Obscured sources and Supergiant Fast X-ray Transients: new classes of high mass X-ray binaries
aa r X i v : . [ a s t r o - ph ] O c t Chin. J. Astron. Astrophys. Vol. 0, No. 0, (200x) 000–000( ) Chinese Journal ofAstronomy andAstrophysics
Obscured sources and Supergiant Fast X-ray Transients:new classes of high mass X-ray binaries
Sylvain Chaty ⋆ Laboratoire AIM, CEA/DSM - CNRS - Universit´e Paris Diderot, DAPNIA/Serviced’Astrophysique, Bˆat. 709, CEA-Saclay, FR-91191 Gif-sur-Yvette Cedex, France
Received October 24, 2018; accepted October 24, 2018
Abstract
A new type of high-energy binary systems has been revealed by the
INTEGRAL satellite. These sources are in the course of being unveiled by meansof multi-wavelength optical, near- and mid-infrared observations. Among thesesources, two distinct classes are appearing: the first one is constituted of in-trinsically obscured high-energy sources, of which IGR J16318-4848 seems to bethe most extreme example. The second one is populated by the so-called super-giant fast X-ray transients, with IGR J17544-2619 being the archetype. We re-port here on multi-wavelength optical to mid-infrared observations of a sampleof 21
INTEGRAL sources. We show that in the case of the obscured sources ourobservations suggest the presence of absorbing material (dust and/or cold gas)enshrouding the whole binary system. We finally discuss the nature of these twodifferent types of sources, in the context of high energy binary systems.
Key words:
X-ray binaries; Visible; Near infrared; Infrared;
INTEGRAL ;IGR J16318-4848; IGR J17544-2619
The
INTEGRAL observatory has performed a detailed survey of the galactic plane and theISGRI detector on the IBIS imager has discovered many new high energy sources, most of allreported in Bird et al. (2007) (see also http://isdc.unige.ch/ ∼ rodrigue/html/igrsources.html ).The most important result of INTEGRAL to date is the discovery of many new high energysources –concentrated in the Galactic plane, and in the Norma arm (see e.g. Chaty & Filliatre2005)–, exhibiting common characteristics which previously had rarely been seen. Many of themare high mass X-ray binaries (HMXBs) hosting a neutron star orbiting around an O/B com-panion, in most cases a supergiant star. They divide into two classes: some of the new sourcesare very obscured, and exhibiting a huge intrinsic and local extinction, and the others are
Based on observations collected at the European Southern Observatory, Chile (proposals ESO N ◦ ⋆ E-mail: [email protected]
S. Chaty
HMXBs hosting a supergiant star and exhibiting fast and transient outbursts: an unusual char-acteristic among HMXBs: they are therefore called Supergiant Fast X-ray Transients (SFXTs,Negueruela et al. 2006; Sguera et al. 2005). High-energy observations are not sufficient to revealthe nature of the newly discovered sources, since the
INTEGRAL localisation ( ∼ ′ ) is not accu-rate enough to unambiguously pinpoint the source at other wavelengths. Once X-ray satellitessuch as XMM-Newton , Chandra or Swift provide an arcsecond position, the hunt for the opticalcounterpart of the source is open. However, the high level of absorption towards the galacticplane makes the near-infrared (NIR) domain more efficient to identify these sources. We firstreport on multi-wavelength observations of two sources belonging to each class described above,then give general results on
INTEGRAL sources, before discussing them and concluding.
The multiwavelength observations described here were performed at the European SouthernObservatory (ESO), using Target of Opportunity (ToO) and Visitor modes, in 3 domains:optical (400 −
800 nm) with the EMMI instrument on the 3.5m New Technology Telescope(NTT) at La Silla, NIR (1 − . µ m) with the SOFI instrument on the NTT, and mid-infrared(MIR, 5 − µ m) with the VISIR instrument on Melipal, the 8m Unit Telescope 3 (UT3) of theVery Large Telescope (VLT) at Paranal (Chile). These observations include photometry andspectroscopy on 21 INTEGRAL sources in order to identify their counterparts and the nature ofthe companion star, derive the distance, and finally characterise the presence and temperatureof their circumstellar medium.
IGR J16318-4848 was the first source to be discovered by IBIS/ISGRI on
INTEGRAL on 29January 2003 (Courvoisier et al., 2003).
XMM-Newton observations showed a strong absorp-tion of N H ∼ × cm − (Matt & Guainazzi, 2003). The accurate localisation by XMM-Newton allowed Filliatre & Chaty (2004) to rapidly trigger ToO photometric and spectroscopicobservations in optical and NIR, leading to the discovery of the optical counterpart and tothe confirmation of the NIR one found by Walter et al. (2003). The extremely bright NIRsource (Ks = 7 .
20 magnitudes) exhibits an unusually strong intrinsic absorption in the op-tical of A v = 17 . A v = 11 . ⋆ = 20 . R ⊙ , and anextra component of temperature T = 900 K and radius R = 12 R ⋆ , with A v = 17 . M ⊙ ,we obtain an orbital separation of 50 R ⊙ , smaller than the extension of the extra component,suggesting that this component enshrouds the whole binary system, as would do a cocoon of bscured sources and SFXTs: new classes of HMXBs 3 gas/dust (see Figure 2, left panel). In summary, IGR J16318-4848 is an HMXB system, locatedat a distance between 1 to 6 kpc, hosting a compact object (probably a neutron star) and asgB[e] star (it is therefore the second HMXB with a sgB[e] star, after CI Cam; Clark et al.1999). The most striking facts are i. the compact object seems to be surrounded by absorbingmaterial and ii. the whole binary system seems to be surrounded by a dense and absorbing cir-cumstellar material envelope or cocoon, made of cold gas and/or dust. This source exhibits soextreme characteristics that it might not be fully representative of the other obscured sources. SFXTs constitute a new class of sources identified among the recently discovered
INTEGRAL sources, whose common characteristics are: they exhibit rapid outbursts lasting only hours,a faint quiescent emission, their high energy spectra require a BH or NS accretor, and theyhost O/B supergiant companion stars. IGR J17544-2619, a bright recurrent transient X-raysource discovered by
INTEGRAL on 17 September 2003 (Sunyaev et al., 2003), seems to bethe archetype of this class of sources. Observations with
XMM-Newton have shown that itexhibits a very hard X-ray spectrum, and a relatively low intrinsic absorption (10 cm − ,Gonz´alez-Riestra et al. 2004). Its bursts last for hours, in-between bursts it exhibits long quies-cence periods, which can reach more than 150 days (Zurita Heras et al. in prep.). The compactobject is probably a neutron star (in’t Zand, 2005). Pellizza et al. (2006) managed to get op-tical/NIR ToO observations only one day after the discovery of this source. They identified alikely counterpart inside the XMM-Newton error circle, confirmed by
Chandra accurate localiza-tion. Spectroscopy showed that the companion star was a blue supergiant of spectral type O9Ib,with a mass of 25 − M ⊙ and temperature of T ∼ ⋆ = 21 . R ⊙ . They derived an absorption A v = 5 . . ∼ INTEGRAL sources and discussion
To better characterize this population, Chaty et al. (2007) and Rahoui et al. (2007) studied asample of 21
INTEGRAL sources belonging to both classes described above. Some results arereported in Table 1. The optical/NIR study, through an accurate astrometry, photometry andspectroscopy, allowed Chaty et al. (2007) to identify the counterpart, and to show that mostof these systems are HMXBs, containing massive and luminous early-type companion stars.By combining MIR photometry, and fitting their optical–MIR spectral energy distributions,Rahoui et al. (2007) showed that i. most of these sources exhibit an intrinsic absorption andii. three of them exhibit a MIR excess, that they suggest to be due to the presence of a co-coon of dust and/or cold gas enshrouding the whole binary system (see also Chaty & Rahoui2006). Nearly all the
INTEGRAL
HMXBs for which both spin and orbital periods have beenmeasured are located in the upper part of the Corbet diagram (Corbet, 1986): they are windaccretors, typical of supergiant HMXBs, and X-ray pulsars exhibiting longer pulsation peri-ods and higher absorption (by a factor ∼
4) as compared to the average of previously knownHMXBs (Bodaghee et al., 2007). This extra absorption might be due to the presence of a cocoonof dust/cold gas enshrouding the whole binary system in the case of the obscured sources: the
S. Chaty λ F λ ( W / m ) λ ( µ m)IGR J16318-4848 star+duststar onlydust only 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 0.1 1 10 100 λ F λ ( W / m ) λ ( µ m) IGR J17544-2619 Fig. 1
Optical to MIR SEDs of IGR J16318-4848 (left) and IGR J17544-2619 (right),including data from ESO/NTT, VISIR on VLT/UT3 and
Spitzer (Rahoui et al.,2007). IGR J16318-4848 exhibits a MIR excess, interpreted by Rahoui et al. (2007)as the signature of a strong stellar outflow coming from the sgB[e] companion star(Filliatre & Chaty, 2004). On the other hand, IGR J17544-2619 is well fitted withonly a stellar component corresponding to the O9Ib companion star spectral type(Pellizza et al., 2006).intrinsic properties of the supergiant companion star could therefore explain some propertiesof these sources. However, fundamental differences exist between obscured sources and SFXTs,which might be explained by the geometry of the binary systems, and/or the extension of thewind/cocoon enshrouding either the companion star or the whole system. Indeed, obscuredsources are naturally explained by a compact object orbiting inside a cocoon of dust and/orcold gas, while the fast X-ray behaviour of SFXTs needs a clumpy stellar wind environment, toaccount for fast and transient accretion phenomena (see Figure 2, left and right panels respec-tively, and Chaty & Rahoui 2006). These results show the existence in our Galaxy of a dominantpopulation of a previously rare class of high-energy binary systems: supergiant HMXBs, someexhibiting a high intrinsic absorption (Chaty et al. 2007; Rahoui et al. 2007). A careful studyof this population, recently revealed by
INTEGRAL , will provide a better understanding of theformation and evolution of short-living HMXBs. Furthermore, stellar population models willhenceforth have to take these objects into account, to assess a realistic number of high-energybinary systems in our Galaxy. Our final word is that only a multiwavelength study can allowto reveal the nature of the obscured high-energy sources.
Acknowledgements
SC would like to thank the organisers for their invitation to report on theseexciting results on newly discovered
INTEGRAL sources, and for organising an interesting workshop,in a nice place, fruitful to arise scientific discussions and new ideas. SC is grateful to Juan Antonio ZuritaHeras for a careful rereading of the manuscript, and to an anonymous referee for useful comments.
References
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Fig. 2
Scenarios illustrating two possible configurations of
INTEGRAL sources, witha neutron star orbiting around a supergiant star on a circular orbit (left image), andon an excentric orbit (right image), accreting from the clumpy stellar wind of thesupergiant. The accretion of matter is persistent in the case of the obscured sources,as in the left image, where the compact object orbits inside the cocoon of dust en-shrouding the whole system. On the other hand, the accretion is intermittent in thecase of SFXTs, which might correspond to a compact object on an excentric orbit,as in the right image. A 3D animation of these sources is available on the website
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S. Chaty
Table 1
Results on the sample of
INTEGRAL sources, more details are given inChaty et al. (2007). We indicate respectively the name of the sources, the region ofthe Galaxy in the direction of which they are located, their spin and orbital pe-riod, the interstellar, optical-IR and X-ray derived column density respectively (inunits of 10 cm − ), their spectral type, nature and reference. Type abbreviations:AGN = Active Galactic Nucleus, B = Burster, BHC = Black Hole Candidate, CV= Cataclysmic Variable, D = Dipping source, H = High Mass X-ray Binary sys-tem, IP = Intermediate polar, L = Low Mass X-ray Binary, O = Obscured source,P = Persistent source, S = Supergiant Fast X-ray Transient, T: Transient source,XP: X-ray Pulsar. Reference are: c: Chaty et al. (2007), co: Combi et al. (2006), f:Filliatre & Chaty (2004), h: Hannikainen & et al. (2007), m1: Masetti et al. (2004)m2: Masetti et al. (2006) n1: Negueruela et al. (2005), n2: Negueruela et al. (2006),n3: Nespoli et al. (2007), p: Pellizza et al. (2006), t: Tomsick et al. (2006). Source Reg P s (s) P o (d) Nh is Nh IR Nh X SpT Type RefIGR J16167-4957 No 2.2 0.23 0.5 A0 CV/IP t,m2IGR J16195-4945 No 2.18 2.9 7 OB H?/S?/O tIGR J16207-5129 No 1.73 2.0 3.7 BOI H/O t,m2IGR J16318-4848 No 2.06 3.3 200 sgB[e] H/O/P fIGR J16320-4751 No 1250 8.96(1) 2.14 6.6 21 sgOB H/XP/T/O cIGR J16358-4726 No 5880 2.20 3.3 33 sgB[e]? H/XP/T/O cIGR J16393-4643 No 912 3.6875(6) 2.19 2.19 24.98 BIV-V? H/XP/T cIGR J16418-4532 No 1246 3.753(4) 1.88 2.7 10 sgOB? H/XP/S cIGR J16465-4507 No 228 2.12 1.1 60 B0.5I H/S n1IGR J16479-4514 No 2.14 3.4 7.7 sgOB H/S? cIGR J16558-5203 - - - - - - Sey1.2 AGN m2IGR J17091-3624 GC 0.77 1.03 1.0 L L/BHC cIGR J17195-4100 GC 0.77 0.08 CV/IP t,m2IGR J17252-3616 GC 413 9.74(4) 1.56 3.8 15 sgOB H/XP/O cIGR J17391-3021 GC 1.37 1.7 29.98 O8Iab(f) H/S/O n2IGR J17544-2619 GC 165? 1.44 1.1 1.4 O9Ib S pIGR J17597-2201 GC 1.17 2.84 4.50 L L/B/D/P cIGR J18027-1455 - - - - - - Sey1 AGN m1,coIGR J18027-2016 GC 139 4.5696(9) 1.04 1.53 9.05 sgOB H/XP/T cIGR J18483-0311 GC 21.05 18.55 1.62 2.45 27.69 BeV? H/XP cIGR J19140+0951 13.558(4) 1.68 2.9 6 sgB0.5I H/O n3,h
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DISCUSSIONNIELS LUND:
Are all the SFXT sources heavily absorbed? The animation seems to indicatea connection between the heavily absorbed sources and the SFXT sources.