 "SFXTs: Eine neue Klasse von Röntgendoppelsternen"
INTEGRAL (Main)
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Supergiant Fast X-ray Transients (SFXTs)
K1 
Supergiant Fast X-Ray Transient Sources
Zum Thema:
Supergiant Fast X-Ray Transient Sources | |
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K1.1
SFXT IGR J16479-4514
K1.2
Supergiant Fast X–ray Transients with Swift
K1.3
SFXTs with Swift: Rise to the outburst in IGR J16479-4514
| IGR J16479-4514 — fX(20–60 keV) = 10-9 erg cm-2 s-1 | |
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Authors: Romano, P.; Sidoli, L.; Mangano, V.; Vercellone, S.; Kennea, J.A.; Cusumano, G.; Krimm, H.A.; Burrows, D.N.; Gehrels, N. |
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Journal-ref: ApJ 680 (2008) L137 [0805.2089  ] |
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Title: Monitoring Supergiant Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514 |
| Abstract:
IGR J16479-4514 is a Supergiant Fast X-ray Transient (SFXT), a new class of High Mass X-ray Binaries, whose
number is rapidly growing thanks to the observations of the Galactic plane performed with the INTEGRAL
satellite.
IGR J16479-4514 has been regularly monitored with Swift/XRT since November 2007, to study the quiescent emission, the outburst properties and their recurrence. A new bright outburst, reaching fluxes above fX(20–60 keV) = 10-9 erg cm-2 s-1, was caught by the Swift/BAT. Swift immediately re-pointed at the target with the narrow-field instruments so that, for the first time, an outburst from a SFXT where a periodicity in the outburst recurrence is unknown could be observed simultaneously in the 0.2-150 keV energy band. The X-ray emission is highly variable and spans almost four orders of magnitude in count rate during the Swift/XRT observations covering a few days before and after the bright peak. The X-ray spectrum in outburst is hard and highly absorbed. The power-law fit resulted in a photon index of G = 0.98, and in an absorbing column density NH of ~ 5 × 1022 cm-2. These observations demonstrate that in this source (similarly to what was observed during the 2007 outburst from the periodic SFXT IGR J11215-5952), the accretion phase lasts much longer than a few hours. 1. Introduction
Supergiant Fast X–ray Transients (SFXT) are a new class of High Mass X–ray Binaries associated with blue
supergiant companions, several members of which were discovered thanks to the INTEGRAL observations of the
Galactic plane (Sguera et al. 2005). They are sources with transient X–ray emission concentrated in short and
bright flares (with a typical duration of a few hours), a peak luminosity in the range of
LX ~ 1036-37 erg s-1 and a quiescent level of
1032 erg s-1. The short duration flaring activity is part of a
longer accretion phase at a lower level (Romano et al. 2007).
IGR J16479-4514 is a hard X-ray transient discovered by INTEGRAL in 2003 August (Molkov et al. 2003). Hard X–ray activity was observed on August 8 and 9, at a level of ~ 12mCrab (18–25 keV), while the following day the flux increased by a factor of ~2. Other outbursts caught with INTEGRAL in 2003 and 2004–2005were reported by Sguera et al. (2005) and Sguera et al. (2006), respectively, with peak fluxes above 10-9 erg cm-2 s-1 (20–60 keV). The recurrent and short outbursts observed from this source led to a suggestion that it belongs to the SFXT class. A frequent hard X–ray (E>20 keV) flaring activity was recently discussed by Walter & Zurita Heras (2007), who report on 27 short (duration <15 ks) flares and on 11 long (>15 ks) flares in archival INTEGRAL data, spanning times from 2003 January 11 to 2005 December 2, with variable fluxes. Rahoui et al. (2008) classify the optical companion as an O8.5I star located at ~4.9 kpc. This, together with the highly variable X–ray flux, confirms the classification as a SFXT. In these sources, a subclass of HMXBs, the accretion onto the compact object is very likely through the strong wind from the supergiant donor. The orbital parameters of IGR J16479-4514 as well as the nature of the compact object are still unknown, although the X–ray spectral properties are similar to those of accreting pulsars. Thus, the compact object is probably a neutron star, as in IGR J11215-5952,which hosts a pulsar with a period of ~187 s (Swank et al. 2007). References Rahoui, F., Chaty, S., Lagage, P., et al. 2008, | |
K1.4
IGR J17544-2619 & XTE J1739-302 in Outburst
| IGR J17544-2619 (d = 3.6 kpc) — XTE J1739-302 (d = 2.7 kpc) — Outbursts | |
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Authors: Sidoli, L.; Romano, P.; Mangano, V.; Cusumano, G.; Vercellone, S.; Kennea, J. A.; Paizis, A.; Krimm, H. A.; Burrows, D. N.; Gehrels, N. |
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Journal-ref: ApJ (2008) [0808.3085 ] |
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Title: Monitoring Supergiant Fast X-ray Transients with Swift. III.
Outbursts of the prototypical SFXTs IGR J17544-2619 and XTE J1739-302 |
| Abstract:
IGR J17544-2619 and XTE J1739-302 are considered the prototypical sources of the new class of
High Mass X-ray Binaries, the Supergiant Fast X-ray Transients (SFXTs).
These sources were observed during bright outbursts on 2008 March 31 and 2008 April 8, respectively, thanks to an on-going monitoring campaign we are performing with Swift, started in October 2007. Simultaneous observations with XRT and BAT allowed us to perform for the first time a broad band spectroscopy of their outbursts. The X-ray emission is well reproduced with absorbed cutoff powerlaws, similar to the typical spectral shape from accreting pulsars. IGR J17544-2619 shows a significantly harder spectrum during the bright flare (where a luminosity in excess of LX = 1 × 1036 erg s-1 is reached) than during the long-term low level flaring activity (1033-34 erg s-1), while XTE J1739-302 displayed the same spectral shape, within the uncertainties, and a higher column density during the flare than in the low level activity. The light curves of these two SFXTs during the bright flare look similar to those observed during recent flares from other two SFXTs, IGR J11215-5952 and IGR J16479-4514, reinforcing the connection among the members of this class of X-ray sources. 1. Introduction
IGR J17544-2619 and XTE J1739-302 are confirmed members of the new sub-class of High Mass X–ray Binaries,
the Supergiant Fast X–ray Transients (SFXTs), whose members have been mainly discovered with the INTEGRAL
satellite (see e.g. Sguera et al. 2005).
SFXTs are characterized by X–ray transient emission during short (a few hours long) flares reaching a few 1036-37 erg s-1 and they are associated with blue supergiant companions (e.g. Negueruela et al. 2006b and Smith et al. 2006). The quiescent state in SFXTs have been observed only in a few sources and is characterized by a soft spectrum and X–ray luminosity at a level of 1032 erg s-1, thus a very large dynamic range of about 103-4 has been observed. The short duration bright flares are part of a longer accretion phase at a lower level (Romano et al. 2007). When not in outburst, these sources spend most of their lifetime in accretion at an intermediate (and flaring) level of X–ray luminosity, of 1033-34 erg s-1 (Sidoli et al. 2008b). The spectral properties are reminiscent of those of accreting pulsars, thus it is likely that several members of the class are actually hosting neutron stars, although the spin period has been measured only in two SFXTs (AX J1841.0–0536, Bamba et al. 2001; IGR J11215–5952, Swank et al. 2007). 
IGR J17544-2619 was discovered (Sunyaev et al. 2003) with IBIS/ISGRI on-board INTEGRAL on 2003 September
17 during a 2 hour flare reaching 160 mCrab (18–25 keV). During a Chandra observation, both the quiescence level
and the onset of an outburst was caught (in’t Zand 2005), observing a dynamic range as large as 104.
The optical counterpart is an O9Ib star (Pellizza et al. 2006) located at 3.6 kpc (Rahoui et al. 2008). Several other bright flares have been observed with INTEGRAL in 2003, 2004 and 2005, with flare durations ranging from 0.5 to about 10 hours, reaching peak fluxes of 400 mCrab (20–40 keV). More recently, two new outbursts were detected with the Swift satellite, on 2007 November 8 (Krimm et al. 2007) and on 2008 March 31 (Sidoli et al. 2008a), 144 days apart. The flux at peak observed with Swift/BAT was 165 mCrab (20–40 keV).
XTE J1739-302 was discovered with RXTE after a short outburst in August 1997 (Smith et al. 1998), and
displayed a spectrum well fitted with a bremsstrahlung model with a temperature
of ~ 22 keV, reaching a peak flux of
fX(2–25 keV) ~ 4 × 10-9 erg cm-2 s-1. Later,
several other short flares were observed with RXTE/PCA (Smith et al. 2006). The optical
counterpart is an O8I star (Negueruela et al. 2006a) located at 2.7 kpc (Rahoui et al. 2008).
Upper limits to the quiescent emission were placed with ASCA observations (Sakano et al.
2002) at a level of < 10-12 erg cm-2 s-1. Bright outbursts
(up to 300 mCrab) were detected with IBIS/ISGRI in 2003 March, and 2004 March (Sguera et al. 2006). Frequent
flaring activity with INTEGRAL has been reported by Walter & Zurita Heras (2007). Recently, it triggered the
Swift Burst Alert Telescope (BAT). An immediate slew allowed us to monitor the brightest part of a flare at soft
energies (Romano et al. 2008a) with the Swift X-ray Telescope (XRT). This outburst was also observed by the
INTEGRAL/JEM-X monitor, which detected a flare starting 5 hours before the flares seen with Swift
(Chenevez et al. 2008).
4. Discussion
The X–ray spectroscopy shows that these two SFXTs, which are considered the prototypes of this new class of
HMXBs, have different properties during the bright flares. IGR J17544-2619 is one order of magnitude less
absorbed than XTE J1739-302, and displays a significantly flatter spectrum below 10 keV.
Different mechanisms have been proposed to explain the bright and short duration flaring activity in this new class of sources. Some models are related to the structure of the supergiant. Sidoli et al. (2007) explain the outbursts as being due to enhanced accretion onto the neutron star when it crosses, moving along the orbit, an equatorial wind disk component from the supergiant companion. Depending on the thickness and truncation of this supposed disk wind and on its inclination with respect to the orbital plane of the binary system, the compact object will cross once or twice in a periodic or quasi-periodicmanner the disk, undergoing outbursts. In the framework of this model, the geometry, the structure of this disk wind and its inclination with respect to the line of sight could explain the variability in the local absorbing column density, even during different outbursts (as observed several times in XTE J1739-302) and compared with the low level activity. References Bamba, A., et al. 2001, PASJ 53, 1179 | |
K1.5
IGR J17544-2619 & XTE J1739-302 in out-of-outburst
| IGRJ17544-2916 and XTEJ1739-302 — LX(out-of-flare) = 1033-34 erg s-1 | |
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Authors: Sidoli, L.; Romano, P.; Cusumano, G.; Mangano, V.; Vercellone, S.; Paizis, A.; Pellizzoni, A.; Kennea, J.A.; Burrows, D.N.; Krimm, H.A.; Gehrels, N.; Guidorzi, C.; Evans, P.A. |
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Journal-ref: 7th INTEGRAL Workshop (2008) [0810.5446 ] |
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Title: Swift monitoring of Supergiant Fast X-ray Transients: the out-of-outburst behaviour and the flares from IGR J17544-2916 and XTE J1739-302 |
| Abstract:
Supergiant Fast X-ray Transients (SFXTs) are a sub-class of High Mass X-ray Binaries (HMXBs) associated with
OB supergiant companions and displaying transient X-ray activity. This behaviour is quite surprising
since HMXBs hosting supergiants were known to be persistent sources, until the INTEGRAL discoveries obtained
by means of the monitoring of the Galactic plane.
We have been performing a monitoring campaign with Swift of four SFXTs with the main aim of characterizing both the long-term behaviour of these transients and the properties during bright outbursts. Here we discuss the properties of the X-ray emission observed outside the outbursts as well as the flares observed from two SFXTs: IGRJ17544-2916 and XTEJ1739-302. Contrarily to what previously thought, Swift allowed us to discover that SFXTs spend most of the time in accretion at a low level, even outside the bright outbursts, with an accretion luminosity of 1033-34 erg s-1, and that the quiescent level at ~1032 erg s-1 is a much rarer state. 1. Supergiant Fast X–ray Transients before Swift
The Galactic plane monitoring performed with the INTEGRAL satellite led to the discovery
of several new sources [2]. Some of them displayed sporadic, recurrent, bright
and short flares, with a typical duration of a few hours and reaching a peak luminosity of
1036-37 erg s-1 (Sguera et al, 2005, 2006; Negueruela et al. 2006).
Refining the INTEGRAL positions at arcsec level with X–ray follow-up observations, allowed the association with OB supergiant companions (e.g. Halpern et al. 2004; Pellizza et al. 2006; Masetti et al. 2006; Negueruela et al. 2006; Nespoli et al. 2008). The similarities of the SFXTs with the properties of accreting pulsars suggest that the majority of these transients are indeed HMXBs hosting a neutron star, although only in three SFXTs X–ray pulsations have been discovered: • IGR J11215–5952 (Pspin ~ 186.8 s, Swank et al. 2007); • AX J1841.0–0536 (Pspin ~ 4.7 s, Bamba et al. 2001) and • IGR J18483–0311 (Pspin ~ 21 s, Sguera et al. 2007). References [1] Bamba, A., et al. 2001, PASJ 53, 1179 | |
K2.1
IGR J08408-4503
| IGR J08408-4503 — d ~ 2.7 kpc — LX(flare) = 6 × 1035 erg s-1 | |
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Authors: J-C Leyder, R. Walter, M. Lazos, N. Masetti, N. Produit |
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Journal-ref: A&A 465 (2007) L35-L38 [0712.1200 ] |
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Title: Hard X-ray flares in IGR J08408-4503 unveil clumpy stellar winds |
| Abstract:
• Context : A 1000-s flare from a new hard X-ray transient, IGR J08408-4503, was observed by INTEGRAL on May 15, 2006 during the real-time routine monitoring of IBIS/ISGRI images performed at the INTEGRAL Science Data Centre. The flare, detected during a single one-hour long pointing, peaked at 250 mCrab in the 20-40 keV energy range. • Aims : Multi-wavelength observations, combining high-energy and optical data, were used to unveil the nature of IGR J08408-4503. • Methods : A search in all INTEGRAL public data for other bursts from IGR J08408-4503 was performed, and the detailed analysis of another major flare is presented. The results of two Swift Target of Opportunity observations are also described. Finally, a study of the likely optical counterpart, HD 74194, is provided. • Results : IGR J08408-4503 is very likely a supergiant fast X-ray transient (SFXT) system. The system parameters indicate that the X-ray flares are probably related to the accretion of wind clumps on a compact object orbiting about 1013 cm from the supergiant HD 74194. The clump mass loss rate is of the order of 10-6M  /yr.
• Conclusions : Hard X-ray flares from SFXTs allow to probe the stellar winds of massive stars, and could possibly be associated with wind perturbations due to line-driven instabilities. 1. Introduction
A new hard X-ray transient, IGR J08408–4503, was discovered by INTEGRAL on May 15, 2006 during the real-time
routine monitoring of IBIS/ISGRI 20–40 keV images performed at the INTEGRAL Science Data Centre (ISDC). It was
detected during a one-hour long pointing, and further analysis of the lightcurve with a 100-s binning showed
that the source flared for ~1000 s, with a peak flux of 250 mCrab in the 20–40 keV energy range.
Analysis of the public INTEGRAL archive shows that there was at least another outburst from this source on July 1, 2003. A first Swift target of opportunity observation was carried out on IGR J08408–4503 on May 22, 2006. Preliminary studies of ESO archival optical spectra of the star HD 74194, which is lying at the Swift/XRT position, were performed by Masetti et al. (2006), who pointed out that this star is likely associated with IGR J08408–4503. | |
K2.2
Multiple flaring activity
| IGR J08408-4503 — Porb ~ 35 d | |
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Authors: P. Romano, L. Sidoli, G. Cusumano, P.A. Evans, L. Ducci, H.A. Krimm, S. Vercellone, K.L. Page, A.P. Beardmore, D.N. Burrows, J.A. Kennea, N. Gehrels, V. La Parola, V. Mangano |
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Journal-ref: MNRAS (2008) [0810.1180 ] |
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Title: Multiple flaring activity in the supergiant fast X-ray transient IGR J08408-4503 observed with Swift |
| Abstract:
IGR J08408-4503 is a supergiant fast X-ray transient discovered in 2006 with
a confirmed association with a O8.5Ib(f) supergiant star, HD 74194.
We report on the analysis of two outbursts caught by Swift/BAT on 2006 October 4 and 2008 July 5, and followed up at softer energies with Swift/XRT. The 2008 XRT light curve shows a multiple-peaked structure with an initial bright flare that reached a flux of fX(2-10 keV) = 1 × 10-9 erg cm-2 s-1, followed by two equally bright flares within 75 ks. The spectral characteristics of the flares differ dramatically, with most of the difference, as derived via time-resolved spectroscopy, being due to absorbing column variations. We observe a gradual decrease of the NH, derived with a fit using absorbed power law model, as time passes. We interpret these NH variations as due to an ionization effect produced by the first flare, resulting in a significant decrease in the measured column density towards the source. The durations of the flares, as well as the times of the outbursts suggest that the orbital period is ~35 days, if the flaring activity is interpreted within the framework of the Sidoli et al. 2007 model with the outbursts triggered by the neutron star passage inside an equatorial wind inclined with respect to the orbital plane. 1. Introduction
IGR J08408–4503 belongs to the class of the supergiant fast X–ray transients (SFXTs), which are transient
sources in high mass X–ray binaries (HMXBs) associated with blue supergiant companions
(see e.g., Smith et al. 2006). Several members of this class have been discovered since the launch in 2002
of the INTEGRAL satellite, thanks to its monitoring of the Galactic plane performed with a large field of
view and a good sensitivity at hard X-rays. Indeed, several of these transients are highly absorbed in X–rays,
and previous missions had failed to detect them.
Optical spectroscopy of HD 74194 just a few days after the 2006 May 15 flare revealed variability in the Hα profile and a radial velocity variation in the He I and He II absorption lines with an amplitude of about 35 km s-1. The study of three additional flares observed with INTEGRAL and Swift by Götz et al. (2007) led these authors to suggest that the orbital period of IGR J08408–4503 is probably of the order of 1 yr, the spin period of the putative neutron star could be of order of hours, and the surface magnetic field is probably around 1013 G. References Blay P., et al., 2008, A&A, [0806.4097 | |
K3.1
AX J1749.1-2733
| AX J1749.1-2733 — Porb ~ 185 d — Prot = 132 s | |
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Authors: D.I. Karasev, S.S. Tsygankov, A.A. Lutovinov |
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Journal-ref: MNRAS (2008) [0801.3247 ] |
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Title: Discovery of X-ray Pulsations from the HMXB Source AX J1749.1-2733 |
| Abstract:
We are reporting a discovery of X-ray pulsations from the source AX J1749.1-2733 with the period of ~132 s
based on the XMM-Newton data obtained in March 2007.
The observed pulse profile has a double-peaked structure with the pulse fraction of about 25-30 % in the 3-10 keV energy band. We have also found that a periodicity with practically the same period has been detected from the source by the IBIS telescope onboard the INTEGRAL observatory during an outburst on Sept. 9, 2003 in the 20-50 keV energy band. Due to the double-peaked pulse profile, there is an additional peak on both periodograms of nearly ~66 s, therefore we have also investigated the possibility that the last value is the true pulse period. The source spectrum obtained by the XMM-Newton observatory in the soft energy band is being heavily absorbed (NH ~ 2 × 1023 cm-2) due to a strong intrinsic absorption in the binary system that leads to the conclusion that AX J1749.1-2733 is a new transient X-ray pulsar in the high mass X-ray binary system. References Corbet R., 1986, MNRAS 220, 1047 Sakano M., Koyama K., Maeda Y., 2002, ApJ 138, 19 Sguera V., Bazzano A., Bird A. et al., 2006, ApJ 646, 452 | |
K3.2
Magnetars in High Mass X-ray Binaries? The Case of SFXTs
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Authors: Bozzo, E.; Falanga, M.; Stella, L. |
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Journal-ref:ApJ 683 (2008) 1031 [0805.1849 ] |
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Title: Are There Magnetars in High Mass X-ray Binaries? The Case of SuperGiant Fast X-Ray Transients |
| Abstract:
In this paper we survey the theory of wind accretion in high mass X-ray binaries hosting a magnetic neutron star
and a supergiant companion. We concentrate on the different types of interaction between the inflowing
wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star
surface is largely inhibited; these include the inhibition through the centrifugal and magnetic barriers.
Expanding on earlier work, we calculate the expected luminosity for each regime and derive the conditions under which transition from one regime to another can take place. We show that very large luminosity swings (~104 or more on time scales as short as hours) can result from transitions across different regimes. The activity displayed by supergiant fast X-ray transients, a recently discovered class of high mass X-ray binaries in our galaxy, has often been interpreted in terms of direct accretion onto a neutron star immersed in an extremely clumpy stellar wind. We show here that the transitions across the magnetic and/or centrifugal barriers can explain the variability properties of these sources as a results of relatively modest variations in the stellar wind velocity and/or density. According to this interpretation we expect that supergiant fast X-ray transients which display very large luminosity swings and host a slowly spinning neutron star are characterized by magnetar-like fields, irrespective of whether the magnetic or the centrifugal barrier applies. Supergiant fast X-ray transients might thus provide a new opportunity to detect and study magnetars in binary systems. 1. Introduction
High mass X-ray binaries (HMXBs) consist of a collapsed object, usually a magnetic neutron star (NS), that
accretes matter from an OB companion star. Mass transfer takes place because of the intense stellar wind
from the OB star, part of which is captured by the collapsed object.
Many HMXBs are transient systems that remain at low X-ray luminosity levels (1032-33 erg s-1) most of the time and undergo outbursts lasting from weeks to months. During these outbursts they display nearly identical properties to those of persistent HMXBs. Transient systems usually comprise a Be star donor and relatively long, moderately eccentric orbits, such that the star sits deep in its Roche lobe and stellar wind capture is the only mechanism through which mass transfer takes place. The occurrence of the outbursts is likely associated to variations in the stellar wind of the Be star, such as shell ejection episodes, or build up of matter around the resonant orbits in the slow equatorial wind component (van den Heuvel & Rappaport 2005). However, there are characteristics of the outbursts that are difficult to interpret if accretion onto the neutron star surface takes place unimpeded also in quiescence; these are (a) the large outburst to quiescence X-ray luminosity swing (factor of ~1000 or larger) and (b) the presence in a given source of low-luminosity (Type I) outbursts recurring close to periastron and, at different times, of high-luminosity (Type II) outbursts that last for several orbital cycles and display little (if any) X-ray flux variations associated to the orbital phase. These characteristics of Be transients can be explained if the accretion rate (and thus X-ray luminosity) variations that are produced by the stellar wind alone, could be amplified by some “gating” mechanism. Since most Be star HMXB transients contain relatively fast spinning X-ray pulsars, such mechanism has been identified with the centrifugal barrier that results from the rotation of the neutron star magnetosphere (Stella et al. 1986). References Stella, L., White, N.E., & Rosner, R. 1986, ApJ 308, 669 van den Heuvel, E.P.J. & Rappaport, S. 1987, in Physics of Be stars (CUP), 291 | |
K3.3
SAX J1818-1703
| SAX J1818-1703 — Porb = 30 d | |
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Authors: A.J. Bird, A. Bazzano, A.B. Hill, V.A. McBride, V. Sguera, S.E. Shaw, H.J. Watkins |
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Journal-ref: MNRAS (2008) L [0810.5696 ] |
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Title: Discovery of a 30 day period in the supergiant fast X-ray transient SAX J1818.6-1703 |
| Abstract:
SAX J1818.6-1703 has been characterised as a Supergiant Fast X-ray Transient system on the basis of several
INTEGRAL/IBIS detections since the original BeppoSAX Wide Field Camera detection.
Using IBIS/ISGRI, Swift/BAT and archival observations, we show that in fact SAX J1818.6-1703 exhibits emission on a period of 30 days, with a high degree of recurrence. SAX J1818.6-1703 is therefore the second SFXT shown to exhibit periodic outbursts, but with a considerably shorter period than the other known system, IGR J11215-5952. 1. Introduction
In just over 5 years, the IBIS instrument on board the INTEGRAL gamma-ray satellite has revolutionised our
classical view of High Mass X-ray binaries (HMXB). Specifically, IBIS has discovered many new HMXBs
hosting massive OB supergiant stars (the so-called Supergiant High Mass X-ray binaries, SGXB); in just a
few years their population has been almost tripled thanks to the IBIS identification of two previously
unrecognized subclasses of SGXBs: the highly obscured SGXBs (Kuulkers 2005) and the Supergiant Fast X-ray
Transients (SFXTs; Sguera et al. 2005, 2006, Negueruela et al. 2006).
Around 9 SFXTs are known to date. They spend most of the time in a low level of X-ray activity characterized by X-ray luminosity values in the range 1032-33 erg s-1, well below the persistent bright state of other classical SGXBs (~1036 erg s-1). Only occasionally do they display bright and fast X-ray outbursts with a duration no longer than a few days (typically only a few hours) and a dynamical range in flux of 103-4 erg s-1. A number of hypotheses have been proposed to explain the peculiar flaring behaviour of SFXTs, both as a class of objects on their own, and in the context of persistently accreting supergiant X-ray binaries. Some of the models proposed invoke structure in the wind of the supergiant companion. This structure could be either in the form of clumping of spherically symmetric outflow from the supergiant donor (Leyder et al. 2007; Walter & Zurita Heras 2007), or in the form of an equatorially density enhanced wind from the supergiant, inclined at some angle to the orbit of the neutron star (Sidoli et al. 2007). Futhermore, variations in the orbital eccentricity (Negueruela et al. 2008) can be used to explain the difference between supergiant X-ray binary systems showing transient as opposed to persistent X-ray emission. Fast outbursts from SFXTs are relatively rare occurances (i.e. a few are detected per year), and long periods of inactivity are interspersed here and there with short flares spaced by irregular intervals of time. Because of this, the identification of periodicity is a very challenging task which can be fullfilled only through long-term and continous monitoring. To date, while recurrent (but non-periodic) outbursts have been observed in many SFXTs, only one, IGR J11215-5952 has been shown to exhibit periodic flaring activity separated by regular intervals of ~ 165 days (Sidoli, et al. 2006; Romano et al. 2007). The identification of periodicity in the outburst behaviour of SFXTs, most likely represents the orbital period of the binary system, and is a key diagnostic for studying the geometry of the system (i.e. orbital radius, eccentricity) and hence for understanding the physical reasons behind their very unusual X-ray behaviour. References | |
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Authors: J.A. Zurita Heras, S. Chaty |
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Journal-ref: A&A (2008) [0811.2941 ] |
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Title: Discovery of an eccentric 30 days period in the supergiant X-ray binary SAX J1818.6-1703 with INTEGRAL |
| Abstract:
• Context. SAX J1818.6??1703 is a flaring transient X-ray source serendipitously discovered by BeppoSAX in 1998 during an observation of the Galactic centre. The source was identified as a High-Mass X-ray Binary with an OB SuperGiant companion. Displaying short and bright flares and an unusually very-low quiescent level implying intensity dynamical range as large as 103-4, the source was classified as a Supergiant Fast X-ray Transient. • Aims. The mechanism triggering the different temporal behaviour observed between the classical SGXBs and the recently discovered class of SFXTs is still debated. The discovery of long orbits (> 15 d) should help to discriminate between emission models and bring constraints. • Methods. We analysed archival INTEGRAL data on SAX J1818.6-1703. We built short- and long-term light curves and performed timing analysis in order to study the temporal behaviour of SAX J1818.6-1703 on different time scales. • Results. INTEGRAL revealed an unusually long orbital period of 30 ± 0.2 d and an elapsed accretion phase of ~ 6 d in the transient SGXB SAX J1818.6-1703. This implies an elliptical orbit and constraints the possible supergiant spectral type between B0.5–1I with eccentricities e ~ 0.3 - 0.4 (for average fundamental parameters of supergiant stars). During the accretion phase, the source behaved like classical SGXBs. The huge variations of the observed X-ray flux can be explained through accretion of macro-clumps formed within the stellar wind. Our analysis strengthens the model which predicts that SFXTs behave as SGXBs but with different orbital parameters, thus different temporal behaviour. 1. Introduction
Supergiant/X-ray binaries (SGXBs) are high-mass X-ray binaries (HMXB) composed of an OB supergiant companion star
and a compact object (a neutron star (NS) or a black hole). In SGXBs, the compact object orbits within a few days
(3–15 d) of the massive companion in circular (or slightly eccentric) orbits.
Supergiant stars possess a strong stellar wind that is partly captured by the compact object producing an X-ray radiation. This happens either by direct spherical accretion or by Roche-Lobe overflow via an accretion disk on the compact object. As the compact object always orbits within the stellar wind, these systems are persistent X-ray emitting objects and show high variations on short time scales. Very few of these systems were known before INTEGRAL’s launch. The mission allowed the discovery of several new sources that could be identified as SGXBs (e.g. Walter et al. 2006; Chaty et al. 2008). These sources show typical hard X-ray spectra of accreting pulsars and most of them show a strong absorption leading to refer to them as obscured HMXB (Zurita Heras et al. 2006). Among the new SGXBs, INTEGRAL has unveiled a new subclass. Indeed, several newly discovered sources were identified as Galactic X-ray sources with a supergiant companion and displaying a transient behaviour (e.g. IGR J17544-2619, Pellizza et al. 2006). The quiescent level in these systems is of the order 1032-33 erg s-1 (e.g. IGR J08408-4503 Götz et al. 2007; Leyder et al. 2007). The X-ray luminosity increases up to 1036 erg s-1 (as observed in known SGXBs) only during periods of short and luminous flares. The X-ray luminosity remains at a very low level (if not totally absent) during most of the time, except during the flares. These flaring periods last a few hours at most and then the source goes back to an undetectable level of emission (e.g. XTE J1739-302, Smith et al. 2006). Therefore, they received the name of Supergiant Fast X-ray Transient (SFXT, Negueruela et al. 2006). Besides their transient nature, the spectral features of SFXTs are similar to the previously known persistent SGXBs. BeppoSAX discovered the new X-ray transient SAX J1818.6-1703 on March 11, 1998, during a bright flare (in ’t Zand et al. 1998). in’t Zand (2005) reported its possible association with other fast X-ray transients like XTE J1739-302 and IGR J17544-2619, archetype of SFXTs. References | |
K4.1
Galactic hard X-ray sources
Zum Thema | |
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Authors: S. Chaty, F. Rahoui, C. Foellmi, J.A. Tomsick, J. Rodriguez, R. Walter | ||
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Journal-ref: A&A 484 (2008) 783 [0802.1774 ] | ||
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Title: Galactic hard X-ray sources discovered by INTEGRAL brought to light by
multi-wavelength observations. I. The nature of the companion star | ||
| Abstract:
• Context. The hard X-ray INTEGRAL observatory has brought to light an emerging population of highly obscured X-ray binary systems. • Aims. To better understand this newly-discovered population, • Methods. we performed an intensive study of a sample of thirteen INTEGRAL sources, through multi-wavelength optical to NIR photometric and spectroscopic observations, using EMMI and SofI instruments at the ESO NTT telescope. We performed accurate astrometry and identified candidate counterparts for which we give the optical and NIR magnitudes, we determine the spectral type of the companion star, and fit with stellar black bodies the mid-infrared to optical spectral energy distributions of these sources. • Results. By spectroscopic analysis of the most likely candidates we found the spectral types of IGR J16320-4751, IGR J16358-4726, IGR J16479-4514, IGR J17252-3616, IGR J18027-2016: They all host OB type supergiant companion stars, with IGR J16358-4726 likely hosting a sgB[e]. Our spectra also confirm the supergiant O and B nature of IGR J17391-3021 and IGR J19140+0951 respectively. From SED fitting we found that IGR J16418-4532 is a (likely OB supergiant) HMXB, IGR J16393-4643 a (likely BIV-V star) HMXB, and IGR J18483-0311 a likely HMXB system. By accurate astrometry, we rejected the proposed counterparts of IGR J17091-3624 and IGR J17597-2201, and we discovered two new candidate counterparts for each source, both suggesting an LMXB from SED fitting. We confirm the AGN nature of IGR J16558-5203. Finally, we report that NIR fields of four sources of our sample exhibit large-scale regions of absorption. Conclusions. We therefore show that the majority of these systems are high-mass X-ray binaries hosting supergiant companion stars: INTEGRAL is therefore revealing a dominant class of obscured and short-living high-energy binary systems, and we suggest an association of these systems with regions of the Galaxy exhibiting large-scale absorptions.
1. Introduction
The ISGRI detector on the IBIS imager has discovered many new hard X-ray sources, including
binary systems, pulsars and AGNs, all these so-called IGR sources being reported in
Bird et al. (2007) and Bodaghee et al. (2007).
One of the most important achievements of the INTEGRAL observatory to date is that it is revealing hard X-ray sources which were not easily detected in earlier soft X-ray (typically < 10 keV) observations, bringing to light a previously hidden part of a population of highly obscured high-energy binary systems in our Galaxy. These objects share characteristics which previously had rarely been seen (see Dean et al. 2005). They are high-mass X-ray binaries (HMXBs) hosting a neutron star orbiting around an O/B companion star, in some cases a supergiant star (see e.g. Filliatre & Chaty 2004 and Pellizza et al. 2006), some of them being possibly long-period X-ray pulsars. Many of these new sources are highly absorbed, exhibiting column densities higher than about NH = 1023 cm-2, and concentrated in directions tangential to Galactic arms, as for instance the Norma arm (see Chaty & Filliatre 2005), the richest arm of our Galaxy in high-mass star forming regions. This short-living population hosts the likely progenitors of extremely compact binary objects, which are good candidates of gravitational wave emitters, and might constitute a key sample in the understanding of the evolution of high-energy binary systems. Among these high-mass X-ray binaries hosting an O/B supergiant companion star, two classes, which might overlap, seem to appear (Chaty & Rahoui, 2006). The first class is constituted of intrinsically highly obscured hard X-ray sources, exhibiting a huge local extinction. The most extreme example of these sources is the highly absorbed source IGR J16318-4848 (Filliatre & Chaty, 2004). The second class exhibits fast and transient outbursts, with peak fluxes of order fX(20 - 40 keV) = 10-9 erg cm-2 s-1, and lasting only a few hours, this last characteristic is very unusual among HMXBs. For this reason they are called Supergiant Fast X-ray Transients (SFXTs, Negueruela et al., 2006b). These SFXTs exhibit faint quiescent emission, and their hard X-ray spectra require a black hole or neutron star accretor. Among these sources, IGR J17544-2619 (Pellizza et al., 2006) seems to be the archetype of this new class of HMXBs, exhibiting long quiescence periods (Zurita Heras et al. in prep.). References Bird, A.J., Barlow, E.J., Bassani, L., et al. 2006, ApJ, 636, 765 | |||
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Authors: F. Rahoui, S. Chaty, P-O Lagage, E. Pantin |
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Journal-ref: A&A 484 (2008) 801 [0802.1770 ] |
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Title: Galactic hard X-ray sources discovered by INTEGRAL brought to light by multi-wavelength observations II.
The environment of the companion star |
| Abstract:
• Context. The INTEGRAL mission has led to the discovery of a new type of supergiant X-ray binaries (SGXBs), whose physical properties differ from those of previously known SGXBs. Those sources are in the course of being unveiled by means of multi-wavelength X-rays, optical, near- and mid-infrared observations, and two classes are appearing. The first class consists of obscured persistent SGXBs and the second is populated by the so-called supergiant fast X-ray transients (SFXTs). • Aims. We report here mid-infrared (MIR) observations of the companion stars of twelve SGXBs from these two classes in order to assess the contribution of the star and the material enshrouding the system to the total emission. Methods. We used data from observations we carried out at ESO/VLT with VISIR, as well as archival and published data, to perform broad-band spectral energy distributions of the companion stars and fitted them with a combination of two black bodies representing the star and a MIR excess due to the absorbing material enshrouding the star, if there was any. • Results. We detect a MIR excess in the emission of IGR J16318-4848, IGR J16358-4726, and perhaps IGR J16195-4945. The other sources do not exhibit any MIR excess even when the intrinsic absorption is very high. Indeed, the stellar winds of supergiant stars are not suitable for dust production, and we show that this behaviour is not changed by the presence of the compact object. Concerning IGR J16318-4848 and probably IGR J16358-4726, the MIR excess can be explained by their sgB[e] nature and the presence of an equatorial disk around the supergiant companion in which dust can be produced. Moreover, our results suggest that some of the supergiant stars in those systems could exhibit an absorption excess compared to isolated supergiant stars, this excess being possibly partly due to the photoionisation of their stellar wind in the vicinity of their atmosphere. References Bird, A. J.; Malizia, A.; et al. 2007, ApJS 170, 175 | |
K4.2
Obscured HMXB & SFXT (IR Observations)
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Author: Sylvain Chaty |
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Journal-ref: A Population Explosion (2008) [0805.1486 ] |
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Title: Obscured High Mass X-Ray Binaries and Supergiant Fast X-ray Transients: Infrared Observations of INTEGRAL Sources |
| Abstract:
A new type of high-energy binary system has been revealed by the INTEGRAL satellite.
These sources are being unveiled by means of multi-wavelength optical, near-
and mid-infrared observations.
Among these sources, two distinct classes are appearing: the first one is constituted of intrinsically obscured high-energy sources, of which IGR J16318-4848 seems to be the most extreme example.
The second one is populated by the so-called supergiant fast X-ray transients, with IGR J17544-2619 being the archetype.
We report here on multi-wavelength optical to mid-infrared observations of a sample constituted of 21 INTEGRAL sources. We show that in the case of the obscured sources our observations suggest the presence of absorbing material (dust and/or cold gas) enshrouding the whole binary system. We finally discuss the nature of these two different types of sources, in the context of high energy binary systems. 1. Introduction
The INTEGRAL observatory has performed a detailed survey of the Galactic plane. The
ISGRI detector on the IBIS imager has discovered many new high energy sources, most of
which have been reported in Bird et al. (2007).
The most important result of INTEGRAL to date is the discovery of many new high energy sources – concentrated in the Galactic plane, and in the Norma arm (see e.g. Chaty & Filliatre 2005) – exhibiting common characteristics which previously had rarely been seen. Many of them are high mass X-ray binaries (HMXBs) hosting a neutron star orbiting around an O/B companion, in most cases a supergiant star. They divide into two classes: some of the new sources are very obscured, exhibiting a huge intrinsic and local extinction, and the others are HMXBs hosting a supergiant star and exhibiting fast and transient outbursts – an unusual characteristic among HMXBs. These are therefore called Supergiant Fast X-ray Transients (SFXTs, Negueruela et al. 2006; Sguera et al. 2005). High-energy observations are not sufficient to reveal the nature of the newly discovered sources, since the INTEGRAL localisation (~ 2') is not accurate enough to unambiguously pinpoint the source at other wavelengths. Once X-ray satellites such as XMM-Newton, Chandra, or Swift provide an arcsecond position, the hunt for the optical counterpart of the source is open. However, the high level of absorption towards the galactic plane makes the near-infrared (NIR) domain more efficient for identifying these sources. References Bird, A. J.; Malizia, A.; et al. 2007, ApJS 170, 175 | |
K4.3
IGR J16207-5129: An Obscured and Non-Pulsating HMXB
| IGR J16207-5129 — d ~ 6 kpc — Porb >> 1 day (?) — fX(20–40 keV) ~ 3 millicrab — NH ~ 1023 cm-2 | |
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Authors: Tomsick, J.A.; Chaty, S.; Rodriguez, J.; Walter, R.; Kaaret, P.; Tovmassian, G. |
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Journal-ref: ApJ (2008) [0812.2975 ] |
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Title: An XMM-Newton Spectral and Timing Study of IGR J16207-5129: An Obscured and Non-Pulsating HMXB |
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Abstract:
We report on a 12 hr XMM-Newton observation of the supergiant High-Mass X-ray Binary IGR J16207-5129.
This is only the second soft X-ray (0.4-15 keV, in this case) study of the source since it was discovered
by the INTEGRAL satellite.
The average energy spectrum is very similar to those of neutron star HMXBs, being dominated by a highly absorbed power-law component with a photon index of 1.15. The spectrum also exhibits a soft excess below 2 keV and an iron Ka line emission line at 6.39 keV. For the primary power-law component, the column density is NH = 1.2 × 1023 cm-2, indicating local absorption, likely from the stellar wind, and placing IGR J16207-5129 in the category of obscured IGR HMXBs. The source exhibits a very high level of variability with an rms noise level of ~ 64% in the 0.0001 to 0.05 Hz frequency range. Although the energy spectrum suggests that the system may harbor a neutron star, no pulsations are detected with a 90% confidence upper limit of 2% in a frequency range from 0.0001 to 88 Hz. We discuss similarities between IGR J16207-5129 and other apparently non-pulsating HMXBs, including other IGR HMXBs as well as 4U 2206+54 (but see 0812.2365) and 4U 1700-377. 1. Introduction
The hard X-ray imaging by the INTEGRAL satellite (Winkler et al. 2003) has been uncovering a large number of
hard X-ray sources. Since INTEGRAL’s launch in 2002 October, 550 sources have been detected by the IBIS
instrument in the ~20–50 keV band. Included in these sources are 236 “IGR” sources that were unknown or at
least not well-studied prior to INTEGRAL.
An important result from the INTEGRAL mission has been the discovery of a relatively large number of High-Mass X-ray Binaries (HMXBs). There are 37 IGR sources that have been classified as HMXBs (and it should be noted that about 1/3 of the IGR sources are still unclassified). The IGR HMXBs are interesting both for the large number of new systems as well as the specific properties of these systems. These include a new class of “Supergiant Fast X-ray Transients” (Negueruela et al. 2006) that are HMXBs that can exhibit hard X-ray flares that only last for a few hours while the X-ray flux changes by orders of magnitude (in ’t Zand 2005; Sguera et al. 2006). Many of the IGR HMXBs are also extreme in having a high level of obscuration (NH ~ 1023-24 cm-2) due to material local to the source (Walter et al. 2006; Chaty et al. 2008). For both the SFXTs and the obscured HMXBs, it is thought that a strong stellar wind is at least partially responsible for their extreme X-ray properties (Filliatre & Chaty 2004; Walter et al. 2006; Walter & Zurita Heras 2007). In this study, we focus on X-ray observations of IGR J16207–5129, which was discovered in the Norma region of the Galaxy relatively early-on in the INTEGRAL mission (Walter et al. 2004; Tomsick et al. 2004). Although it is a relatively faint hard X-ray source at 3.3±0.1 millicrab in the 20–40 keV band (Bird et al. 2007), it has been consistently detected by INTEGRAL as well as in X-ray follow-up observations by Chandra and XMM-Newton (this work), indicating that it is a persistent source. The Chandra observation provided a sub-arcsecond position that allowed for the identification of an optical counterpart with R = 15.38±0.03 and an IR counterpart with Ks = 9.13±0.02 (Tomsick et al. 2006). Based on the optical/IR Spectral Energy Distribution, Tomsick et al. (2006) found that the system has a massive O- or B-type optical companion. Optical and IR observations confirmed this and indicate a supergiant nature for the companion (Masetti et al. 2006; Negueruela & Schurch 2007; Rahoui et al. 2008), and Rahoui et al. (2008) estimated a source distance of ~4.1 kpc. With further IR spectroscopy, the spectral type of the companion was narrowed down to B1 Ia and a source distance of 6.1 kpc was estimated. (Nespoli, Fabregat & Mennickent 2008). References | |
| Literatur zu "" | |||
| Sguera, V.; Bazzano, A.; Bird, A. J.; et al. | 2006 | ApJ 646, 452 |
"Unveiling Supergiant Fast X-Ray Transient Sources with INTEGRAL"
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| J-C Leyder, R. Walter, M. Lazos, N. Masetti, N. Produit | 2007 | A&A 465, L35-L38 |
"Hard X-ray flares in IGR J08408-4503 unveil clumpy stellar winds"
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| Romano, P.; Sidoli, L.; Mangano, V.; et al. | 2008 | ApJ 680, L137 |
"Fast X-ray Transients with Swift. Rise to the outburst in IGR J16479-4514"
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| Rahoui, F.; Chaty, S.; Lagage, P.-O.; Pantin, E. | 2008 | A&A 484, 801 |
"Galactic hard X-ray sources. II. The environment of the companion star"
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| Chaty, S.; Rahoui, F.; Foellmi, et al. | 2008 | A&A 484, 783 |
"Galactic hard X-ray sources. I. The nature of the companion star"
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| E. Bozzo, L. Stella, G. Israel, et al. | 2008 | ApJ 683, 1031 |
"Are There Magnetars in HMXBs? The Case of SFXTs"
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 | H. Heintzmann | ( Eintrag vom 20.1.2009) |
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