A&A 443, 223-230 (2005)
DOI: 10.1051/0004-6361:20053438
L. Sidoli1 - N. La Palombara1 - T. Oosterbroek2 - A. N. Parmar3
1 - Istituto di Astrofisica Spaziale e Fisica Cosmica "G. Occhialini'', IASF/INAF,
via Bassini 15, 20133 Milano, Italy
2 -
Science Payload and Advanced Concepts Office, ESA, ESTEC,
Postbus 299, 2200 AG, Noordwijk, The Netherlands
3 -
Research and Scientific Support Department of ESA, ESTEC,
Postbus 299, 2200 AG Noordwijk, The Netherlands
Received 16 May 2005 / Accepted 2 July 2005
Abstract
We report the results of two XMM-Newton observations of the
ultra-compact low-mass X-ray binary 4U 1850-087 located in the galactic
globular cluster NGC 6712. A broad emission feature at 0.7 keV was
detected in an earlier ASCA observation and explained as the
result of an unusual Ne/O abundance ratio in the absorbing
material local to the source. We find no evidence for this feature
and derive Ne/O ratios in the range 0.14-0.21, consistent with
that of the interstellar medium. During the second observation,
when the source was 10% more luminous, there is some
evidence for a slightly higher Ne/O ratio and additional
absorption. Changes in the Ne/O abundance ratio have been detected
from another ultra-compact binary, 4U 1543-624. We propose that
these changes result from an X-ray-induced wind which is
evaporated from an O and Ne rich degenerate donor. As the source
X-ray intensity increases so does the amount of evaporation and
hence the column densities and abundance ratio of Ne and O.
Key words: accretion, accretion disks - stars: individual: 4U 1850-087 - stars: neutron - Galaxy: globular clusters: individual: NGC 6712
ASCA and BeppoSAX observations of ultra-compact (
h)
low-mass X-ray binaries (LMXBs) have revealed two possible
spectral differences compared to the longer period systems. These
are (1) the presence of a discrete spectral feature near 0.7 keV
(attributed to Ne) in the ASCA spectra (Juett et al. 2001)
of X1850-087, X1543-624,
X0614+01 and X0918-549 (the orbital periods
of the last 2 sources are unknown, but their optical faintness is
consistent with an ultra-compact nature) and (2) their best-fit
parameter values when fitted with a disk-blackbody and Comptonized
continuum (Sidoli et al. 2001). In the case of the
ultra-compact sources X0512-401,
X1820-303, X1850-087 and
X1832-330, fits to the BeppoSAX spectra give
significantly lower disk-blackbody temperatures than for other
LMXB. In addition, the Comptonization seed photon temperatures
appear consistent with those of the inner disk regions.
The LMXBs located in the globular clusters NGC 6652
(Parmar et al. 2001) and Terzan 5 (Heinke et al. 2003) show very similar spectral properties, suggesting
an ultra-compact nature for these binary systems (see
Verbunt 2005 for a review).
Table 1: XMM-Newton on-axis observation log of 4U 1850-087. Two observations were performed. The MOS1, MOS2 and pn cameras all used the medium thickness filter.
The X-ray burst source X1850-087 (Swank et al. 1976)
is an X-ray binary located in the galactic globular cluster
NGC 6712, the least concentrated amongst those that host a luminous
X-ray source. A short period (20.6 minutes) UV modulation was
discovered with HST from the likely optical counterpart (Anderson
et al. 1993), implying a degenerate companion of
0.04
(Homer et al. 1996). The source is located
6
or
core radii from the cluster
center (Hertz & Grindlay 1983). Another UV-excess star was
discovered in the core of NGC 6712 with the ESO Very Large Telescope
(Ferraro et al. 2000), a few arcsec away from the LMXB. The
presence of these two interacting binaries inside the core of the
low-density cluster NGC 6712 suggests that the interaction of the
cluster with the disk and bulge of our Galaxy during numerous
orbital passages plays a role in the formation of LMXBs in
globular clusters (Ferraro et al. 2000). Moreover, there is
evidence supporting a scenario where NGC 6712 was much more
massive in the past and that it experienced a significant mass
evaporation produced by the tidal force due to interactions with
our Galaxy (see, e.g., Paltrinieri et al. 2001, and
references therein).
EXOSAT observations of 4U 1850-087 revealed a complex spectrum. The
best-fit was obtained with a model consisting of a power-law with
a photon index, ,
of 0.4 with an exponential cut-off at
1 keV, together with a blackbody with a temperature, kT,
of 2.4 keV and absorption,
,
of
<
cm-2 (Parmar et al. 1989). A thermal
bremsstrahlung (kT = 1.7 keV) absorbed by
cm-2 is a good approximation of the ROSAT Position Sensitive
Proportional Counter spectrum (Verbunt et al. 1995). During a
BeppoSAX survey of the bright LMRXB located in galactic globular
clusters (Sidoli et al. 2001) the 0.3-50 keV spectrum was
fit with a disk-blackbody and Comptonized continuum with
cm-2, an inner disk temperature,
,
of 0.6 keV, an inner projected radius of
5 km
(for an assumed NGC 6712 distance of 6.8 kpc, Harris 1996),
a temperature, kT0, of the input "seed'' photons, of 0.8 keV
(consistent with the inner disk-blackbody temperature), an
electron temperature,
,
of 70 keV, and an optical
depth,
,
of 1.7. The 0.1-100 keV luminosity was
erg s-1.
Analysis of the ASCA Solid-state Imaging Spectrometer (SIS) data
(Juett et al. 2001) from 4U 1850-087 revealed the presence of a
spectral feature near 0.7 keV. A good fit to these data was found
with an absorbed kT = 0.4 keV blackbody together with a
power-law with
when the relative abundances of O
and Ne were allowed to vary. Both components are absorbed by
cm-2, with a relative (to solar)
O/H abundance of
and Ne/H abundance of
.
The authors interpreted this excess absorption as
due to neutral Ne-rich material local to the binary. Preliminary
results from the 0.4-2 keV 4U 1850-087 XMM-Newton RGS spectra were reported
in Sidoli et al. (2004), who found no evidence for an
anomalous Ne/O abundance ratio. Recently, analysis of Chandra Low-Energy Transmission Grating Spectrometer (LETGS) data
confirmed this result (Juett & Chakrabarty 2005),
measuring a Ne/O ratio of
consistent with that
expected from the interstellar medium (ISM) of 0.18 (Wilms et al.
2000).
Here we report the results of XMM-Newton observations performed in
order to investigate the nature of the 0.7 keV feature found with
ASCA. We use the following updated values for the globular cluster
NGC 6712 parameters (Paltrinieri et al. 2001): a distance
of kpc (note that a previous estimate for the distance
was 6.8 kpc, Harris 1996) and a reddening
.
Adopting the relation
,
and
cm-2
(Predehl &
Schmitt 1995) the optical reddening translates into an ISM
column density of (
cm-2 to NGC 6712.
The XMM-Newton Observatory (Jansen et al. 2001) includes
three 1500 cm2 X-ray telescopes each with an European Photon
Imaging Camera (EPIC) at the focus. Two of the EPIC imaging
spectrometers use MOS CCDs (Turner et al. 2001) and one
uses a pn CCD (Strüder et al. 2001). Behind two of the
telescopes there are Reflection Grating Spectrometers (RGS,
0.35-2 keV; den Herder et al. 2001). XMM-Newton observed 4U 1850-087 twice, due to visibility problems, in 2003 September and October,
about 12 days apart (see Table 1 for the observation
details).
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Figure 1: EPIC pn lightcurves in two energy bands (S, 0.3-2 keV and H, 2-10 keV) and hardness ratios, H/S, during the first ( left) and second observation ( right). Time is in hours of 2003 September 27 (for the first observation) and of the 2003 October 9 (for the second observation). The binning is 64 s. |
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Data were reprocessed using version 6.1 of the Science Analysis
Software (SAS). Known hot, or flickering, pixels and electronic
noise were rejected using the SAS. The latest response matrices
were used (updated to 2004-12-03, which should improve the
agreement between the MOS and pn below 1 keV,
Saxton 2004), while the ancillary response files were
generated using the SAS task arfgen. Spectra were selected
from single events only (pattern 0) for the MOS1 timing mode (only
pattern 0 has been calibrated in this instrument mode) while for
MOS2 patterns from 0 to 12 and for the pn patterns from 0 to 4 were selected. Source counts were extracted from circular regions
of 40
radius centered on 4U 1850-087 for the pn and the MOS2.
With the SAS task epatplot we verified that pn Small Window
data are not significantly affected by pile-up, whereas in both
MOS spectra pile-up was evident. Thus, we minimized the effects of
pile-up by extracting MOS2 events in an annulus outside of a 10
radius core of the 4U 1850-087 point spread function, and
MOS1 events from a wide column outside the central 15
.
A
comparison between MOS1 and MOS2 spectra revealed that after this
selection the source spectral shapes observed by the two
instruments were similar. We use the pn for the determination of
the source flux. Background counts were obtained from similar
regions offset from the source position. The backgrounds do not
show any evidence for flaring activity, so the entire nominal
exposure times were considered. For both observations, the RGS
spectra were analyzed as produced by the pipeline processing
performed by XMM-Newton Survey Science Centre.
In order to ensure applicability of the
statistic, the
extracted spectra were rebinned such that at least 20 counts per
bin were present and such that the energy resolution was not
over-sampled by more than a factor 3. Note that no systematic
uncertainties were added to the spectra. All spectral
uncertainties and upper-limits are given at 90% confidence for
one interesting parameter.
Lightcurves for the two observations in soft (0.3-2 keV) and hard (2-10 keV) energy ranges were extracted in order to search for variability and hardness ratio variations (Fig. 2). The source was slightly harder and more intense during the second observation. Thus, the two observations were analyzed separately. Since within each individual observation the source does not show evidence for intensity or hardness variations (see Fig. 1), we can safely consider two separate spectra extracted from each of the two observations without making any further selections. The statistical quality of the data, combined with the length of the observations, does not allow for a meaningful search for periods around the optical period.
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Figure 2: Hardness ratio (H = 2-10 keV; S = 0.3-2 keV counts) versus total intensity (S+H, 0.3-10 keV) for the two pn observations (squares mark the first observation, circles the second). The binning is 512 s. |
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We performed separate spectral analysis for the two XMM-Newton observations. We first studied the pn, MOS1 and MOS2 spectra in the energy range 0.3-12 keV. We noticed that the pn spectrum, in both the observations, showed a significant departure from the MOS1 and MOS2 shapes, especially around 0.6 keV, where a large excess is present only in the residuals of the pn spectrum (Fig. 3). A significant departure of the pn spectrum in this energy range is present also compared with the RGS1 and RGS2. Note that this excess is not at the same energy of the feature present in the ASCA SIS spectra, which was interpreted as due to additional absorption by neutral Ne. Smaller differences between the pn and MOS cameras are also present at other energies, below 0.4 keV, and up to about 1.7 keV. Uncertainties in the calibration of the pn Small Window mode below 2 keV are reported in Kirsch et al. (2004), although the use of the latest updated response matrices should reduce these differences (Saxton 2004). Examination of the pn background spectra does not reveal any features at these energies, so we are confident that they are not due to improper background subtraction. Moreover, both observations show similar shapes for the structured residuals. Thus, we restricted the pn energy range to 1.7-12 keV, where there are no significant differences between the MOS1, MOS2 and pn spectra.
In summary, we used the following energy ranges: 0.4-2 keV for RGS1 and RGS2, 0.3-8 keV for the MOS1 (because of the low statistics at high energy), 0.3-10 keV for the MOS2, and 1.7-12 keV for the pn. The RGS spectra in both observations do not show evidence for edges or emission features. We investigated the 0.3-12 keV 4U 1850-087 spectra by simultaneously fitting the RGS1, RGS2, MOS1, MOS2 and pn spectra of each of the two observations individually. Factors were included in the spectral fitting to allow for normalization uncertainties between the instruments. In the spectral fitting XSPEC version 11.2 was used, and the interstellar abundances of Wilms et al. (2000) were used in the photoelectric absorption models.
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Figure 3: Residuals (in units of standard deviations) when fitting the 2003 September MOS and pn spectra with a single absorbed power-law model. There is a different structure of the residuals in the pn (filled circles), compared with MOS1 (crosses) and MOS2 (triangles). A similar structured excess is present in the 2003 October spectrum. |
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LMXB X-ray spectra are generally fit with two component models, a
black-body or a multicolor disk-blackbody component to account for
the low-energy emission (originating from the accretion disk or
the neutron star surface), and a high-energy component which is
usually modeled with power-law, cut-off power-law, or Comptonized
components, to account for the high-energy emission thought to be
produced in a corona. In order to check if a soft component was
required by the data we tried first with the simplest model
consisting of an absorbed power-law (photoelectric absorption
model PHABS in XSPEC). The fits resulted in positive
residuals between 0.3-0.7 keV and reduced
for
1098 degrees of freedom (d.o.f.), and
for 969
d.o.f. for the first and the second observations, respectively.
Adding a blackbody improved the fits (reduced
for
1096 d.o.f., and
for 967 d.o.f.), but structured
residuals at low-energies remain.
Using a disk-blackbody component instead of the
blackbody resulted in better fits, with reduced
for
1096 d.o.f., and
for 967 d.o.f. for the first and
second observations, respectively.
The absorption resulting from these fits was always higher than
that derived from the optical reddening to NGC 6712. As well as the
interstellar absorption in the direction of the globular cluster,
modeled with PHABS with
fixed at
21 cm-2, we added another multiplicative
component to investigate whether an ionized absorber could be
present ( ABSORI model in XSPEC). We fixed the Fe
abundance of the absorber to the NGC 6712 value (
),
and linked the photon index of the ionizing continuum to that of
the power-law component. The fit resulted in an un-ionized cold
absorbing medium, with a best-fit ionization
in the
range 0.5-2 (where L is the ionizing luminosity, n the
density of the absorbing medium, and R is the distance of the
obscuring material to the source). Thus, we do not consider
further the presence of any ionized absorber.
We next tried partial covering ( PCFABS model in
XSPEC), absorbing both the disk-blackbody and power-law continuum
components. We included a PHABS component with fixed at
cm-2 to account for the
interstellar absorption. The fit resulted in additional absorption
in the range 6-
cm-2 for the two observations,
with a covering factor
95% for reduced
for
1095 d.o.f., and
for 966 d.o.f. for the first and
the second observations, respectively. The disk-blackbody
temperatures were 0.6 and 0.3 keV (with the innermost radii of the
accretion disk
km
and
km) while
and
(with power-law normalizations of 0.060
+0.001
-0.008 and
photons keV-1 cm-2 s-1 at 1 keV, for the first and
the second observations, respectively).
In order to investigate the Ne/O abundance ratio, we used a
variable abundance absorption model ( VPHABS in XSPEC),
with the elemental abundances set to the ISM values of Wilms et al. (2000) except for those of O, Ne and Fe
which were fixed to zero. Their absorption effect has been replaced
with three edges (O-K, Fe-L, Ne-K edges) with energies fixed at 0.54, 0.71 and 0.87 keV, and edge depths allowed to vary.
In this way we could also account for a local iron abundance likely
different from the cosmic value.
We fit the
spectra with 3 different two-components models for the continuum:
(1) a disk-blackbody ( DISKBB in XSPEC, Mitsuda et al. 1984) and a power-law, (2) a blackbody and a power-law,
and (3) a disk-blackbody and a high-energy Comptonized component
( COMPTT in XSPEC, Titarchuk 1994). This latter
model was considered since it has been successfully fit to almost
all the broad-band spectra of the galactic globular cluster
LMXBs (Sidoli et al. 2001). Since the BeppoSAX best-fit
electron temperature,
,
of
70 keV is well above
the XMM-Newton upper energy threshold, it was fixed to 70 keV for
the fits performed here.
All 3 continuum models fit well and the best-fit continuum
parameters are given in Table 2. All 3 models give
similar 0.5-10 keV source luminosities of
erg s-1 and
erg s-1 for a
distance of 8 kpc for the first and second observations,
respectively. Table 3 gives the measured Ne/O abundance ratios. These are all similar to the ISM value of 0.18
of Wilms et al. (2001). The equivalent column densities due to O,
Ne and Fe were then estimated by modeling the absorption as
occurring from 3 edges with energies fixed at 0.54, 0.71 and
0.87 keV together with a narrow absorption line at an energy of
0.53 keV (to account for O I ISM absorption). The resulting edge
depths, columns and equivalent hydrogen columns are listed in
Table 3 and shown in Fig. 4.
In Fig. 5 we show the best fit spectra
during the two observations.
As a final test, we tried fitting the spectra with the ASCA model,
with the Ne and O abundances fixed at the ASCA best-fit values
(Juett et al. 2001), letting all
the other parameters vary in the usual way. We obtained
unacceptable fits with reduced
for 1096 d.o.f., and
for 967 d.o.f. with structured residuals evident
below 1 keV (see Fig. 6).
Table 2:
Best-fit continuum parameters when the RGS1, RGS2, pn,
MOS1 and MOS2 4U 1850-087 spectra were fit simultaneously.
dbb = disk-blackbody, pow = power-law, ctt = Comptonization
model COMPTT in XSPEC.
is the
power-law photon index,
the inner disk temperature,
is the inner disk radius for a
distance of 8 kpc, i is the disk inclination angle and
is the blackbody radius. For the COMPTT model,
is the temperature of the "seed'' photons,
is the electron temperature (fixed at 70 keV) and
is the plasma optical depth.
Table 3:
Photoelectric absorption results towards 4U 1850-087 (see
Table 2 for the continuum emission parameters). The
edge energies were fixed at 0.54, 0.71 and 0.87 keV for O, Fe and
Ne, respectively.
is the absorption depth.
is the element column density (in units of
1017 cm-2) calculated using the Henke et al. (1993)
cross sections.
is the hydrogen column density implied
by
,
in units of 1021 cm-2, assuming the ISM
abundances of Wilms et al. (2000).
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Figure 4:
The hydrogen column densities (in units of
1021 cm-2) implied by the optical depths of the Ne, O and Fe
edges, from the first ( left) and second observations ( right),
depending on the different models assumed for the continua
(Table 3). Shadowed regions mark the 90% confidence
range for ![]() |
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Figure 5: The 0.3-12 keV 4U 1850-087 count spectra (together with the residuals in units of standard deviation) from the two observations (left panel shows the first observation, the right panel the second). The continua have been modeled with a disk-blackbody and a power-law (see Tables 2 and 3 for the continuum and line parameters, respectively). |
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Figure 6: The 0.3-12 keV 4U 1850-087 count spectra (together with the residuals in units of standard deviations) from the two observations (left panel shows the first observation, the right panel the second), fit with the ASCA model with the Ne and O abundances fixed at the values reported in Juett et al. (2001). |
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We report the results of two XMM-Newton observations of 4U 1850-087 performed
about 12 days apart. The spectra require a soft emission
component, which is slightly better described by a multi-color
disk-blackbody than a blackbody. At higher energies, a power-law
provides a good fit the spectra. The photon index is similar to
that measured during the ASCA observation (Juett et al. 2001), while it is significantly softer than during the
later Chandra observation (Juett & Chakrabarty 2005).
This may be due to the fact that in fitting the Chandra LETGS
spectrum the low-energy absorption was fixed to the ISM value
towards the globular cluster. We note, however, that the source
luminosities during the XMM-Newton and Chandra observations were
similar, whilst during the ASCA observation the source was almost
a factor 2 brighter with a 0.5-10 keV luminosity of
erg s-1.
The total low-energy absorption resulting from the fits is similar
in both the XMM-Newton observations and is 4-
21 cm-2, depending on the model adopted for the continuum. There is
evidence for extra-absorption in the line of sight, since the
best-fit total
is always significantly higher than the
optically derived value in the direction of the host globular
cluster of (
21 cm-2. Thus the intrinsic
absorption ranges from 2 to
21 cm-2, depending
on the continuum model assumed. In the Comptonization model a
lower column density is required because of the turnover at low
energies present in the model. The presence of neutral
extra-absorption local to the source is also confirmed by a good
fit when using the partial covering fraction absorption model,
which indicate that the central source is absorbed by a neutral
medium with a covering factor of
95% and an intrinsic
hydrogen column density in the range 6-
21 cm-2 for the two observations.
We adopted a variable absorption model together with three edges
(O-K, Ne-K and Fe-L) in order to measure the column density of the
Ne, O and Fe in the line of sight, since the ASCA spectrum suggests
an excess absorption of neutral Ne-rich material local to the
source (Juett et al. 2001). Other ultra-compact X-ray
binaries (
h) display over-abundances of neutral
Ne from the absorption effects in ASCA spectra. Moreover, during
XMM-Newton and Chandra observations an anomalously high Ne/O
abundance ratio has been observed in a a number of other
ultra-compact binaries, indicative of neutral Ne overabundance
(e.g., 4U 0614+091, Paerels et al. 2001; 4U 1543-624
and 2S 0918-549, Juett & Chakrabarty 2003). Thus, it has
been proposed that the donor stars in some ultra-compact binary systems
are Ne-rich white dwarfs (e.g., Yungelson et al. 2002;
Bildsten 2002).
The high resolution RGS 4U 1850-087 spectra presented here do not show any prominent emission features or other absorption edges, besides those of O-K, Ne-K and Fe-L. Our study of the absorbing Ne and O toward 4U 1850-087 reveals an Ne/O abundance ratio (see Table 3) which is consistent with the ISM value of 0.18 (Wilms et al. 2000). This result is in agreement with the Chandra observation performed in 2002 (Juett & Chakrabarty, 2005) but contrary to the earlier ASCA measurement (Juett et al. 2001).
If the measured elemental column densities
(see
Table 3) are converted to equivalent H column
densities using the Wilms et al. (2000) ISM abundances, we
can compare the
resulting from the overall shape of
the XMM-Newton spectra (dashed regions in Fig. 4 include the 90% uncertainties on the
resulting from the fit). For
each continuum model, for both observations, there seems to be a
discrepancy between the total
and the equivalent
calculated from the elemental column densities
.
This
could indicate an over-abundance of Ne and O and a sub-solar
abundance of Fe. This under-abundance may be explained by the low
Fe abundance of the host globular cluster. Alternatively, it is
possible that the uncertainties on the derived hydrogen column
densities could be underestimated. Indeed, Paerels et al. (2000)
point out that the photoelectric cross-sections could have a 30%
uncertainty which is large enough to account for this discrepancy.
There is no evidence for any temporal variability of the Ne, O and
Fe column densities between the two XMM-Newton observations (within 90% uncertainty), although we note that the column densities are
systematically higher in the second observation. Also the Ne/O
ratio, although always compatible within uncertainties with the
standard ISM value, is on average higher during the second
observation. The 4U 1850-087 spectrum during the two XMM-Newton observations
differs in the total 0.5-10 keV luminosity (see
Table 2) with the second observation being 10%
more luminous than the first. This suggests a possible correlation
between the Ne/O abundance ratio and the X-ray source luminosity,
and a possible explanation for the different Ne/O ratios observed
with XMM-Newton, Chandra, and ASCA. Note that during the ASCA
observation, where evidence for a strong Ne overabundance was
reported, the source luminosity was almost twice that during the
XMM-Newton and Chandra observations.
Juett & Chakrabarty (2003) proposed that ionization could
play a role in the variable abundance ratios.
We suggest another
possible mechanism which could help in the understanding of the
local extra-absorption and its metal abundance. Maccarone et al. (2004) studied the irradiation-induced stellar winds in
X-ray binaries in order to explain different X-ray spectra from
LMXBs located in globular clusters with different metallicities.
An evaporative wind can be produced even in LMXBs with degenerate
companions, where part of the radiation produced from the central
source illuminates the donor star (e.g., Ruderman et al. 1989). This wind could contribute to the observed
column density toward the X-ray source, leading to an intrinsic
column density of
cm-2 (see Eq. (7) in
Maccarone et al. 2004). During the XMM-Newton observations we
found a comparable amount of extra-absorption towards 4U 1850-087. We
suggest that a wind evaporated from the degenerate companion could
be responsible for the intrinsic absorption observed. Using the
4U 1850-087 observed parameters, and Eq. (7) of Maccarone et al. (2004), we derive a wind velocity of
cm s-1, which may be confined to the
binary (the escape velocity is
108 cm s-1). A
higher source luminosity would translate into a larger
contribution by the wind from the degenerate donor, which is
likely to be rich in Ne and O. We suggest that this mechanism
could contribute to the different abundance ratio observed from
4U 1850-087 with XMM-Newton, Chandra, and ASCA. This could possibly help in
explaining why during higher source luminosity intervals, higher
Ne/O abundance ratios are observed. We note that another
ultra-compact X-ray binary, 4U 1543-624, displays a variable
Ne/O abundance ratio. Juett & Chakrabarty (2003) measured an Ne/O
abundance ratio of
with Chandra, and
with XMM-Newton. The higher Ne/O abundance ratio was
observed when the source was more luminous, as appears to be the
case with 4U 1850-087.
Theoretical models for the formation of white dwarfs predict a Ne/O abundance ratio in the range 0.2-0.4 (e.g., Deloye & Bildsten 2002; Segretain et al. 1994; Gutierrez et al. 1996), which is low compared with the Ne/O ratios observed with ASCA in 4U 1850-087 (or with Chandra in 4U 1543-624). On the other hand, Yungelson et al. (2002), studying the formation of Ne-enriched donors in ultracompact X-ray binaries, point out that the abundance of neon in the nucleus of the dwarf may be underestimated by a factor of 3 (Isern et al. 1991), which makes the theoretically predicted Ne/O ratios agree better with the higher observed values.
Acknowledgements
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).