absorbed component mainly observed
5 keV (the
covered or high-energy component) and dominating the total spectrum.
We propose that the covered component results from the compact object
being embedded within the dense circumstellar wind emitted from the
sgB[e] companion star, in agreement with the picture drawn from
optical spectroscopy of the source (Robinson et al. 2002). The
large range of observed
at X-ray wavelengths could reflect the
complexity of the B[e] star environment in which the compact object is
traveling. Regions with different physical properties may be crossed,
depending e.g., on the distance of the compact object from the sgB[e]
star or from its equatorial plane. This environment may vary with time
as well. It may also be modified by the X-rays emitted in the vicinity
of the compact object, which are probably variable themselves.
We suggest two possible origins for the low-energy component. First,
it could be due to X-ray emission from the sgB[e] star itself. The
X-ray emission from OB stars is intrinsically soft (up to
4 keV, Long & White 1980). Orlandini et al. (2000) estimate that
the X-ray luminosity of the companion star in XTE J0421+56 could be
5
erg s-1, while
Robinson et al. (2002, Sect. 2.4) estimate that the sgB[e] star
could emit up to 1034 erg s-1 in the 0.2-4.0 keV band. The
0.2-4.0 keV luminosity observed from XTE J0421+56 during the XMM-Newton
observation is
erg s-1 at 5 kpc. Thus, we cannot
exclude that the low-energy emission, or part of it, originates from
the companion star.
Another possibility is that the low-energy component is due to the
scattering of higher-energy X-ray photons in a wind or ionized
corona such as observed in some low-mass X-ray binaries. The flux of
the low-energy component in XTE J0421+56 is about 10% of the total
0.5-10 keV flux. In dipping, eclipsing or accretion disk
corona sources, the ratio observed between the flux attributed to
scattered emission and the total flux is usually 5% (see
e.g., Parmar et al. 1986). Thus, at least a part of the low-energy
emission could be due to scattering in XTE J0421+56. We note however
that corona have been observed in low-mass X-ray binaries that are
much brighter than XTE J0421+56. So the possible scattering region in XTE J0421+56 may differ in nature and formation from those observed in low-mass
X-ray binaries. The scattering region in XTE J0421+56 could be linked to the
wind emitted by the B[e] companion star. Emission from the companion
star and scattering could both play a role in the low-energy emission
observed from XTE J0421+56.
XTE J0421+56 is the first identified member of a new class of HMXB with sgB[e] companion. It is the only known system in which the compact object is immersed in a dense and complex circumstellar wind. Further multiwavelength observations of this source are needed to explore the geometry and the emission processes involved in this system. Many other stars showing the B[e] phenomenon, and especially sgB[e] stars, could host a compact object. Due to their low X-ray luminosity and absorbed spectra, such objects are unlikely to have been identified in previous low-energy (0.1-2.5 keV) sky surveys such as conducted by ROSAT, and we await future medium energy X-ray surveys to detect further members of this class.
Acknowledgements
This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA). L. Boirin acknowledges an ESA Fellowship. We thank Rudi Much, Igor Antokhin and Simon Rosen for providing very useful help analyzing the XMM-Newton Optical Monitor data.