Issue |
A&A
Volume 519, September 2010
|
|
---|---|---|
Article Number | A6 | |
Number of page(s) | 13 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/201014095 | |
Published online | 06 September 2010 |
The supergiant fast X-ray transients XTE J1739-302 and IGR J08408-4503 in quiescence with XMM-Newton
E. Bozzo1 - L. Stella2 - C. Ferrigno1 - A. Giunta2,3 - M. Falanga4 - S. Campana5 - G. Israel2 - J. C. Leyder6
1 - ISDC - Science Data Centre for Astrophysics, University of Geneva,
Chemin d'Écogia 16, 1290 Versoix, Switzerland
2 - INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00044
Rome, Italy
3 - Dipartimento di Fisica - Università di Roma Tor Vergata, via della
Ricerca Scientifica 1, 00133 Rome, Italy
4 - International Space Science Institute (ISSI) Hallerstrasse 6, 3012
Bern, Switzerland
5 - INAF - Osservatorio Astronomico di Brera, via Emilio Bianchi 46,
23807 Merate (LC), Italy
6 - Institut d'Astrophysique et de Géophysique de l'Université de
Liège, 17 allée du 6 août, 4000 Liège, Belgium
Received 18 January 2010 / Accepted 9 April 2010
Abstract
Context. Supergiant fast X-ray transients are a
subclass of high mass X-ray binaries that host a neutron star accreting
mass from the wind of its OB supergiant companion. They are
characterized by an extremely pronounced and rapid variability in
X-rays, which still lacks an unambiguous interpretation. A number of
deep pointed observations with XMM-Newton have
been carried out to study the quiescent emission of these sources and
gain insight into the mechanism that causes their X-ray variability.
Aims. We continued this study by using three XMM-Newton
observations of the two supergiant fast X-ray transient prototypes XTE J1739-302
and IGR J08408-4503 in quiescence.
Methods. An in-depth timing and spectral analysis of
these data have been carried out.
Results. We found that the quiescent emission of
these sources is characterized by both complex timing and spectral
variability, with multiple small flares occurring sporadically after
periods of lower X-ray emission. Some evidence is found in the XMM-Newton
spectra of a soft component below 2 keV, similar to that observed in the
two supergiant fast X-ray transients AX J1845.0-0433 and
IGR J16207-5129 and in many other high mass X-ray binaries.
Conclusions. We suggest some possible
interpretations of the timing and spectral properties of the quiescent
emission of XTE J1739-302 and IGR J08408-4503
in the context of the different theoretical models proposed to
interpret the behavior of the supergiant fast X-ray transients.
Key words: X-rays: binaries - stars: individual: XTE J1739-302 - stars: individual: IGR J08408-4503 - stars: neutron - X-rays: stars
1 Introduction
Supergiant fast X-ray transients are a subclass of supergiant X-ray
binaries (SGXBs) that host a neutron star (NS) accreting from the wind
of its OB supergiant companion (SFXT, Sguera
et al. 2005). In contrast to the previously known
supergiant X-ray binaries, SGXBs (i.e., the so-called ``classical
SGXBs'', see e.g., White
et al. 1995), SFXTs are characterized by a
pronounced transient-like activity. These sources undergo few hours
long (as opposed to weeks-months long) outbursts with peak X-ray
luminosities of 1036 erg s-1,
and exhibit a large dynamic range in X-ray luminosity (
104
between outburst and quiescence; Walter
& Zurita Heras 2007). The origin of this extreme
variability is still debated, and different models have been developed
to interpret it. One of these models involves a NS accreting matter
from the extremely clumpy wind of its supergiant companion (Walter &
Zurita Heras 2007; in't Zand 2005; Negueruela
et al. 2008). According to this interpretation, the
sporadic capture and accretion of these clumps by the compact object
can produce the observed fast X-ray flares. To reach the required X-ray
luminosity swing, very high density clumps are required (a factor 104-105
denser than the homogeneous stellar wind, Walter
& Zurita Heras 2007). Numerical simulations of
supergiant star winds indicate that these high density clumps might be
produced by instabilities in the wind (Runacres
& Owocki 2002; Oskinova
et al. 2007, and references therein). Bozzo et al. (2008)
proposed that the X-ray variability of the SFXT sources might be driven
by centrifugal and/or magnetic ``gating'' mechanisms that can halt most
of the accretion flow during quiescence, and only occasionally permit
direct accretion onto the NS (see also Grebenev
& Sunyaev 2007). The properties of these gating
mechanisms depend mainly on the NS magnetic field and spin period. At
odds with the extremely clumpy wind model, in the gating scenario a
transition from the regime in which the accretion is (mostly) inhibited
to that in which virtually all the captured wind material accretes onto
the NS requires comparatively small variations in the stellar wind
velocity and/or density, and can easily give rise to a very large
dynamic range in X-ray luminosity. Yet another model was proposed for
IGR J11215-5952, so far the only SFXT to display regular
periodic outbursts, which envisages that the outbursts take place when
the NS in its orbit crosses a high density equatorial region in the
supergiant's wind (Sidoli
et al. 2007).
Except for their peculiar X-ray variability, SFXT sources
share many properties with the previously known SGXBs. Measured orbital
periods in SFXTs range from 3 to 30 days
(IGR J16479-4514: 3.32 days,
IGR J17544-2619: 4.92 days; IGR J18483-0311:
18.5 days; SAX J1818.6-1703: 30.0 days; Clark
et al. 2009; Jain et al. 2009; Zurita Heras
& Chaty 2009; Bird et al. 2009; Sguera
et al. 2007), and are thus similar to those of other
SGXBs. The only exception is the SFXT IGR J11215-5952, which
has an orbital period of 165 days (Sidoli
et al. 2007). The relatively high eccentricities
inferred for two SFXTs (0.3-0.7,
Rahoui
et al. 2008; Zurita Heras & Chaty 2009)
suggest that these systems might be characterized by somewhat more
elongated orbits than classical SGXBs. The spin period of the NS hosted
in these sources has been measured only in four cases, the periods
ranging from 4.7 to 228 s (IGR J1841.0-0536:
4.7 s; IGR J1843-0311: 21 s;
IGR J16465-4507: 228 s; IGR J11215-5952:
186.78 s; Bamba
et al. 2001; Swank et al. 2006; Sguera
et al. 2007; Lutovinov et al. 2005),
and thus being similar to the spin periods measured in the classical
SGXBs. However, owing to the limited duration of most observations, it
is not possible to exclude that a number of SFXTs have much longer spin
periods (see e.g., Bozzo
et al. 2008; Smith et al. 1998).
A number of pointed XMM-Newton observations of several SFXTs were carried out to study the quiescent emission of these sources and gain insight into the mechanism that drives their peculiar X-ray activity. During the observation of the SFXT IGR J16479-4514, XMM-Newton captured the source undergoing a very steep luminosity decay from the end of an outburst to a much lower state. The latter was interpreted in terms of an eclipse of the source by the companion star (Bozzo et al. 2009; Jain et al. 2009; Bozzo et al. 2008). This observation revealed that in at least one case the X-ray variability of a SFXT was due to the obscuration by the companion star. In the case of IGR J18483-0311, XMM-Newton helped identify pulsations in the quiescent X-ray flux of this source (Giunta et al. 2009), and thus provided strong support for the idea that SFXTs also accrete matter during their quiescent states (see e.g., Sidoli et al. 2007; Bozzo et al. 2008).
To study the low level emission of SFXT sources, we present in this paper quiescent state XMM-Newton observations of the prototypical SFXTs XTE J1739-302 and IGR J08408-4503. In Sect. 2, we summarize previous observations of these sources, and in Sects. 3 and 4 we present our data analysis and results. In particular, we find that
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Figure 1: Selection of the high background intervals during the two XMM-Newton observations (Epic-PN camera) of XTE J1739-302 (OBS1 on the left and OBS2 in the middle) and the observation of IGR J08408-4503 (on the right). In each case, we reported the source lightcurve not corrected for the selection of the good time intervals and not subtracted for the background in the 0.2-12 keV (upper panel) and 10-12 keV energy band (middle panel), and the count rate of the total FOV in the 10-12 keV energy band (lower panel). In all cases, the time bin is 100 s. Only the observational intervals in which the total FOV count rate in the 10-12 keV energy band was below the threshold indicated with a dashed line were considered for the timing and spectral analysis of the sources. |
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2 The sources
2.1 XTE J1739-302
XTE J1739-302 is a SFXT
prototype, and was discovered with RXTE during a
bright outburst in 1997 (Smith
et al. 1998). The identification of its supergiant
companion led to the determination of the source distance at
2.7 kpc (Rahoui
et al. 2008). Several outbursts from this source
were detected later with RXTE (Smith
et al. 2006), and INTEGRAL (Sguera
et al. 2006; Blay et al. 2008; Lutovinov
et al. 2005; Sguera et al. 2005). XTE J1739-302
was observed in outburst with Swift /BAT
on three occasions, on 2008 April 8 (Sidoli
et al. 2009a), on 2008 August 13 (Sidoli et al. 2009b),
and on 2009 March 10 (Romano
et al. 2009c). In only the first two cases, Swift
slewed to the source and observations with the X-ray Telescope, XRT,
were carried out. During the 2008 April 8 outburst, XRT observed XTE J1739-302
387 s
after the BAT trigger. These data showed that the source was rapidly (
1000 s)
decreasing in intensity, and the X-ray spectrum (0.3-10 keV)
could be reproduced well by using an absorbed (
cm-2)
power law (hereafter, PL) model (photon index
). The
0.5-100 keV X-ray luminosity was
erg s-1.
Sidoli et al. (2009a)
also performed an analysis of the Swift broad
band (0.3-60 keV) spectrum of XTE J1739-302
during this outburst, and found that this spectrum could be reasonably
well described by using either a power law with a cutoff at high energy
(
13 keV),
or a Comptonizing plasma model ( COMPTT in XSPEC).
For the outburst of 2008 August 13, XRT data were obtained
starting from
390 s
after the BAT trigger, and revealed a more complex behavior than that
observed during the previous event (Sidoli
et al. 2009b). A time-resolved analysis showed that
the source X-ray spectrum could be fit equally well by using an
absorbed PL or a black-body (BB) model with constant photon index or
temperature (
,
keV), and a varying
absorption column density (in the range 3-
cm-2).
The combined XRT+BAT broad band (0.3-60 keV) spectrum could be
well fit by using either a model of Comptonization of seed photons in a
hot plasma ( COMPTT in XSPEC)
or a BMC model. The BMC comprises a BB component and a component
accounting for the Comptonization of the BB due to thermal and/or
dynamical (bulk) Comptonization. The 0.1-100 keV X-ray
luminosity derived from the simultaneous XRT+BAT spectrum was
erg s-1.
On 2009 March 10, XTE J1739-302 again
triggered BAT (Romano et al.
2009c). On this occasion, Swift did not
perform any quick slew towards the source and XRT data were accumulated
only
1.5 h
after the BAT trigger. At this time, the source was already much
fainter (X-ray luminosity
7
1034 erg s-1,
2-10 keV), and the XRT spectrum could be reproduced well by
using an absorbed power-law model (
cm-2,
).
Little is known about the quiescent emission of XTE J1739-302.
An ASCA observation in 1999 did not detect the
source and placed a 3
upper limit on its X-ray luminosity of
erg s-1
(exposure time
13 ks,
Sakano et al. 2002).
A
5 ks
Chandra observation in 2001 caught the source in a
relatively low luminosity state (
erg s-1)
and the X-ray spectrum was fit well by using an absorbed power-law
model (
cm-2,
;
Smith et al. 2006).
Based on a monitoring program with Swift, Romano et al. (2009b)
carried out the first study of the long-term variation in the quiescent
emission from XTE J1739-302. They
identified three different quiescent states of the source, with a
2-10 keV X-ray luminosity of 1035,
,
and
erg s-1,
respectively. The X-ray spectra of these states could be reasonably
well described by using an absorbed power-law model, with absorption
column densities and photon indices in the range (0.3-3.6)
1022 cm-2 and
0.5-1.6, respectively. Alternatively, these spectra could also be fit
by using a BB model with temperatures and radii in the range
1.3-1.9 keV and 0.02-0.28 km, respectively. With
these results at hand, the authors argued that the quiescent emission
of XTE J1739-302 was most likely
produced by a spot covering a relatively small region of the NS
surface, possibly the NS magnetic polar caps.
2.2 IGR J08408-4503
IGR J08408-4503 was discovered in the
Vela region on 2006 May 15 with INTEGRAL during a
short flare lasting less than 1000 s (Götz
et al. 2007). Its optical counterpart was later
identified as the supergiant star HD 74194 located at
3 kpc, thus confirming that this source belongs to the SFXT
class (Masetti
et al. 2006; Götz et al. 2007). IGR J08408-4503
was observed in outburst two times with INTEGRAL,
and the combined JEM-X and ISGRI spectra were most accurately fit by
using a rather flat cut-off power-law model (
,
keV,
Leyder
et al. 2007; Götz et al. 2007). The
absorption column density was
cm-2,
compatible with the interstellar value in the direction of the source (Dickey & Lockman 1990).
The 0.1-100 keV luminosity of the two outbursts was
and
erg s-1.
IGR J08408-4503 was also caught
in outburst by Swift 4 times between 2006 and
2009 (Romano
et al. 2009a; Barthelmy et al. 2009;
Sidoli
et al. 2009b). During the first outburst, which
occurred on 2006 October 4, Swift /XRT
slewed to the source 2000 s
after the BAT trigger. The combined XRT+BAT spectrum could be
reproduced well by using a cut-off power-law model (
,
keV). The column
density was
cm-2.
The second outburst, which took place on 2008 July 5, was characterized
by complex behavior and comprised two different flares separated by
105 s.
The time-resolved spectral analysis found that the soft X-ray emission
of the source could be reasonably well described by using an absorbed
power-law model with a constant photon index of
1 and a variable absorption column density
ranging from
0.5
to
11
cm-2.
The simultaneous XRT+BAT broad band (0.3-80 keV) spectrum of
the source could be reproduced well using an absorbed cut-off power-law
model, with
keV,
,
and
cm-2.
The third outburst occurred on 2008 September 21. On this occasion, Swift /XRT
slewed to the source
150 s
after the BAT trigger, and time-resolved spectroscopy did not reveal
any variation in the absorption column density. However, the total XRT
spectrum of the observation (exposure time 1160 s) clearly
revealed a previously undetected soft component below
2 keV.
A reasonably good fit to these data could be obtained by using an
absorbed (
cm-2)
power-law (
)
component and a BB at the softer energies (with a temperature of
keV,
and a radius of
1.2 km).
This BB component was also required to fit the combined XRT+BAT broad
band spectrum of the outburst (a single comptonization model, BMC, only
provided a poor fit to the data). Sidoli
et al. (2009b) found that the seed photon
temperature of the BMC component and the BB temperature could not be
linked to the same value in the fit. This was interpreted as being
caused by the presence of two distinct photon populations, a colder one
with a temperature of about 0.3 keV and a radius of
10 km,
and a hotter one with a temperature of 1.4-1.8 keV and a
radius of
1 km.
However, the statistics of the data did not allow the authors to
distinguish which of these two populations was seen directly as a BB
and which one ended up being seed photons for the thermal
Comptonization. The latest outburst from IGR J08408-4503
was caught by Swift /BAT on 2009 August
28. Unfortunately, in this case Swift /XRT
did not perform any follow-up observation, and thus no detailed
spectral or timing information was available in the 0.3-10 keV
energy band.
To date, the quiescent emission of IGR J08408-4503
has remained largely unexplored; the only detection of this source
during a period of low X-ray activity was obtained by two Swift /XRT
follow-up observations carried out in 2006 May 22 and 2007
September 29 (Leyder et al.
2007). The exposure time of each of these observations was
about 4.0 ks,
and only a total of 40 photons could be collected. Assuming a Crab-like
spectrum, the source X-ray flux was estimated to be
erg cm-2 s-1
(0.5-10 keV), corresponding to a luminosity of about
erg s-1
(Kennea & Campana 2006).
3 XMM-Newton data analysis
For the present study, we used two XMM-Newton observations of XTE J1739-302, and one XMM-Newton observation of IGR J08408-4503. During all three observations, the two sources were in quiescence.
XMM-Newton observation data files (ODFs)
were processed to produce calibrated
event lists using the standard XMM-Newton Science
Analysis
System (v. 9.0). We used the EPPROC and EMPROC
tasks for the Epic-PN and the two MOS
cameras, respectively. The event files of the two observations were
filtered to exclude high background time intervals. The effective
exposure time for each observation and camera is given in
Sects. 4.1
and 4.2.
Source lightcurves and spectra were extracted in the
0.2-15 keV band for the Epic-PN and 0.2-10 keV for
the two Epic-MOS cameras.
We extracted the background lightcurves and spectra from the nearest
source-free region to XTE J1739-302 and IGR J08408-4503.
Background and source circles were all
chosen to lie within the same CCD. The difference in extraction areas
between source and background was accounted for by using the SAS BACKSCALE
task for the spectra and the LCMATH task
from
H EASOFT for the lightcurves. All of the
EPIC spectra were rebinned before fitting
so as to have at least 25 counts per bin and, at the same time, prevent
oversampling the
energy resolution by more than a factor of three.
Given the low count rate of the source, the Epic-MOS1 and Epic-MOS2
cameras did not contribute significantly to the spectral analysis; we
therefore discuss in this paper only the spectra from the Epic-PN
camera. Throughout this paper, the errors are given at
90% c.l. (unless stated otherwise).
4 Results
4.1 XTE J1739-302
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Figure 2: XMM-Newton Epic-PN background-subtracted lightcurve of XTE J1739-302 during the observation carried out on 2008 October 1. The upper panel shows the source lightcurve in the 0.2-2 keV energy band, whereas the middle panel gives the lightcurve in the 2-15 keV energy band (the binning time is 300 s). The ratio of the source count rate in the two bands, (2-15 keV)/(0.2-2 keV), versus time is shown in the lower panel. The time intervals in which no data are plotted have been discarded because of high background events. |
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![[*]](/icons/foot_motif.png)
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Figure 3: The same as Fig. 2, but for the XMM-Newton observation carried out on 2009 March 11. The binning time is 300 s. |
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Figure 4: Hardness-intensity diagram for the two XMM-Newton observations of XTE J1739-302 (the upper panel is for the observation carried out in 2008, the lower panel for the observation in 2009). The hardness is defined as the ratio of the count rate in the hard (2-15 keV) to soft (0.2-2 keV) energy band. The points were obtained by using the lightcurves given in Figs. 2 and 3, but, wherever necessary, consecutive bins were rebinned so as to achieve a S/N>5.5 in the hardness ratio. |
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Figure 5: The XMM-Newton spectra of the XTE J1739-302 observation carried out in 2008. The open circles, open squares, and filled circles are from the energy resolved data obtained during the time intervals in which the Epic-PN 0.2-15 keV source count rate was >0.4, 0.1-0.4, <0.1, respectively. The model used for the best fits is an absorbed CUTOFFPL (cutoff energy fixed at 13 keV). The residuals from this fit are shown in the bottom panel. (See the electronic edition of the paper for a color version of this figure.) |
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Table 1: Results of the count-rate-resolved spectral analysis for the two XMM-Newton observations of XTE J1739-302.
The lightcurves of the two observations corrected for the good
time interval selection and background-subtracted are shown in
Figs. 2
and 3. In
both cases, the source displayed a pronounced variability on a
timescale of hundreds of seconds, with small flares occurring
sporadically after periods of lower X-ray emission. During these
flares, the X-ray flux typically increased by a factor of 10-30 (from
a few 10-13 erg cm-2 s-1
up to
10-11 erg cm-2 s-1).
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Figure 6: The same as Fig. 5 but for the observation carried out in 2009. The model used for the best fits is an absorbed CUTOFFPL (cutoff energy fixed at 13 keV). The residuals from these fits are shown in the bottom panel. (See the electronic edition of the paper for a color version of this figure.) |
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Figure 7: The XMM-Newton spectrum of XTE J1739-302 extracted by using the total exposure time of OBS1. The model used for the best fit is an absorbed CUTOFFPL (we did not fix here the cutoff energy). The residuals from this fit are shown in the bottom panel, whereas the middle panel shows the residuals from the fit obtained by using an absorbed CUTOFFPL with the cutoff energy fixed at 13 keV. |
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Figure 8: The same as Fig. 7, but for the observation carried out on 2009 March 11. Here the best-fit model to the data was an absorbed CUTOFFPL model (the cutoff energy was allowed to vary during fitting). |
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Figure 9: An example of an unfolded spectrum of the XMM-Newton observation of XTE J1739-302 carried out in 2008 (OBS1). Here the best fit is obtained by using an absorbed CUTOFFPL plus a MKL component (see Sect 4.1). |
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By leaving the cutoff energy free to vary in the fit, a
significantly closer fit was obtained, with a
/d.o.f. =
1.02/174, 1.09/183 for OBS1 and OBS2, respectively. By using the
F-test, we found that this improvement was highly significant for OBS1
(5.3
)
and somewhat less significant for OBS2 (3.5
). In both cases, the derived
cutoff energy turned out to be much lower (
4 keV, see Table 2) than that
measured previously when the source was in outburst (
13 keV).
Such a low value for the cutoff energy might not be unlikely for an
X-ray pulsar (see Sect. 5). In the
case of OBS1, when an additional spectral component to that of the
CUTOFFPL appears to be clearly significant, we also tried to
investigate the applicability of other spectral models.
Table 2: Spectral fits of the XMM-Newton data of XTE J1739-302.
We again fixed the cutoff energy of the CUTOFFPL component at 13 keV and tried first a phenomenological model including an additional black-body component (BB) at lower energies (<2 keV). We found that both a BB with


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Figure 10: XMM-Newton Epic-PN background-subtracted lightcurve of IGR J08408-4503 during the observation of 2007 May 29. The upper and middle panel shows the source lightcurve in the 0.2-2 keV and 2-15 keV energy bands, respectively. The binning time is 300 s. The ratio of the source count rate in the two bands versus time is shown in the lower panel. |
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We also performed a Fourier analysis of the lightcurves of
OBS1 and OBS2, in search of coherent pulsations, by using the method
described in Israel & Stella
(1996). No significant (above 3
level) signal was detected in either observation. The corresponding 3
c.l. upper limits
to the pulsed fraction (defined as the semi-amplitude of the sinusoid
divided by the source
average count rate), were then computed according to the method
described in Vaughan et al.
(1994).
In OBS1, upper limits at a level of 30%, 20%, and 40% were inferred in
the 0.1-0.2 s, 0.2-50 s, and 50-150 s period
range, respectively. In OBS2, we derived upper limits at a level of
20%, 30%, and 35% for periods in the range 0.03-20 s,
20-50 s, and 0.02-0.03 s, respectively.
4.2 IGR J08408-4503
XMM-Newton observed IGR J08408-4503 on 2007 May 29, with the Epic-PN camera operating in full frame. To identify the high background time intervals we followed the same technique described for XTE J1739-302 (see Sect. 4.1 and Fig. 1). We extracted the Epic-PN lightcurve for the full field of view (FOV) in the 10-12 keV energy band, and set a threshold on the full-FOV count rate in this energy band of 0.45 cts s-1. The total effective exposure time after the good time interval selection for IGR J08408-4503 was 26 ks.
From the lightcurve of the observation (Fig. 10), it is apparent that the variability in the quiescent state of this source was rather similar to that of XTE J1739-302. In particular, the lower panel of Fig. 10 shows that the hardness ratio of IGR J08408-4503 increased with the source count rate.
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Figure 11:
Hardness-intensity diagram for IGR J08408-4503.
The diagram was created by using the 0.2-2 keV and
2-15 keV lightcurves of Fig. 10, but rebinned
so as to achieve a |
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Figure 12:
The same as Figs. 5 and 6 but
for the observation of IGR J08408-4503.
Here the open circles, open squares, and filled circles refer to the
spectrum accumulated during the time intervals of the observation in
which the Epic-PN 0.2-15 keV source rate was >0.2,
0.1-0.2, <0.1 counts/s respectively. The model used for
the best fits comprises a MKL plus a CUTOFFPL component (
|
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Figure 13: The XMM-Newton spectrum of IGR J08408-4503 extracted by using the total exposure time of the observation carried out on 2007 May 29. The model used for the fit is an absorbed CUTOFFPL plus a MKL (the cutoff energy is fixed at 11 keV). The residuals from the fit are shown in the bottom panel. The middle panel shows the residuals of the fit obtained by using only an absorbed CUTOFFPL model (cutoff energy is fixed at 11 keV). |
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Table 3: The same as Table 2, but for the time-resolved spectra of IGR J08408-4503.












By looking at the results of the fits in Table 3, we conclude
that the increase in the source hardness ratio with the count rate can
be most likely ascribed to a variation in the photon index, ,
rather than a change in the absorption column density or in the
properties of the soft component. Indeed, the value of
,
as well as the temperature and normalization of the BB and the
MKL components, are compatible with being constant (to within
the errors) in the three spectra. That most of the changes in the X-ray
flux of the source occurred in the 1.5-10 keV energy band,
where the contribution of the CUTOFFPL component is much greater than
that of the soft component (see Fig. 9),
added support fot the above conclusion.
To further constrain the soft component more tightly, we also
extracted the source spectrum by using the total available exposure
time of the XMM-Newton observation. A fit to this
spectrum with a simple BB or a PL model did not provide an acceptable
result (
/d.o.f. = 4.9/101,
1.8/101, respectively for the BB and the PL model). A CUTOFFPL model
only marginally improved the fit (
/d.o.f. = 1.5/100)
and some structures remained present in the residuals (see
Fig. 13).
To obtain an acceptable fit to the data, we thus used the same spectral
models that we adopted for the rate-resolved analysis. All these models
provided an equivalently good fit to the data. The results of these
fits are given in Table 4 and
discussed in detail in the following section. We note that when fitting
the total spectrum of IGR J08408-4503 we
discarded data in the energy range 0.4-0.6 keV, as we noted
that in this energy range the background was rather high. Including
these points does not affect the best-fit values of the model
parameters, but indicates that the fit with a CUTOFFPL+BB model is
slightly preferable (
/d.o.f. = 1.0/104)
than the CUTOFFPL+MKL model (
/d.o.f. = 1.1/104).
Further XMM-Newton observations of this source
are probably needed to resolve this issue. In Fig. 13, we show the
spectrum of IGR J08408-4503 accumulated
over the entire exposure of the observation and fitted with the
MKL+CUTOFFPL model. In this figure, we also show for comparison the
residuals obtained by fitting the same spectrum with a simple absorbed
CUTOFFPL model (
keV, fixed). The
unfolded spectrum of IGR J08408-4503 is
shown in Fig. 14.
We searched for pulsations in the power spectra of the XMM-Newton
observation of IGR J08408-4503 by using
the same technique described in Sect. 4.1. No
significant (above 3
level) signal was detected in these data. We determined an upper limit
to the pulsed fraction of 25%, 30%, and 40% for periods in the range
0.3-50 s, 50-100 s, and 0.15-0.3 s,
respectively (3
c.l.).
Table 4: The same as Table 3, but for the spectrum of IGR J08408-4503 obtained by using the total available exposure time of the XMM-Newton observation.
5 Discussion
We have presented the first deep pointed observations of the two prototypical SFXTs, XTE J1739-302 and IGR J08408-4503 in quiescence.
![]() |
Figure 14: The unfolded spectrum of the XMM-Newton observation of IGR J08408-4503. The best-fit model represented here is obtained by using an absorbed CUTOFFPL (cutoff energy fixed at 11 keV) plus a MKL component. |
Open with DEXTER |
The two sources exhibited a very complex timing and spectral variability, and we discuss them separately below. Here we also carry out a comparison between their quiescent and outburst emission.
5.1 XTE J1739-302
The two XMM-Newton observations analyzed in
Sect. 4.1
show that the quiescent emission of XTE J1739-302
is characterized by a number of low-intensity flares, taking place
sporadically from a lower persistent emission level. The typical
duration of these flares is a few thousands of seconds, and their X-ray
flux is a factor of 10-30
higher than the persistent quiescent flux. During the time intervals
when the source was at its lowest level of emission, we measured an
X-ray flux of
erg cm-2 s-1
(0.5-10 keV). This corresponds to a luminosity of
erg s-1
at 2.7 kpc, and is among the lowest values of X-ray luminosity
measured from XTE J1739-302 (a factor of
2 lower than
the 3
upper limit reported by Sakano
et al. (2002), and comparable with the lowest
luminosity reported by Romano
et al. 2009b). The total dynamic range of the X-ray
luminosity of XTE J1739-302 between
outburst and quiescence is thus
104
(see Sect. 1).
The hardness intensity diagrams and rate resolved analysis
carried out in Sect. 4
showed that the variations in the X-ray flux measured during the XMM-Newton
observations were accompanied by a change in the spectral properties of
the source, the source hardness ratio increasing significantly with the
count rate. A fit to the rate-resolved spectra with a CUTOFFPL model (
keV Sidoli et al. 2009a)
implies that this behavior originates from a change in the CUTOFFPL
photon index,
,
rather than in a variation of the absorption column density.
These results indicate that the timing and spectral
variability of XTE J1739-302 during the
quiescence is qualitatively similar to that observed during the
outbursts (see Sect. 1).
A change in the PL photon index with the X-ray flux of XTE J1739-302
was first noticed by Smith
et al. (1998). These authors analyzed several
different flares caught by RXTE, INTEGRAL and ASCA, and showed that the
photon index of the hard (2-10 keV) X-ray emission from XTE J1739-302
changed from 0.8, when the source X-ray flux was
erg s-1
(2-10 keV), to 2.0, when the X-ray flux was
erg s-1
(2-10 keV). However, these flares were also characterized by a
significant change in the absorption column density (from 3 to
cm-2).
Similar values of the PL photon index and absorption column density
were reported for the outbursts observed with Swift
from this source (see Sect. 1). In the
long-term monitoring of XTE J1739-302
carried out with Swift, Romano
et al. (2009b) identified three different states in
the source X-ray flux (Sect. 1), and showed
that each of these states could be characterized by a different PL
photon index (in the range 0.5-1.6) and a different absorption column
density (from 0.3 to
cm-2).
Considering all of these results, it seems unlikely that there exists a
single monotonic variation in both the absorption column density and PL
photon index with the source intensity across its quiescent and
outburst state.
When the X-ray spectrum of XTE J1739-302
from the entire XMM-Newton observation is
considered, the interpretation of the emission from this source becomes
even more complicated. In particular, this spectrum cannot be fit
successfully by using a simple absorbed PL or a BB model. We indeed
showed in Sect. 4.1
that these models provided only a poor fit to the data, and a more
refined model was required. Because of the relatively low X-ray flux of
the source, different models could equivalently describe the data. From
a statistical point of view, the CUTOFFPL with a variable cutoff energy
would be preferable, as it provided a very good fit to the data of both
OBS1 and OBS2
and requires a lower number of free parameters with respect to the
others models reported in Table 2. However, this
model would require a cutoff energy much lower (4 keV) than the value determined when
the source was in outburst (
13 keV).
This low value for the cutoff energy might not be unlikely for an X-ray
pulsar (see e.g., the cases of X-Persei and RX J0146.9+6121
, Di Salvo et al. (1998);
Haberl
et al. (1998), and Coburn
et al. (2002) for a review). However, we suggested
that, given our relatively poor knowledge of the quiescent emission of XTE J1739-302
and of SFXTs in general, the applicability of other spectral models
might be worth exploring.
Besides the CUTOFFPL, the quiescent spectrum of XTE J1739-302
might require an additional spectral component at the lower energies
(<2 keV). In Sect. 4.1, we
showed that a fit to the spectra of OBS1 and OBS2 with a CUTOFFPL model
and a fixed keV
would leave some evident structures in the residuals from the fit and
the addition of a BB, or a MKL component can significantly improve the
results. Even if these models require one more free parameter than the
CUTOFFPL with a variable
,
the results we obtained would then be in agreement with those found in
the case of IGR J08408-4503. The
spectrum of this source could, indeed, not be reproduced using a simple
CUTOFFPL model (see below).
A soft spectral component below 2 keV might be
expected in the spectra of the SFXT sources, as this component is very
common in binaries hosting a NS accreting mass from a massive
companion. Hickox et al.
(2004) showed that the detectability of this component
depends mainly on the amount of absorption in the direction of the
sources. According to their results, in the most luminous objects (
erg s-1)
the soft component is produced by reprocessing hard X-rays from the NS
by some optically thick material (e.g., an accretion disk), whereas for
sources with lower luminosities (<1036 erg s-1)
the most likely origin of the soft component is the emission from
either a photoionized or collisionally heated diffuse gas in the binary
system or from the NS surface. In the case of XTE J1739-302,
we found that BB emission from a relatively cold (
0.1 keV) and large (
100 km
equivalent radius) region or from a much hotter (
1 keV) and less
extended (<100 m) spot, or, alternatively, emission
from an optically thin gas (MKL) provided equally good fits to the
data. While the discussion above and the rapid variability observed in
the SFXT would argue against the presence of an accretion disk in these
sources (see also, Bozzo
et al. 2008), we also consider that emission from a
small and hot spot on the NS surface is inconsistent with models of
accretion onto magnetic NS. The accretion flow in SFXT is, indeed,
thought to be quasi-spherical, and expected to penetrate the NS
magnetosphere mainly by means of the Rayleigh-Taylor and the
Kelvin-Helmholtz instability (see e.g., Bozzo
et al. 2008, and references therein). In these
circumstances, the size of the hot-spot over which the accretion takes
place is expected to be inversely proportional to the X-ray luminosity
and might cover a substantial fraction of the NS surface for
erg s-1
(see White et al. 1983,
and references therein).
Taking these results into account, emission from an optically
thin and diffuse gas around the NS seems to be a more reasonable
explanation of the soft spectral component of XTE J1739-302.
According to this interpretation, the emitting region can be estimated
from the normalization of the MKL component (see Table 2)
using the relation (see e.g., Zurita Heras
& Walter 2009, and references therein):
a132/3 cm,
where
is the normalization of the MKL component, D is the source distance,
/a,
a is the binary separation, and
a13=a/1013 cm.
Therefore, the radius
of the emitting region is compatible with the binary separation for a
wide range of values of orbital periods similar to those measured in
other SFXTs. We note also that the properties of this MKL component
would be rather similar to those derived from the spectrum of the SFXT
AX J1845.0-0433 (Zurita Heras
& Walter 2009).
Another possibility that we investigated in Sect. 4.1 is the applicability of a model comprising a power law component and a partial covering to the X-ray spectrum of XTE J1739-302. We concluded that this model can also provide a reasonably good fit to the data. A similar model was proposed to interpret the quiescent XMM-Newton spectrum of the SFXT IGR J16207-5129 (Tomsick et al. 2009) and might provide support for clumpy wind in these sources. In this case, one would expect part of the radiation from the NS to escape absorption by the clumps local to the source and be affected only by interstellar absorption (see, e.g. Walter & Zurita Heras 2007). Finally, we showed that the COMPTT model also provided a reasonable fit to the XMM-Newton spectra of XTE J1739-302, and should thus be considered a valid alternative to the other models discussed above. We note, however, that neither the partial covering nor the COMPTT model could give an acceptable fit to the spectrum of IGR J08408-4503 (see also below). Given the similarities between the two sources, a spectral model that provides acceptable results in both cases should probably be favored (e.g., the CUTOFFPL+MKL model).
5.2 IGR J08408-4503
The XMM-Newton observation of IGR J08408-4503,
detected a very similar behavior to that discussed above for XTE J1739-302.
In the light curve of IGR J08408-4503, a
number of relatively small flares were observed to take place
sporadically on a timescale of few thousands of seconds and were
characterized by an X-ray flux 10-15 times higher than the underlying fainter
persistent emission. The lowest X-ray flux that we measured from this
source was
erg cm-2 s-1
(0.5-10 keV) and corresponds to a luminosity of
erg s-1
(at a distance of 3 kpc), comparable to the value reported by Kennea & Campana (2006).
The total dynamic range in the X-ray luminosity of IGR J08408-4503
between outburst and quiescence is thus
104
(see also Sect. 1).
As for XTE J1739-302, the
hardness intensity diagrams and the rate resolved analysis carried out
in Sect. 4
showed that the variations in the X-ray flux measured during the XMM-Newton
observation were also accompanied by a significant change in the
spectral properties of the source. In contrast to the case of XTE J1739-302,
the rate-resolved spectra of IGR J08408-4503
could not be fit by using a simple CUTOFFPL model. We showed that an
acceptable fit to the data could, instead, be obtained by introducing
an additional relatively cold (
keV) and extended (
100 km)
BB component, or a MKL model (see Tables 3 and 4). We
note that the parameters measured for the MKL component are rather
similar to those derived in the case of XTE J1739-302.
An equivalently good fit was provided by the BMC model. This model has
the same number of free parameters as the CUTOFFPL+BB (see
Table 3),
and would predict similar properties for the temperature and the size
in which the soft photons are produced (the normalization of the BMC
model is defined as the ratio of the source luminosity to the square of
the distance in units of 10 kpc, see e.g. Sidoli et al. 2009b,
and references therein). Similar values of the fit parameters were also
obtained from the analysis of the spectrum of IGR J08408-4503
extracted by using the total available exposure time of the XMM-Newton
observation (see Sect. 4.2 and
Table 4).
According to the discussion in Sect. 5.1, a BB emission with these properties seems unlikely in the case of IGR J08408-4503 and thus we suggest that the CUTOFFPL+MKL model can provide a more reasonable description of the data. We note that in the XMM-Newton spectrum of the supergiant HMXB IGR J16320-4751 a similar soft component was found that could be fit with a BB of 0.07 keV but was attributed to a cloud surrounding the NS (Rodriguez et al. 2006). Following the CUTOFFPL+MKL interpretation, the rate resolved analysis carried out for the observation of IGR J08408-4503 would indicate that the properties of the MKL component do not change significantly with the source count rate and the increase in the hardness ratio observed in Figs. 11 and 10 is most likely due to a change in the CUTOFFPL photon index. Furthermore, no significant variations in the absorption column density were revealed in the different rate-resolved spectra. This is similar to the results discussed above for XTE J1739-302.
As for XTE J1739-302, a comparison between the results of the present XMM-Newton observation and the observations carried out in the same energy band (0.5-10 keV) when this source was in outburst does not indicate a clear correlation between the power law photon index, the absorption column density, and the source X-ray flux (see Sect. 1). It is interesting that the soft component in this source detected by the XMM-Newton observation appears to have a different origin from that detected by Sidoli et al. (2009b) when IGR J08408-4503 was in outburst (see Sect. 2.2). On that occasion, the soft component appeared to be caused by thermal emission from a hot-spot on the NS surface. We note that, even if the soft component observed by XMM-Newton is interpreted in terms of a BB emission, the emitting region derived from the fit is considerably larger than the NS radius and it is thus unlikely that it is produced on the NS surface.
Finally, for both XTE J1739-302
and IGR J08408-4503, we investigated
whether the harder spectral component detected in these sources might
be produced by the X-ray emission from the NS supergiant companion. The
time-averaged X-ray luminosity that we measured from these sources in
quiescence matches quite well the luminosity expected from an isolated
OB supergiant or from colliding winds in a binary containing OB
supergiant stars (see e.g., Gudel
& Nazel 2009, for a review). However, this
interpretation appears to be contrived
for the following reasons. The X-ray spectrum of isolated or colliding
wind binaries with OB supergiant stars is usually described well by a
model comprising one or more thermal components (MKL in XSPEC,
see e.g., Gudel & Nazel 2009).
The softer MKL component has a typical temperature of 0.2-0.7 keV,
and is thus similar to those we detected in IGR J08408-4503
and XTE J1739-302. This component is
thought to be generated by the shocks within the stellar wind. The
hotter MKL component, extending up to several keV, can have a
temperature as high as
1-3 keV
and is characterized by a number of very prominent emission lines (see
also Raassen et al. 2008).
This hard component is usually interpreted in terms of magnetically
confined wind shocks, highly compressed wind shocks, or inverse Compton
scattering of photospheric UV photons by relativistic particles
accelerated within the shocks (Albacete
Colombo et al. 2007). Possible detections of a
non-thermal X-ray emission from OB supergiant stars were reported in
only two cases, but they still lack confirmation (Gudel
& Nazel 2009).
The X-ray spectra of XTE J1739-302
and IGR J08408-4503 were reproduced well
using a CUTOFFPL model, and no prominent emission lines were detected.
The values of the photon index, ,
derived from XMM-Newton are also comparable to
those obtained previously when the sources were in outburst, thus
suggesting that a common mechanism produces their harder X-ray
component. Furthermore, the relatively rapid X-ray variability (of
period few thousands seconds) observed in the lightcurves of IGR J08408-4503
and XTE J1739-302 is not reminiscent of
the typical X-ray variability of the OB stars, which, when present,
takes place on longer timescales (tens of ks, see e.g., Albacete Colombo et al. 2007).
We conclude that the harder X-ray emission from XTE J1739-302 and IGR J08408-4503 is most likely due to residual accretion taking place onto the NS at a much lower rate than during outburst.
6 Conclusions
The three XMM-Newton observations that we have
analyzed in the present work, indicate that the quiescent spectra of
the two prototypical SFXT XTE J1739-302
and IGR J08408-4503 are characterized by
two different spectral components, one dominating the spectrum at the
softer energies (2 keV)
and the other one being more prominent above 2 keV.
The properties of the soft component (2 keV) could be
reasonably well constrained in the case IGR J08408-4503,
where the absorption column density was relatively low (<1022 cm-2),
whereas in the case of XTE J1739-302 the
detection of this component is less significant. However, the
similarity in the timing and spectral behavior observed in the
quiescent state of the two sources argues in favor of adopting the same
spectral model for both of them. We suggested that the model comprising
a CUTOFFPL component at the higher energies plus a MKL component would
provide a reasonable description of the data and a plausible physical
explanation of the properties observed in the two sources
(Sect. 5).
According to this interpretation, the MKL component would represent the
contribution to the total X-ray emission of the shocks in the wind of
the supergiant companion.
The results of the fits with this model to the data of the three
observations inferred a temperature of the MKL component and an
emitting region comparable with the values found also in the case of
the SFXT AX J1845.0-0433.
Similar soft spectral components have been detected in many other HMXBs and SGXBs. In a few cases, the detection of a number of prominent emission lines in the high resolution X-ray spectra of these sources carried out with the gratings onboard Chandra and the RGS onboard XMM-Newton (see e.g., Watanabe et al. 2006) have convincingly demonstrated that these components are produced by the stellar wind around the NS, and proved to be a powerful diagnostic to probe the structure and composition of the stellar wind in these systems. The statistics of the present XMM-Newton observations is far too low to permit a similar in-depth study of the stellar wind in the case of XTE J1739-302 and IGR J08408-4503. Furthermore, because of the relatively low luminosity and the high absorption that characterize the emission of these sources in quiescence, observations at higher spectroscopic resolution are probably too challenging for the present generation of X-ray satellites, and the improved sensitivity of the X-ray spectrometers planned for future X-ray missions (e.g. IXO) is probably required to firmly establish the presence of a soft spectral component in the quiescent emission of the SFXT sources and shed light on its nature.
If the harder X-ray emission (2-10 keV) detected from
the XMM-Newton observations of XTE J1739-302
and IGR J08408-4503 was really produced
by residual accretion as we argued in the previous section, then the
accretion process in these sources would take place over more than 4
orders of magnitude of X-ray luminosity.
This is similar to the results reported for the SFXT
IGR J17544-2619 (Rampy
et al. 2009) and, possibly, for the SFXT
SAX J1818.6-1703 (in the latter case the origin of the lowest
quiescent emission remains unclear, Bozzo
et al. 2009). In the case of
IGR J17544-2619, Rampy
et al. (2009) ascribed the high dynamic range in the
X-ray luminosity to the accretion of clumps from the wind of the
supergiant star with a high density contrast with respect to the
surrounding homogeneous wind. However, it was also suggested that a
similar variability might result from the transition across different
accretion regimes onto the NS (Bozzo
et al. 2008). We note that a similar scenario can be
envisaged for interpreting the variations in the X-ray flux observed
during the multiple small flares detected in the present observations.
Even though they took place at a much lower luminosity level than the
brightest outbursts (a factor of
103-104),
our analysis showed that all these events shared a number of similar
timing and spectral properties. In particular, the timescales on which
the smaller flares develop is comparable with the decay timescale of
the source luminosity during the outbursts (see Sect. 1), and the
spectral photon indices and absorption column densities measured from
the XMM-Newton observations are also in
qualitative agreement with those reported previously when the sources
were observed at a much higher X-ray luminosity level (see
Sect. 2).
It is thus most likely that the transitions between the lower quiescent
states and the small flares detected by XMM-Newton
from XTE J1739-302 and IGR J08408-4503
might have been triggered by the same mechanism that sometimes gives
rise to the brightest outbursts (i.e., the accretion of clumps from the
stellar wind and/or the transition between different accretion regimes
of the NS, see Sect. 1).
In contrast to the case for the SFXT IGR J18483-0311, we did not detect any pulsation in the quiescent emission of either XTE J1739-302 or IGR J08408-4503, and provided in Sects. 4.1 and 4.2 the corresponding upper limits to the spin periods and pulsed fractions we were able to infer from the present data.
Deep pointed observations of SFXTs in quiescence are still required in order to understand the origin of the peculiar X-ray variability of these sources and distinguish between different models proposed to interpret their behavior.
AcknowledgementsE.B. thanks N. Schartel and the XMM-Newton staff for the rapid schedule of the XMM-Newton observation of XTE J1739-302 after the outburst occurred on 2009 March 10, and R. Farinelli for helpful discussions. We thank the anonymous referee for useful comments.
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Footnotes
- ...
cameras
- See http://xmm2.esac.esa.int/docs/documents/CAL-TN-0018.pdf.
- ...
thread
- See also http://xmm.esac.esa.int/sas/current/documentation/threads/PN_spectrum_thread.shtml.
- ... IGR J16207-5129
- This source was classified as an intermediate SFXT by Walter & Zurita Heras (2007).
- ...
RX J0146.9+6121
- Note that these authors used an absorbed power-law with exponential high-energy cutoff (HIGHECUT*PL in XSPEC) to fit their data.
- ... luminosity
- That we did not find any evidence for X-ray eclipses in the three XMM-Newton observations that we analyzed (this is unlike the case of IGR J16479-4514, see Sect. 1) is also consistent with this interpretation.
All Tables
Table 1: Results of the count-rate-resolved spectral analysis for the two XMM-Newton observations of XTE J1739-302.
Table 2: Spectral fits of the XMM-Newton data of XTE J1739-302.
Table 3: The same as Table 2, but for the time-resolved spectra of IGR J08408-4503.
Table 4: The same as Table 3, but for the spectrum of IGR J08408-4503 obtained by using the total available exposure time of the XMM-Newton observation.
All Figures
![]() |
Figure 1: Selection of the high background intervals during the two XMM-Newton observations (Epic-PN camera) of XTE J1739-302 (OBS1 on the left and OBS2 in the middle) and the observation of IGR J08408-4503 (on the right). In each case, we reported the source lightcurve not corrected for the selection of the good time intervals and not subtracted for the background in the 0.2-12 keV (upper panel) and 10-12 keV energy band (middle panel), and the count rate of the total FOV in the 10-12 keV energy band (lower panel). In all cases, the time bin is 100 s. Only the observational intervals in which the total FOV count rate in the 10-12 keV energy band was below the threshold indicated with a dashed line were considered for the timing and spectral analysis of the sources. |
Open with DEXTER | |
In the text |
![]() |
Figure 2: XMM-Newton Epic-PN background-subtracted lightcurve of XTE J1739-302 during the observation carried out on 2008 October 1. The upper panel shows the source lightcurve in the 0.2-2 keV energy band, whereas the middle panel gives the lightcurve in the 2-15 keV energy band (the binning time is 300 s). The ratio of the source count rate in the two bands, (2-15 keV)/(0.2-2 keV), versus time is shown in the lower panel. The time intervals in which no data are plotted have been discarded because of high background events. |
Open with DEXTER | |
In the text |
![]() |
Figure 3: The same as Fig. 2, but for the XMM-Newton observation carried out on 2009 March 11. The binning time is 300 s. |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Hardness-intensity diagram for the two XMM-Newton observations of XTE J1739-302 (the upper panel is for the observation carried out in 2008, the lower panel for the observation in 2009). The hardness is defined as the ratio of the count rate in the hard (2-15 keV) to soft (0.2-2 keV) energy band. The points were obtained by using the lightcurves given in Figs. 2 and 3, but, wherever necessary, consecutive bins were rebinned so as to achieve a S/N>5.5 in the hardness ratio. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: The XMM-Newton spectra of the XTE J1739-302 observation carried out in 2008. The open circles, open squares, and filled circles are from the energy resolved data obtained during the time intervals in which the Epic-PN 0.2-15 keV source count rate was >0.4, 0.1-0.4, <0.1, respectively. The model used for the best fits is an absorbed CUTOFFPL (cutoff energy fixed at 13 keV). The residuals from this fit are shown in the bottom panel. (See the electronic edition of the paper for a color version of this figure.) |
Open with DEXTER | |
In the text |
![]() |
Figure 6: The same as Fig. 5 but for the observation carried out in 2009. The model used for the best fits is an absorbed CUTOFFPL (cutoff energy fixed at 13 keV). The residuals from these fits are shown in the bottom panel. (See the electronic edition of the paper for a color version of this figure.) |
Open with DEXTER | |
In the text |
![]() |
Figure 7: The XMM-Newton spectrum of XTE J1739-302 extracted by using the total exposure time of OBS1. The model used for the best fit is an absorbed CUTOFFPL (we did not fix here the cutoff energy). The residuals from this fit are shown in the bottom panel, whereas the middle panel shows the residuals from the fit obtained by using an absorbed CUTOFFPL with the cutoff energy fixed at 13 keV. |
Open with DEXTER | |
In the text |
![]() |
Figure 8: The same as Fig. 7, but for the observation carried out on 2009 March 11. Here the best-fit model to the data was an absorbed CUTOFFPL model (the cutoff energy was allowed to vary during fitting). |
Open with DEXTER | |
In the text |
![]() |
Figure 9: An example of an unfolded spectrum of the XMM-Newton observation of XTE J1739-302 carried out in 2008 (OBS1). Here the best fit is obtained by using an absorbed CUTOFFPL plus a MKL component (see Sect 4.1). |
Open with DEXTER | |
In the text |
![]() |
Figure 10: XMM-Newton Epic-PN background-subtracted lightcurve of IGR J08408-4503 during the observation of 2007 May 29. The upper and middle panel shows the source lightcurve in the 0.2-2 keV and 2-15 keV energy bands, respectively. The binning time is 300 s. The ratio of the source count rate in the two bands versus time is shown in the lower panel. |
Open with DEXTER | |
In the text |
![]() |
Figure 11:
Hardness-intensity diagram for IGR J08408-4503.
The diagram was created by using the 0.2-2 keV and
2-15 keV lightcurves of Fig. 10, but rebinned
so as to achieve a |
Open with DEXTER | |
In the text |
![]() |
Figure 12:
The same as Figs. 5 and 6 but
for the observation of IGR J08408-4503.
Here the open circles, open squares, and filled circles refer to the
spectrum accumulated during the time intervals of the observation in
which the Epic-PN 0.2-15 keV source rate was >0.2,
0.1-0.2, <0.1 counts/s respectively. The model used for
the best fits comprises a MKL plus a CUTOFFPL component (
|
Open with DEXTER | |
In the text |
![]() |
Figure 13: The XMM-Newton spectrum of IGR J08408-4503 extracted by using the total exposure time of the observation carried out on 2007 May 29. The model used for the fit is an absorbed CUTOFFPL plus a MKL (the cutoff energy is fixed at 11 keV). The residuals from the fit are shown in the bottom panel. The middle panel shows the residuals of the fit obtained by using only an absorbed CUTOFFPL model (cutoff energy is fixed at 11 keV). |
Open with DEXTER | |
In the text |
![]() |
Figure 14: The unfolded spectrum of the XMM-Newton observation of IGR J08408-4503. The best-fit model represented here is obtained by using an absorbed CUTOFFPL (cutoff energy fixed at 11 keV) plus a MKL component. |
Open with DEXTER | |
In the text |
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