A&A 461, L9-L12 (2007)
DOI: 10.1051/0004-6361:20066527
LETTER TO THE EDITOR
S. Carpano1 - A. M. T. Pollock1 - J. Wilms2 - M. Ehle1 - M. Schirmer3
1 - XMM-Newton Science Operations Centre, ESAC, European
Space Agency, Apartado 50727, 28080 Madrid, Spain
2 -
Dr. Remeis Sternwarte, Astronomisches Institut der FAU Erlangen-Nürnberg,
Sternwartstr. 7, 96049 Bamberg, Germany
3 -
Isaac Newton Group of Telescopes, 38700 Santa Cruz de La Palma, Spain
Received 9 October 2006 / Accepted 6 November 2006
Abstract
Context. Wolf Rayet/black hole binaries are believed to exist as a later evolutionary product of high-mass X-ray binaries. Hundreds of such binaries may exist in galaxies, but only a few of them are close enough to be observed as X-ray binaries. Only a couple of candidates have been reported so far.
Aims. Based on XMM-Newton observations, we report the positional coincidence of the brightest X-ray source in NGC 300 (NGC 300 X-1) with a Wolf-Rayet candidate. Temporal and spectral analysis of the X-ray source is performed.
Methods. We determine an accurate X-ray position of the object, and derive light curves, spectra and flux in four XMM-Newton observations.
Results. The positions of the X-ray source and the helium star candidate coincide within
.
The X-ray light curves show irregular variability. During one XMM-Newton observation, the flux increased by about a factor of ten in 10 h. The spectrum can be modelled by a power-law with
with additional relatively weak line emission, notably around 0.95 keV. The mean observed (absorbed) luminosity in the 0.2-10 keV band is
2
.
Conclusions. NGC 300 X-1 is a good candidate for a Wolf-Rayet/black-hole X-ray binary: its position coincides with a Wolf-Rayet candidate and the unabsorbed X-ray luminosity reached
,
suggesting the presence of a black hole.
Key words: galaxies: individual: NGC 300 - X-rays: binaries - stars: Wolf-Rayet
A helium-star/compact-object X-ray binary was suggested by
van den Heuvel & de Loore (1973) and van den Heuvel (1983) to explain the
nature of Cyg X-3, and discussed further by
van Kerkwijk (1995). Such systems are believed to be a late
evolutionary product of high-mass X-ray binaries. HMXBs are composed
of an early-type star and a neutron-star or black-hole (BH) compact object.
Material from the strong wind of the star is accreted onto the compact
object with the emission of X-ray radiation. When the secondary star
exhausts its core-hydrogen, it expands and begins to overflow its
Roche-Lobe. A common envelope forms and the compact object starts to
spiral in. If the hydrogen envelope is expelled before the compact
object has reached the helium core, a stable binary system with a short orbital period is formed. Ergma & Yungelson (1998) showed, by means of
population synthesis, that a few hundred such black holes with helium
star companions may form in our galaxy. The orbital periods of the
majority exceed 10 h, with a maximum at 100 h. However,
following Illarionov & Sunyaev (1975), the formation of accretion disks from
the strong stellar wind of a Wolf-Rayet (WR) star is possible only for
short orbital periods of (<10 h). The number of such systems is
expected to be extremely small (Ergma & Yungelson 1998).
In the Galaxy, Cyg X-3 is a good candidate for such a binary system.
This X-ray source is one of the most luminous in the Galaxy with
erg s-1 and has a Wolf-Rayet companion
star designated WR 145a (van Kerkwijk et al. 1992). A second candidate,
IC 10 X-1 (
1038 erg s-1) has been
observed in the starburst galaxy IC 10
(Bauer & Brandt 2004; Wang et al. 2005). Although two other much less luminous
candidates in the LMC were suggested by Wang (1995) from ROSAT observations, Portegies Zwart et al. (2002) and Townsley et al. (2006) concluded,
partially on the basis of more recent Chandra data, that these two objects are more likely to be colliding-wind binaries. In this paper
we report the discovery of a third high-luminosity candidate for a WR/BH
X-ray binary in the galaxy NGC 300.
NGC 300 is a dwarf spiral galaxy, belonging to the Sculptor galaxy
group, located at a distance of 1.88 Mpc (Gieren et al. 2005). Because it is
almost face-on oriented and has a low Galactic column density of
(Dickey & Lockman 1990), it
is well suited for studies of stellar content. Based on deep VLT-FORS
narrow-band imaging, Schild et al. (2003) detected 58 WR star
candidates in the central region of the galaxy, of which 16 were
already spectroscopically confirmed (Breysacher et al. 1997; Schild & Testor 1991).
Their technique consisted of taking images through two interference
filters with central wavelengths at
,
containing WR emission lines, and at
as a continuum reference, with band
widths of
and
,
respectively. WR candidates were
selected as having peak intensities of at least
in the
difference (
-
)
image.
The X-ray source population of NGC 300 has been studied with ROSAT (Read & Pietsch 2001) and XMM-Newton (Carpano et al. 2005). With mean observed
luminosities of
(Read & Pietsch 2001) and
(Carpano et al. 2005), the brightest X-ray source has been
suggested to be a black hole of about
.
In this Letter, we
report the positional coincidence of this source with one of the
WR candidates reported by Schild et al. (2003). We refer to the
X-ray source as NGC 300 X-1 and to the Wolf-Rayet star as WR-41
following Schild et al. (2003). The remainder of the Letter is
organised as follows. Section 2 briefly describes the XMM-Newton observations and data reduction. In Sect. 3, we report
the spatial coincidence of NGC 300 X-1 and the WR 41. Temporal and
spectral analysis of the XMM-NewtonX-ray source is shown in
Sect. 4, while a discussion of our results is given in
Sect. 5.
To date, NGC 300 has been observed four times with XMM-Newton on 2000 December 26 for 32 ks EPIC-pn time; on 2001 January 01 for 40 ks; on 2005 May 22 for 35 ks; and on 2005 November 25 for 35 ks during revolutions 0192, 0195, 0998 and 1092, respectively. For each observation, the EPIC-MOS (Turner et al. 2001) and EPIC-pn (Strüder et al. 2001) cameras were operated in full frame mode with the medium filter. The data reduction was identical to that performed in our analysis of the previous XMM-Newton observations (Carpano et al. 2006,2005), except that version 7.0.0 of the XMM-Newton Science Analysis System (SAS) and the most recent calibration files were used.
High fluxes of proton flares were observed during revolutions 0192 and 0998. After screening the MOS data for flares using the standard
procedures described by the XMM-Newton team,
a total of 30 ks of low background data remained in each
revolution. A more detailed description of the XMM-Newton data reduction
can be found in Carpano et al. (2005) for the first two XMM-Newton observations
and Carpano et al. (2006) for the last two.
An accurate XMM-Newton X-ray position of NGC 300 X-1 was derived considering both statistical and systematic errors. Statistical errors depend on the brightness of the source. For a source as bright as NGC 300 X-1, they are small compared with the systematic errors in the X-ray reference frames of each observation. Although merging all four XMM-Newton observations would in principle reduce the statistical errors in the absence of systematic errors, we chose to use only the data of revolution 0195, where the source was bright and well separated from local instrumental features, such as chip gaps, which affected the other observations to some extent.
Initial estimates of the position of NGC 300 X-1 and its statistical
error were derived using the SAS edetect_chain task,
which performs maximum-likelihood source detection. The source was
detected with a likelihood L of 44833. Probabilities P, are
related to maximum likelihood values L, by the relation
.
The statistical position error is of
(or
with
the merged data). The
systematic errors were tackled with the SAS task eposcorr,
which correlates X-ray source positions with those of their optical
counterparts, as explained by Carpano et al. (2005). For this, we used 12 sources
inside the galaxy disk that have clear optical counterparts.
The systematic shift
(in the sense X-ray minus optical position) was
in right ascension and
in declination.
The revised coordinates of NGC 300 X-1 are therefore
and
,
with an uncertainty
of
.
![]() |
Figure 1:
15'' ![]() ![]() |
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Figure 1 shows the optical (B-band) image of the field
taken with the 2.2 m MPG/ESO telescope on La Silla. See
Carpano et al. (2005) and Schirmer et al. (2003) for a description of the
optical data and their reduction. The circle represents the 2 uncertainty of the X-ray position, while the box is centred on the
position of WR-41, derived by Schild et al. (2003). We redetermined the
coordinates of the Wolf-Rayet star in the broad B-band image, using the IDL find
routine, which is part of the idlphot photometry
library
implemented from an early Fortran version of the DAOPHOT aperture
photometry package (Stetson 1987). The revised coordinates of
WR-41 are
and
.
The absolute astrometric
accuracy of the optical image is about
.
The difference between the position of NGC 300 X-1
and the revised position of WR-41 is thus
(or
using the position of Schild et al. (2003)). The spatial coincidence is
comparable to that found for IC 10 X-1,
(
,
Bauer & Brandt 2004), which lies at a distance of 0.8 Mpc.
Figure 2 shows the XMM-Newton light curves of NGC 300 X-1
separately for revolutions 0192, 0195, 0998 and 1092, incorporating
background-corrected MOS and pn data. The gaps in the data of
revolutions 0192 and 0998 correspond to periods of high soft-energy
proton flux. The time bin size is 300 s. The light curve of
NGC 300 X-1 is modulated by short-term (few 1000 s), large-amplitude
(factors of 4-5) variations in all four observations. The same
type of short- and long-term variations has been reported for IC 10 X-1
(Bauer & Brandt 2004; Wang et al. 2005). During revolution 0195, the flux increased
by a factor of 10 within 10 h, and the highest observed luminosity,
assuming isotropic emission, was
.
The corresponding unabsorbed luminosity
is
1
.
Using periodograms
and epoch-folding, no short-time periodic signal was found on
timescales between 5 s and 30 ks.
![]() |
Figure 2: EPIC-pn and -MOS 0.2-10.0 keV light curve of NGC 300 X-1 in revolutions 0192, 0195, 0998 and 1092 from top to bottom. Periods of high background have been excluded from the data. The horizontal lines show the mean values. Times start from the beginning of each observation. |
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Figure 3 shows the pn and MOS spectra of NGC 300 X-1. For
clarity, we have plotted the total spectra from the combination of all
four observations. We first tried to fit single power-law models to
the data, where parameters were left free in each observation, yielding
.
From the residuals shown at the bottom of
Fig. 3, some excess around 0.95 keV is evident. Adding
a Gaussian line significantly improved the fit,
for
degrees of freedom. The
best-fitting parameters of the absorbed combined power-law and single
Gaussian-line model are shown in Table 1 for each
observation separately.
is the
equivalent column density of neutral hydrogen,
the photon
index,
the energy of the line and
its width, and
and
are the normalisation constants for the
Gaussian line and the power-law component, respectively. The
corresponding 0.2-10 keV flux, absorbed and
unabsorbed luminosities are shown in the last three
rows. Uncertainties are given at 90% confidence level.
Allowing a variable photon index between
observations does not improve the fit or change spectral parameters
within the errors except for revolution 0998, where
and
.
The mean observed luminosity increased by more than a factor of two
from the lowest state in revolution 0192 to the highest in revolution 1092. When the source was brighter, the intrinsic absorption, as well
as the emission line/continuum ratio, increased. In revolution 0192,
the equivalent hydrogen column, ,
was compatible with the
galactic value (
),
implying little or no intrinsic absorption in the low state.
We also tried to fit the data using a power-law model combined with thermal
emission from a collisionally ionized plasma modelled by apec
in XSPEC (version 11.3.2). This thermal emission includes both lines and continuum
emissivities with a best-fit temperature of 0.86
+0.03-0.04 keV. This
model is statistically indistinguishable from the power-law plus
Gaussian line model.
![]() |
Figure 3:
EPIC-pn (black) and -MOS (red and green) spectra of NGC 300 X-1 added for the four XMM-Newton observations. Thespectrum is fitted with an absorbed power-law model. Bottom: residuals expressed in ![]() |
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Table 1: Results of the spectral fits for NGC 300 X-1, using an absorbed power-law model and a Gaussian line (phabs*(gauss+power), in XSPEC).
WR-41 is a Wolf-Rayet candidate discovered by Schild et al. (2003). Using
narrow band imaging techniques, they measured that
the flux of the source at
,
where strong WR emission lines are observed, has an excess of 0.95 mag over the
continuum. Its relative magnitude is
,
which corresponds, at a distance of
1.88 Mpc, to an absolute magnitude of
.
Following Schild et al. (2003), objects such as WR-41 are
candidates for weak-line single WN-type stars.
We have shown in this Letter that the brightest X-ray source in the nearby galaxy NGC 300, NGC 300 X-1, is likely to be the X-ray counterpart of WR-41. This would imply that the system is a good candidate for membership in the rare class of WR/BH X-ray binaries. Irregular flux variations were observed in the four light curves, with a particularly large increase observed over about 10 h during XMM-Newton revolution 0195. The origin of these variations is not clear, but likely due to variations in the accretion rate and/or variable absorption in the strong stellar wind of the donor star. We also note that no obvious eclipse is visible in any of the light curves. As WR/BH X-ray binaries should have small orbital periods <10 h, we interpret the absence of eclipses as a possible indication of face-on orientation of the system.
In the context of a WR/BH X-ray binary, the weak discrete emission
observed in the spectra of NGC 300-X-1, which is likely to be composed
of several unresolved lines, may arise from reprocessing by the
photoionized stellar wind: the X-rays emitted around the black hole
are ionizing the high density wind of the Wolf-Rayet star. This may
explain why this emission is more pronounced when the X-ray flux is higher.
The dense wind is probably also responsible for the intrinsic
absorption that increases as the X-ray source gets brighter. Similar
ideas explain the weak emission lines observed in Cyg X-3
(e.g., Kawashima & Kitamoto 1996; Paerels et al. 2000) and in other wind accretors.
Faint lines have also been observed in the X-ray spectrum of the
candidate IC 10 X-1 (Bauer & Brandt 2004). We note that the contribution
of the WR star to the X-ray flux is negligible as typical
X-ray luminosities are about
for single stars
and
for binaries
(e.g., Pollock 1987).
To conclude, NGC 300 X-1 and WR-41 are good candidates for membership
in the very rare class of Wolf-Rayet/black hole X-ray binaries. This
conclusion is supported by the spatial coincidence of the sources as
well as the high maximum X-ray luminosity near
.
As the nature of WR-41
currently relies on narrow-band photometric techniques, we encourage
optical observers to acquire the spectra necessary to confirm the
identification of WR-41 as a Wolf-Rayet star.
Acknowledgements
This paper is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly finded by ESA Member States and NASA, and on observations made with ESO Telescopes at the La Silla observatory and retrieved from the ESO archive.