A&A 403, 901-916 (2003)
DOI: 10.1051/0004-6361:20030420
M. Sasaki
- W. Pietsch
- F. Haberl
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, Postfach 1312, 85741 Garching, Germany
Received 11 December 2002 / Accepted 17 March 2003
Abstract
Based on XMM-Newton EPIC data of four pointings towards the
Small Magellanic Cloud (SMC), results on timing and spectral analyses of 16
known high mass X-ray binaries (HMXBs) and HMXB candidates in the SMC are
presented. We confirm the pulse periods of four sources which were known to
show pulsations. In addition, two new X-ray pulsars are discovered:
XMMU J005605.2-722200 with
s and
RX J0057.8-7207 with
s.
Due to the low Galactic foreground absorption, X-ray binary systems in the
Magellanic Clouds are well suited for studies of
the soft component in their X-ray spectrum.
Spectral analysis reveals soft emission besides a power law component in the
spectra of three sources. The existence of
emission lines in at least one of them
corroborates
the thermal nature of this emission with temperatures of 0.2-0.3 keV
and heavy element abundances lower than solar.
For the HMXB SMC X-2 which was in a low luminosity state,
we determine a flux upper limit of
erg cm-2 s-1 (0.3-10.0 keV).
Furthermore, two new sources (XMMU J005735.7-721932 and
XMMU J010030.2-722035) with hard spectrum and emission line objects
as likely optical counterparts are proposed as new X-ray binary candidates.
Key words: X-rays: galaxies - X-rays: binaries - stars: neutron - Magellanic Clouds
After the discovery of X-ray emission from the Magellanic Clouds (MCs) in 1970 (Price et al. 1971), surveying observations of each MC were performed by different X-ray observatories. As for the Small Magellanic Cloud (SMC), source catalogues were created from observations with Einstein (Seward & Mitchell 1981; Bruhweiler et al. 1987; Wang & Wu 1992), ROSAT (Kahabka et al. 1999; Haberl et al. 2000; Sasaki et al. 2000), and ASCA (Yokogawa et al. 2000).
The analysis of these X-ray sources has shown, that a large number of X-ray bright objects belongs to the class of X-ray binaries (XRBs) in which a neutron star or a black hole forms a binary system with a companion star. In these systems, mass is accreted from the donor star onto the compact object. X-ray binaries can be divided into low mass X-ray binaries and high mass (or massive) X-ray binaries (HMXBs), depending on the mass of the companion star. Therefore, the identification of optical counterparts of the X-ray sources is crucial for the understanding of the nature of these sources. Furthermore, HMXBs form two subgroups with either an OB supergiant or a Be star as donor. A detailed catalogue of HMXBs was compiled by Liu et al. (2000). Negueruela & Coe (2002) performed high resolution spectroscopy of optical counterparts of HMXBs in the Large Magellanic Cloud (LMC) and studied the population of HMXBs. In the Milky Way or in the LMC, the fraction of Be/X-ray binary systems (Be/XRB) is 60-70% of all HMXBs, whereas more than 90% of the HMXBs in the SMC turned out to be Be systems (Haberl & Sasaki 2000, and references therein).
Since pulsed X-ray emission can be observed from neutron star HMXBs, these sources are also called X-ray binary pulsars. Based on ASCA, RXTE, ROSAT and Beppo SAX observations, more than 20 X-ray binary pulsars have been discovered in the SMC so far (Haberl & Sasaki 2000; Yokogawa et al. 2000, and references therein). Moreover, in one of the first observations of XMM-Newton (Jansen et al. 2001), pulsed emission from another HMXB was found, which was identified with a Be star (Sasaki et al. 2001).
In order to improve our understanding of the X-ray source population in the SMC, we proposed and analysed pointed observations of the SMC by XMM-Newton and performed spectral and temporal studies of detected sources. In this paper, we focus on the class of HMXBs and present the results on each HMXB and candidate in the observed fields.
Table 1: XMM-Newton data used for the analysis.
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Figure 1: DSS image of the SMC and the position of the EPIC field of view of the XMM-Newton observations listed in Table 1. The discontinuity seen in the DSS image is an artifact. |
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For AO-1 of XMM-Newton, we proposed observations of eight fields in the SMC in order to study the X-ray binary population (PI: W.P.). Two observations of this proposal were performed. During the first observation (ID 00842008), the telescope pointed towards the HMXB SMC X-2 in the south of the main galaxy. In order to perform source detection and analysis in the whole field of view, we used data from the European Photon Imaging Cameras EPIC PN (Strüder et al. 2001), EPIC MOS1, and EPIC MOS2 (Turner et al. 2001). The observation was performed with all the EPIC cameras in full frame mode. For EPIC PN the thin filter was used, whereas for the EPIC MOS cameras medium filters were chosen. The next observation, ID 00842001, covered a region in the north of the SMC. The CCD read out modes and the filters of the EPIC cameras were the same as in the first observation.
Moreover, we searched the XMM-Newton Science Data Archive for public data of
the SMC, suitable for our purposes. We found two data sets (ID 01100002 and 01357206) of fields in the north of the SMC, slightly
overlapping with each other as well as with the pointing ID 00842001.
The details of the observations are summarised in Table 1.
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Figure 2: HR1 plotted over HR2 and HR2 over HR3 with errors for all the sources in Table 2 except for SMC X-2. Circle is used to mark the source No. 5 which is an AGN, and triangle for source No. 11 which is either an XRB or an AGN. |
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All the data were processed with the XMM-Newton Science Analysis System
(SAS) version 5.3.3.
For source detection, the events were separated into four energy bands:
B1 = 0.3-1.0 keV, B2 = 1.0-2.0 keV, B3 = 2.0-4.5 keV,
and B4 = 4.5-10.0 keV. In all these bands, images were created
and source detection was performed using the sliding window and maximum
likelihood methods of the SAS. Detections with likelihood of existence (ML) higher than 10.0 were accepted as real sources. This corresponds to
the probability
for the existence
of the source. Hardness ratios were computed using the source counts in different
bands:
The detected sources were cross-correlated with catalogues of Einstein sources (Wang & Wu 1992) as well as ROSAT sources detected by PSPC (Kahabka et al. 1999; Haberl et al. 2000) and by HRI (Sasaki et al. 2000) instruments. The positions of the X-ray sources were plotted on Digitized Sky Survey DSS2 (red) images of this field in order to find probable optical counterparts. The optical sources were also verified by cross-correlating the X-ray source list with the USNO-A2.0 catalogue produced by the United States Naval Observatory (Monet 1996, 1998). In addition, we compared the source positions to the entries in the Optical Gravitational Lensing Experiment OGLE-II project list of variable sources in the Magellanic Clouds (Zebrun et al. 2001). For five out of 15 sources, correlations with an OGLE object were found. Finally, the source list was cross-correlated with the list of emission line objects in the SMC (Meyssonnier & Azzopardi 1993, [MA93]). The existence of an emission line star at the position of a hard X-ray source indicates that the source might be an X-ray binary system with a Be-star companion.
The complete set of source lists will be presented in another paper.
Here, we shall concentrate on the HMXBs and candidates in the four fields.
The results on the eighteen sources are summarised in Table 2.
The table includes the X-ray
source coordinates, 1
positional error, count rate,
likelihood for
detection in the total band, flux, hardness ratios (see Eq. (1)),
pulse period with 1
error (Sect. 2.2), and
identifications.
Correlations with ROSAT sources, OGLE objects, and emission line objects
(Meyssonnier & Azzopardi 1993) can be found as well. The sources are sorted
by RA and Dec (J2000.0), and the entry numbers are used in the following.
The positional errors which are given in this table are statistical errors.
The systematic error of the X-ray position
is about 3
-4
(Barcons et al. 2002).
In order to calculate the flux, model parameters resulting from
the spectral analysis (see Sect. 2.2)
were used for all sources.
In the following, the source number in the ROSAT HRI catalogue of the SMC (Sasaki et al. 2000) is given as RH NNN, and the number in the ROSAT PSPC catalogue (Haberl et al. 2000) as RP NNN. The entry number in the list of Haberl & Sasaki (2000, [HS2000]) is also mentioned using the format [HS2000] NN (also see Table 2).
Table 2: Detected HMXBs and candidates in the fields of the SMC observed by XMM-Newton.
Table 2: continued.
Table 2: continued.
Table 3: Spectral parameters for sufficiently bright sources.
As the very first step of data analysis, we checked the EPIC PN data for incorrect time information. It has been reported that in some cases, there are time jumps of 1 s in the EPIC PN data which were not corrected in the SAS processing. Since we didn't find any event which indicated such a time jump, we could proceed without any countermeasure.
After selecting the events for each source, they were analysed using the XANADU software package distributed by the High Energy Astrophysics Science Archive Research Center (HEASARC). It contains the packages XRONOS for timing analysis and XSPEC for spectral fitting.
Based on EPIC PN data, period search was carried out with XRONOS after
correcting the photon arrival times for solar system barycentre.
If a peak
was found in the power spectrum indicating pulsations, a more detailed epoch
folding search was performed around the preliminary value.
Once we got the rough value for the pulse period, the distribution around this value was fitted with a Lorentz profile and the
maximum of the Lorentz profile
was determined together with the 1
error. Finally, folded light
curves were created in three energy bands: B1 = 0.3-1.0 keV,
B2 = 1.0-2.0 keV, B3+4 = 2.0-10.0 keV.
In addition, the ratio of the count rates in the harder band to the softer
band (B2/B1 and B3+4/B1+2, with B1+2 = 0.3-2.0 keV)
were computed to illustrate the changes in the hardness ratios with pulse
phase. Note that these hardness ratios are different to the numbers defined
in Eq. (1).
Except for sources which were too faint, spectra were extracted for each
source. These spectra were modelled with a power law component together with
the fixed Galactic foreground absorbing column density of
cm-2 (Dickey & Lockman 1990)
and a free column density
:
From the spectral models for the emission we were able to estimate the flux
of the sources in the ROSAT band (0.1-2.4 keV).
The flux was calculated from the fitted models, except for
four sources which were too faint:
For No. 02, a power law spectrum with
and
cm-2(Yokogawa et al. 2001) was assumed to estimate the flux upper limit.
For Nos. 01, 09, and 12,
and
cm-2 were adopted.
The resulting luminosity
was used to create a long term light curve of all the ROSAT and the new
XMM-Newton data. In the light curves, crosses are
used for ROSAT PSPC data, triangles for ROSAT HRI data, and dots for XMM-Newton
EPIC data. Upper limits determined from ROSAT observations are plotted as
arrows. For the distance to the SMC, a mean value of 60 kpc was
assumed (see review by van den Bergh 1999).
In this section, we present the results on individual sources. All sources, which were detected in the four data sets and were proven to be HMXBs or candidates, are listed in Table 2.
RX J0051.7-7341 which has been suggested as an XRB candidate by
Kahabka et al. (1999) was only detected in MOS1/2 data. In the PN data
the source was located on a bad column. It is faint,
so neither spectral nor timing analysis was performed for this source.
The PSPC count rate of
s-1
(Kahabka et al. 1999) during the ROSAT observation corresponds to
XMM-Newton MOS (medium filter) count rate of about
s-1. This means that the
luminosities of the source during the ROSAT and XMM-Newton observations
were comparable (see Table 2).
SMC X-2 was one of the first three X-ray sources which were
discovered in the SMC (Clark et al. 1978). It was also detected in the
HEAO 1 A-2 experiment (Marshall et al. 1979), but not in the Einstein IPC
survey (Seward & Mitchell 1981). In ROSAT observations, this transient
source was detected only once (Kahabka & Pietsch 1996). It is thought to be
a Be/XRB, since a Be-star was found as its optical counterpart
(Murdin et al. 1979). In early 2000, the RXTE All-Sky Monitor detected
an outburst at the position of SMC X-2 (Corbet et al. 2001)
and a pulse period of 2.374
0.007 s was determined
(Corbet & Marshall 2000; Torii et al. 2000).
In the XMM-Newton data (Obs. ID 00842008), there was no detection with
ML > 10 (see Sect. 2.1)
at the position of SMC X-2 which was apparently in low luminosity
state during the XMM-Newton observation. Therefore, we performed source
detection using the maximum likelihood routine at the position of the
optical counterpart (SIMBAD):
RA = 00 54
33.4
,
Dec = -73
41
04
(J2000.0).
Since we set the ML limit lower, the source was detected with a
likelihood of ML = 3.4.
The 3
upper limit count rate obtained from the ML source detection
routine is
s-1.
The source counts were highest in the B3
band (2.0-4.5 keV).
In order to estimate the flux upper limit, spectral parameters derived by
Yokogawa et al. (2001) from the ASCA spectrum during the outburst were
used:
Photon index
for a power law spectrum absorbed by a column
density of
cm-2. This results in an
upper limit for the
un-absorbed flux of
erg cm-2 s-1, corresponding
to
erg s-1(0.3-10.0 keV) during the XMM-Newton observation in Oct. 2001.
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Figure 3: Folded light curves and long term light curve of RX J0054.9-7226 (source No. 3). The hardness ratio is the ratio between the count rates in harder band and the count rates in softer band (Sect. 2.2). See text for the symbols used for the long term light curve. |
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RX J0054.9-7226 is known to be an X-ray binary pulsar with a pulse
period of 58.969
0.001 s
(Marshall et al. 1998; Santangelo et al. 1998) and is the only source in our
sample, for which the orbital period has been measured: 65 d
(Lochner et al. 1999).
In the timing analysis of the new XMM-Newton data, the pulse period was
verified to be 59.00
0.02 s. The folded light curves show variations
especially above 1.0 keV, and there is no significant change in hardness
ratios (Fig. 3). As can be seen in the long term light curve,
compared to ROSAT data, the source was observed in low luminosity state.
Due to the low flux, the statistics of the spectrum were not high enough and
the spectrum is thus
not discussed here. However, the results of the
spectral analysis was used to estimate the flux of the source
(see Table 2). The
optical counterpart, a Be-star, is identified with the variable star
OGLE 00545617-7226476 (Zebrun et al. 2001).
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Figure 4: Folded light curves of XMMU J005605.2-722200 (source No. 4). Hardness ratio as in Fig. 3. |
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The error circle of the Einstein source 2E 0054.4-7237
includes an emission line object.
Therefore, it was suggested as a Be/XRB candidate ([HS2000]).
In the XMM-Newton data, a source consistent with the position of the emission
line object was detected (XMMU J005605.2-722200) and pulsations
from this source was discovered. XMMU J005605.2-722200 is most
likely consistent with 2E 0054.4-7237.
The period is 140.1
0.3 s. As can be seen in Fig. 4,
the pulses in the soft band are narrower than in the harder band.
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Figure 5: Spectrum of 2E 0055.6-7241 (source No. 5). |
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2E 0055.6-7241 had been suggested as an XRB candidate by
Kahabka et al. (1999).
Timing analysis revealed no pulsations of the X-ray source.
Also on longer timescales no flux change was verified:
The ROSAT PSPC count rate was
s-1
(Kahabka et al. 1999), corresponding to a count rate of
s-1 for XMM-Newton
EPIC PN (thin1 filter). This value is
similar to the count rate of the XMM-Newton observation,
which is
s-1.
The X-ray spectrum is shown in Fig. 5. It has a photon
index of
,
which is higher than for
other sources of our sample, and highest absorbing column
density of
cm-2(also see Table 3).
A difference to other sources is
also seen in the hardness ratios, as the
source has a relatively high HR1 and lower values
of HR2 and HR3 (HR1 = +0.49
0.05, HR2 = -0.29
0.05,
HR3 = -0.36
0.08, also see Fig. 2).
The high absorption makes HR1 positive,
whereas HR2 and HR3 are negative
due to steeper power law spectrum.
On the DSS2 (red) image, there is a source at the X-ray position, which coincides with the variable object OGLE 00571981-72253375 (Zebrun et al. 2001) with B = 19.7 and R = 17.8 (USNO-A2.0 0150-00625436), i.e. B - R = 1.9. Sasaki et al. (2000) have shown, that all the HMXBs and candidates in the SMC HRI catalogue have 14 < R < 18 and -2 < B - R < 3, whereas e.g. AGNs have R > 16 and B - R > 0. Both the optical magnitudes and the X-ray spectra indicate that this source might as well be an AGN. Spectroscopy of the optical counterpart by Dobrzycki et al. (2003) showed that this object is a z = 0.15 quasar located behind the SMC.
The pulse period of AX J0058-720 was determined from the ASCA data as 280.4
0.3 s
(Yokogawa & Koyama 1998), which we confirmed in the XMM-Newton data:
281.1
0.2 s. It shows strong pulses in the softer bands and its
spectrum becomes harder during the "off'' time (Fig. 6).
The residuals of the power law fit (Table 3 and
Fig. 6) indicate the existence of an additional soft
component. The source has been suggested as a HMXB candidate due to the
likely optical counterpart, which is an emission line object ([HS2000]).
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Figure 6: Folded light curves, spectrum, and long term light curve (0.1-2.4 keV) of AX J0058-720 (source No. 7). Hardness ratio and symbols as in Fig. 3. |
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Figure 7: Folded light curves, spectrum, and long term light curve (0.1-2.4 keV) of RX J0057.8-7207 (source No. 8). Hardness ratio and symbols as in Fig. 3. |
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RX J0057.8-7207 is a HMXB candidate with an emission line object
suggested as a likely optical counterpart ([HS2000]). We discovered pulsations
in the new XMM-Newton data and derived
a pulse period of 152.34
0.05 s.
For this source, a period of 152.098
0.016 s was independently
found in Chandra data by Macomb et al. (2003).
The folded light curves in Fig. 7
show, that there are correlated flux variations in all bands with a
significant minimum at phase 0.4. Especially in the hard band, there is a
slow increase and fast decay. Therefore, the hardness ratio falls off at
phase 0.2 and increases slowly after phase 0.7.
The source spectrum is
well reproduced by a power law spectrum (see Table 3) with a
significant absorption within the SMC or the source itself.
As can be seen in the long term light curve, there was a weak flare observed by
ROSAT, whereas the XMM-Newton observation was performed in a low
luminosity state, 5.3 times
lower than the maximum observed by ROSAT.
The source corresponding to the optically identified
HMXB RX J0058.2-7231 is very faint, so that
no timing analysis could be performed. However, the hardness
ratios HR1, HR2, and HR3 indicate, that this source has a hard spectrum.
Its optical counterpart is a variable Be star in the SMC,
OGLE 00581258-7230485 (Zebrun et al. 2001).
From the ROSAT HRI count rate of
s-1
(Sasaki et al. 2000) we estimated the corresponding XMM-Newton
EPIC PN (thin1 filter) count rate:
s-1.
The source was about 3.6 times brighter when it was detected by ROSAT than
when it was observed by XMM-Newton.
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Figure 8: Spectrum and long term light curve (0.1-2.4 keV) of RX J0059.3-7223 (source No. 10). Symbols for the long term light curve as in Fig. 3. |
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RX J0059.3-7223 has been suggested as an XRB candidate by Kahabka et al. (1999). It was observed by XMM-Newton in two pointings. Its spectrum mainly consists of a power law component typical for a HMXB with additional features (Fig. 8). For this source no pulsations were detected. At its position, there is the variable star OGLE 00592103-7223171 (Zebrun et al. 2001), which is suggested as the optical counterpart. Its magnitudes are B = 17.4 and R = 14.6 (USNO-A2.0 0150-00660299), which gives B - R = 2.8. The R magnitude in particular is characteristic for a HMXB (see Sect. 3.5).
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Figure 9: Spectrum and long term light curve (0.1-2.4 keV) of RX J0100.2-7204 (source No. 11). Symbols for the long term light curve as in Fig. 3. |
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At the position of the XMM-Newton detection corresponding to
RX J0100.2-7204, a very faint object can
be found on the DSS2 (red) image.
However, there is no entry in the USNO-A2.0 catalogue for this source.
We also looked for information in different catalogues using BROWSE of
the HEASARC archive, but could not
find the magnitudes of this optical source.
The X-ray source was suggested as an XRB candidate by
Kahabka et al. (1999). The spectrum of the source is a power law
with
and absorbing column
density of
cm-2
(Fig. 9).
Since the probable optical counterpart is very faint and the power law
photon index is higher than for most of the other sources presented here,
it can not be
ruled out that this source is an AGN (also see Table 3).
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Figure 10: Long term light curve (0.1-2.4 keV) of RX J0101.0-7206 (source No. 13). Symbols as in Fig. 3. |
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The Be/X-ray binary RX J0101.0-7206 showed a luminosity
of
erg s-1 in the ROSAT band (0.1-2.4 keV)
during two XMM-Newton observations. It was about 60 times fainter than at the
maximum observed by ROSAT (Fig. 10).
Pulsations with a period of 304.49
0.13 s were discovered in Chandra data
(Macomb et al. 2003). This period could not be verified in the
XMM-Newton observation, because the source was too faint.
Edge & Coe (2003) presented results on the optical analysis of
likely counterparts, discussing two objects (Nos. 1 and 4) in the ROSAT PSPC
error circle. They conclude that the optical counterpart is object No. 1
which is confirmed to be a Be star. This object is also the only optical source,
which can be found on the DSS image within the XMM-Newton 1
error
circle.
The ROSAT source RX J0101.3-7211 is the first X-ray
binary pulsar of which the discovery was based on XMM-Newton data. It was
covered in two additional observations finding the source again in a low
intensity state. The pulse
period of 455
2 s (Sasaki et al. 2001) was
verified in the new data of the observation ID 01100002: 452.2
0.5 s.
During the observation
ID 01357206, the source was too faint for a timing analysis.
The folded light curves show strong variation in all bands
(Fig. 11). The spectrum of the source becomes harder during
pulse minimum. The spectrum is well fitted with a soft thermal component
described by a MEKAL model
(
kT = 0.20+0.09-0.06 keV)
with a low metal abundance
(
times solar)
and a power law component
absorbed by a high column density (Table 3).
The optical counterpart (OGLE 01012064-7211187) is a Be-star.
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Figure 11: Folded light curves, spectra, and long term light curve (0.1-2.4 keV) of RX J0101.3-7211 (source No. 14). Hardness ratio and symbols as in Fig. 3. For the spectra, solid lines are used for the data of the obs. ID 01100002, and dashed lines for obs. ID 01357206. |
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Figure 12: Spectra and long term light curve (0.1-2.4 keV) of RX J0101.6-7204 (source No. 15). For the spectra, solid lines are used for the data of the obs. ID 01357206, and dashed lines for obs. ID 01100002. Symbols for the long term light curve as in Fig. 3. |
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The Be/XRB candidate RX J0101.6-7204 with an emission line star at the ROSAT PSPC and HRI positions ([HS2000]), was observed in two XMM-Newton pointings. Its spectrum and long term light curve are shown in Fig. 12. The spectrum can be modelled as a moderately absorbed power law. No pulsations were discovered.
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Figure 13: Folded light curves, spectrum, and long term light curve (0.1-2.4 keV) of AX J0103-722 (source No. 16). Hardness ratio and symbols as in Fig. 3. |
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For the Be/X-ray binary AX J0103-722
a pulse period of 345.2
0.1 s was determined by
Israel et al. (1998). In the XMM-Newton data, pulsations were confirmed
with a period of 341.7
0.4 s. The folded light curves show strong
variation below 2.0 keV (Fig. 13), whereas
in the hard band, the variations are strongly reduced.
The spectrum is well reproduced with a power law and a thermal component
(see Table 3).
The MEKAL model for the thermal component yields
kT = 0.27-0.07+0.08 keV and metal abundances of
with respect to solar.
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Figure 14: Spectrum and long term light curve (0.1-2.4 keV) of RX J0103.6-7201 (source No. 17). Symbols for the long term light curve as in Fig. 3. |
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For the HMXB candidate RX J0103.6-7201 ([HS2000]),
an acceptable fit was obtained for the spectrum
with a power law and a thermal component (Table 3).
Modelling the thermal component with MEKAL, we obtained
kT = 0.27-0.04+0.03 and metal abundances of
times solar. Since the source was bright, we also used the VMEKAL model
instead of the MEKAL model, which allows to determine the
abundance for each of the elements. This resulted
in an improvement of the fit, the model reproducing the peaks
around 0.6 and 0.9 keV.
The photon indices
and the absorbing column densities
for both fits are comparable, as can be seen in Table 3.
Also the temperature values kT agree well for
MEKAL and VMEKAL within the 1
errors.
The spectrum with the power law + VMEKAL fit is shown
in Fig. 14.
The comparison to ROSAT data shows that this source was in high
luminosity state during the XMM-Newton observation with
1036 erg s-1 (0.3-10.0 keV). In spite of
the high photon statistics with 3,300 counts, no pulsations were discovered.
Also the analysis of the events separated into soft, medium, and hard band
revealed no pulsations.
RX J0105.9-7203 is a HMXB candidate, coinciding with an emission
line object. Since the source was
very faint during the XMM-Newton observation, the photon statistics are very
low and no timing analysis was possible.
The PSPC count rate derived from the ROSAT observation was
s-1
(Haberl et al. 2000), corresponding to
s-1 for XMM-Newton EPIC PN
(thin1 filter). With a count rate
of
s-1
(Table 2), the source was fainter
during the XMM-Newton observation.
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Figure 15: Spectrum of XMMU J005735.7-721932 (source No. 6). |
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To identify a HMXB, it is crucial to find an optical counterpart and confirm that it is an early-type star. If an emission line object is found at the position of a hard X-ray source and other objects are ruled out as counterpart, the source is presumably a Be/XRB. Cross-correlating the XMM-Newton source list with the emission line star catalogue of Meyssonnier & Azzopardi (1993), we discovered two new sources which met these criteria.
XMMU J005735.7-721932 (source No. 6)
is found at the position of [MA93]1020 and
likely coincides with the source No. 19 in Yokogawa et al. (2000).
XMMU J010030.2-722035 (source No. 12) is associated with [MA93]1208.
Both X-ray sources are very faint.
Only for XMMU J005735.7-721932, we had enough
counts to extract a spectrum (Fig. 15).
The best fit model is a moderately absorbed power law (Table 3).
Furthermore, Chandra data showed that this source has pulsed
emission with a period of 564.81
0.41 s (Macomb et al. 2003).
This period could not be confirmed in the XMM-Newton data.
As indicated by the hardness ratio HR1, these sources have a hard spectrum.
Therefore, these two sources are suggested as new Be/XRB candidates.
For further investigation, we need to perform follow-up
observation in the optical band in order to verify if the emission line
objects are Be stars.
![]() |
Figure 16: Histogram of luminosities [erg s-1] of the HMXBs and candidates in the XMM-Newton band (0.3-10.0 keV). The upper limit for SMC X-2 is shown with dashed line. |
Open with DEXTER |
The comparison between the XMM-Newton sources detected with ML > 10 and other
X-ray catalogues
(Wang & Wu 1992; Kahabka et al. 1999; Haberl et al. 2000; Sasaki et al. 2000)
demonstrates that we detected all known HMXBs and candidates which exist in the
four observed fields, except for SMC X-2 which was very faint.
SMC X-2 was marginally detected with a likelihood of ML = 3.4, and we
derived an upper limit of
erg s-1 (0.3-10.0 keV).
The luminosities of all the other sources in the 0.3-10.0 keV
band are higher than
erg s-1
at an assumed distance
of 60 kpc (van den Bergh 1999),
as is shown in
Fig. 16. As we have seen in the long term light curves, most
of the sources were in quiescence during the XMM-Newton observations,
whereas they were mostly detected during outburst by previous missions.
This indicates that all known HMXBs in the SMC have luminosities higher than
erg s-1 in quiescence and can be detected by
XMM-Newton in observations with an exposure of about 15 ks.
Consequently, we have an extensive set of HMXBs for studying their
properties.
In order to visualise the spectral characteristics of the HMXBs, we plotted the hardness ratios HR1, HR2, and HR3 in Fig. 2. The high absorption in XRBs causes positive values for HR1, while HR2 and HR3 have small absolute values around zero. AGNs typically show steeper X-ray spectra than HMXBs. Therefore, the two source classes can be disentangled using hardness ratios. This can also be applied to classification work on other nearby galaxies.
X-ray spectra of high mass X-ray binaries below 10 keV can be in general
modelled as a
power law with a photon index of
.
For HMXBs located far away
from the Galactic plane or in the Magellanic Clouds, the interstellar
absorption in the line of sight is low, and an additional soft spectral
component was discovered in supergiant X-ray binary systems like
SMC X-1 (Marshall et al. 1983; Woo et al. 1995) or LMC X-4
(Woo et al. 1996), as well as in Be/X-ray binary systems like
RX J0059.2-7138 (Kohno et al. 2000) or
EXO 053109-6609.2 (Haberl et al. 2003).
In our sample of HMXBs and candidates in the SMC, pulsations were confirmed for six sources. Studying the pulsations and hardness ratio changes in different bands, we found that there are different types of pulsations. Furthermore, four out of these six were bright enough to allow us to test the existence of a soft component in their spectra. The pulsating sources of our sample can be divided into four groups:
The low energy component in the spectrum of the supergiant systems SMC X-1 (Marshall et al. 1983) and LMC X-4 (Woo et al. 1996) was modelled as blackbody emission or thermal Bremsstrahlung which arises from the stellar wind of the supergiant, the accretion disk, or the fan-beam of the accretion column close to the neutron star surface. However, Paul et al. (2002) pointed out that a power law nature is most probable for the soft emission. They derived that the pulse shape of the soft emission from SMC X-1 is sinusoidal, similar to the soft energy light curve of Her X-1 (e.g. Oosterbroek et al. 1997). In our Galaxy, the supergiant system Vela X-1 is thought to show emission from the atmosphere and stellar wind of the companion as well as from the gas stream towards the neutron star (Haberl & White 1990, and references therein). High resolution spectroscopy of Galactic HMXBs like Cen X-3 (with Chandra HETG, Wojdowski et al. 2002) or Her X-1 (with XMM-Newton RGS, Jimenez-Garate et al. 2002) resolved fluorescent lines and hydrogen- and helium-like lines of elements from Ne to Fe. The line fluxes of Cen X-3 are consistent with recombination radiation from photo-ionised and collisionally ionised plasma as well as resonant line scattering in photo-ionised plasma (Wojdowski et al. 2002).
As for the Be/X-ray binary systems, Kohno et al. (2000) analysed both ASCA and ROSAT data of RX J0059.2-7138 and found that there is a soft component in the spectrum, which can be modelled as a thermal emission with kT = 0.37 keV. Below 2.0 keV, the source shows no pulsations. Therefore they argue that the soft emission originates from a large region comparable to the full binary system. Using an XMM-Newton observation of a northern field in the LMC, Haberl et al. (2003) extracted emission from the Be/X-ray binary EXO 053109-6609.2 and showed that there are strong pulsations above 0.4 keV. In the spectrum there is a low energy thermal component, which is believed to arise from the equatorial disk around the Be star, illuminated by the X-ray source.
The origin of the soft emission from HMXBs is not clearly understood. One would expect that there are differences between a supergiant and a Be system. Most of the HMXBs which have been studied in detail (since they are located in the Milky Way and therefore closer) are supergiant systems, whereas the sources in the SMC we are confronted with, are Be systems. In Be/XRBs, the neutron star and the Be star are thought to form a binary system with an extended orbit. This makes the stellar material in the equatorial disk around the Be star as the origin of the soft pulsed emission rather implausible. The HMXBs in the MCs are ideal objects to study the soft part of their spectrum, since the absorption by Galactic foreground matter is low in the direction of the MCs. The existence of a soft thermal component in the spectrum and pulsations below 1-2 keV in our data indicates that the size of the origin of the soft emission is not as large as is assumed for e.g. RX J0059.2-7138. In addition to timescales and luminosities, a crucial parameter for the physical processes responsible for this emission is the magnetic field of the neutron star. In order to clarify the conditions in which the soft component is produced, at least we need to get information about the orbital motion and about a possible orbital phase dependence of the total source spectrum as well as the pulsed emission. As for the SMC Be systems discussed here, the orbital period is known only for one source.
In the last few years, the number of known Be/XRBs in the SMC increased
drastically based on temporal studies of hard X-ray sources and optical
observations. In order to identify an X-ray source
as a HMXB and clarify the nature of the mass donor star, we need to perform
spectroscopy of the optical counterpart. Since most of the HMXB candidates
which are known now are correlated to emission line objects, we expect that
additional Be/XRBs will be found in the near future.
This will further increase the ratio between the Be systems and the OB systems among the HMXBs in the SMC.
Be/XRBs are thought to evolve from binary systems in about
yrs, whereas supergiant systems
evolve faster due to the high mass of the companion star. The large number
of Be/XRBs sets constraints on the secondary star formation in the SMC,
making a burst some 107 yrs ago most likely.
We analysed XMM-Newton EPIC PN and MOS 1/2 data of four pointings towards the
SMC. One observation covered the field around the HMXB SMC X-2 in the
south, whereas the fields of view of the other three are located in the
northern
part of the main body of the SMC. In total, there were 15 detections which
were identified as known HMXBs or XRB candidates. For SMC X-2 which
was faint during the observation, a flux upper limit of
erg cm-2 s-1 (0.3-10.0 keV) was derived.
We found two new sources (XMMU J005735.7-721932 and
XMMU J010030.2-722035) which have a hard spectrum and positionally
coincide with emission line objects (Meyssonnier & Azzopardi 1993). These sources
are proposed as new HMXB candidates, probably Be systems.
Four sources in our list were known to show pulsed emission and pulse periods
had been determined in former observations. In this work, the pulse periods
were confirmed for all four sources. Furthermore, we discovered that two
other sources which had been proposed to be Be/XRB candidates, show pulsations:
XMMU J005605.2-722200 with a pulse period of 140.1
0.3 s and
RX J0057.8-7207 with 152.34
0.05 s.
Spectral analysis of the sources was performed. For faint sources,
a good fit was obtained with a single power law spectrum. However, for three
brighter sources, we could show that there is a significant low energy excess
in the spectrum, if we only assume a power law. The spectra indicate emission
line features,
suggesting that the emission is thermal. This soft component was modelled
as thermal emission, yielding temperatures of 0.2-0.3 keV. The abundances
in the emitting plasma are below solar values, but comparable to typical SMC values (Russell & Dopita 1992): for RX J0101.3-7211
it is
times solar, and for AX J0103-722
best fit is obtained with
times solar.
The errors are 1
values.
Only for RX J0103.6-7201 the abundance
is higher with
with respect to solar.
The flux of the sources in the MCs is low compared to the bright
(
erg s-1) HMXBs in
our Galaxy, making it difficult to
perform a detailed analysis of their soft emission. However,
the sources in the MCs have the advantage of low Galactic
absorption. This allows us to study the thermal emission from a large
sample of HMXBs and to increase the understanding of the interaction
between X-rays from the compact object and the ambient stellar matter. It is
also important to verify if there is a change in temperature or emissivity,
which is related to the orbital phase of the binary system.
Due to the improved time resolution and sensitivity, there is a large
detection potential for new pulsating XRBs in further XMM-Newton observations.
Acknowledgements
We would like to thank the anonymous referee for useful comments. The XMM-Newton project is supported by the Bundesministerium für Bildung und Forschung/Deutsches Zentrum für Luft- und Raumfahrt (BMBF/DLR), the Max-Planck Society and the Heidenhain-Stiftung. This research has been carried out by making extensive use of the SIMBAD data base operated at CDS, Strasbourg, France. The Digitized Sky Survey was produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these institutions. This research has made use of data obtained through the High Energy Astrophysics Science Archive Research Center Online Service, provided by the NASA/Goddard Space Flight Center.