How to reveal the mysteries of the most obscured high-energy sources of our Galaxy, discovered by INTEGRAL?
aa r X i v : . [ a s t r o - ph ] O c t How to reveal the mysteries of the most obscured high-energysources of our Galaxy, discovered by
INTEGRAL ? Sylvain Chaty
AIM - Astrophysique Interactions Multi-échelles (UMR 7158 CEA/CNRS/Université Paris 7 Denis Diderot) CEASaclay, DSM/DAPNIA/Service d’Astrophysique, Bât. 709, L’Orme des Merisiers FR-91 191 Gif-sur-YvetteCedex, France, [email protected]
Abstract.
A new type of high-energy binary systems has been revealed by the
INTEGRAL satellite. These sources arein the course of being unveiled by means of multi-wavelength optical, near- and mid-infrared observations. Among thesesources, two distinct classes are appearing: the first one is constituted of intrinsically obscured high-energy sources, of whichIGR J16318-4848 seems to be the most extreme example. The second one is populated by the so-called supergiant fastX-ray transients, with IGR J17544-2619 being the archetype. We report here on multi-wavelength optical to mid-infraredobservations of these systems. We show that in the case of the obscured sources our observations suggest the presence ofabsorbing material (dust and/or cold gas) enshrouding the whole binary system. We then discuss the nature of these twodifferent types of systems.
Keywords:
X-ray binaries; Visible; Near infrared; Infrared;
INTEGRAL ; IGR J16318-4848; IGR J17544-2619
PACS:
INTRODUCTION
The
INTEGRAL observatory has performed a detailed survey of the galactic plane and the ISGRI detec-tor on the IBIS imager has discovered many new high energy sources, most of all reported in [1] (andhttp://isdc.unige.ch/ ∼ rodrigue/html/igrsources.html). The most important result of INTEGRAL to date is thediscovery of many new high energy sources –concentrated in the Galactic plane, and some in the Norma arm (seee.g. [2])–, exhibiting common characteristics which previously had rarely been seen. Most of them are high massX-ray binaries (HMXBs) hosting a neutron star orbiting around an O/B companion, in some cases a supergiant star.They divide into two classes: some of the new sources are very obscured, and exhibiting a huge intrinsic and localextinction, and the others are HMXBs hosting a supergiant star and exhibiting fast and transient outbursts: an unusualcharacteristic among HMXBs: they are therefore called Supergiant Fast X-ray Transients (SFXTs, [3]). High-energyobservations are not sufficient to reveal the nature of the newly discovered sources, since the
INTEGRAL localisation( ∼ ′ ) is not accurate enough to unambiguously pinpoint the source at other wavelengths. Once X-ray satellites suchas XMM-Newton , Chandra or Swift provide an arcsecond position, the hunt for the optical counterpart of the sourceis open. However, the high level of absorption towards the galactic plane makes the near-infrared (NIR) domain moreefficient to identify these sources. We first report on multi-wavelength observations of two sources belonging to eachclass described above, then give general results on
INTEGRAL sources, before discussing them and concluding.
OBSERVATIONS AND RESULTS
The multiwavelength observations described here were performed at the European Southern Observatory (ESO), usingTarget of Opportunity (ToO) and Visitor modes, in 3 domains: optical (400 − m m) with the EMMI instrument onthe 3.5m New Technology Telescope (NTT) at La Silla, NIR (1 − . m m) with the SOFI instrument on the NTT, andMIR (5 − m m) with the VISIR instrument on Melipal, the 8m Unit Telescope 3 (UT3) of the Very Large Telescope(VLT) at Paranal (Chile). These observations include photometry and spectroscopy on 20 INTEGRAL sources in orderto identify their counterparts, the nature of the companion star, derive the distance, and finally characterise the presenceand temperature of their circumstellar medium.
GR J16318-4848: extreme among the obscured high-energy sources
IGR J16318-4848 was the first source to be discovered by IBIS/ISGRI on
INTEGRAL on 29 January 2003 [4].
XMM-Newton observations showed a strong absorption of N H ∼ × cm − [5]. The accurate localisation by XMM-Newton allowed [6] to rapidly trigger ToO photometric and spectroscopic observations in optical and NIR,leading to the discovery of the optical counterpart and to the confirmation of the NIR one found by [7]. The extremelybright NIR source (Ks = .
20 magnitudes) exhibits an unusually strong intrinsic absorption of A v = . A v = . = star = . R ⊙ , and an extra component of temperature T =
900 K, radius R = R star andA v = . IGR J17544-2619: archetype of the Supergiant Fast X-ray Transients
The Supergiant Fast X-ray Transients (SFXTs) constitute a new class of sources identified among the recentlydiscovered
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 they host O/B supergiantcompanion stars. Among these sources, IGR J17544-2619, a bright recurrent transient X-ray source discovered by
INTEGRAL on 17 September 2003 [9], seems to be the archetype. Observations with
XMM-Newton have shown thatit exhibits a very hard X-ray spectrum, and a faint intrinsic absorption (10 cm − ) [10]. Its bursts last for hours,in-between bursts it exhibits long quiescence periods, and a long outburst period of 165 days [3]. The nature of thecompact object is probably a neutron star [11]. [12] managed to get optical/NIR ToO observations only one day afterthe discovery of this source. They identified a likely counterpart inside the XMM-Newton error circle, confirmed by
Chandra accurate localization. Spectroscopy showed that the companion star was a blue supergiant of spectral typeO9Ib, with a mass of 25 − M ⊙ and temperature of T ∼ = star = . R ⊙ . The absorption they derived was A v = . = . ∼ General results on
INTEGRAL sources and discussion
In order to better characterize this population, [13] and [8] have studied a sample of 20
INTEGRAL sourcesbelonging to both classes described above. The optical/NIR study allowed [13] to identify or confirm the identificationof the counterpart, and to show that most of these systems are HMXBs, containing massive and luminous early-typecompanion stars. By fitting the spectral energy distributions of these sources from optical to MIR, [8] showed that i.most of them exhibit an intrinsic absorption and ii. three of them exhibit a MIR excess, that they suggest to be due tothe presence of a cocoon of dust and/or cold gas enshrouding the whole binary system (see also [14]). These resultsconfirm the existence in the Galaxy of a dominant population of a previously rare class of high-energy binary systems, l F l ( W / m ) l ( m 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 l F l ( W / m ) l ( m m) IGR J17544-2619 FIGURE 1.
Optical to MIR SEDs of IGR J16318-4848 (left) and IGR J17544-2619 (right), including data from ESO/NTT, VISIRon VLT/UT3 and
Spitzer [8]. IGR J16318-4848 exhibits a MIR excess, interpreted by [8] as the signature of a strong stellar outflowcoming from the sgB[e] companion star [6]. On the other hand, IGR J17544-2619 is well fitted with only a stellar componentcorresponding to the companion star spectral type (O9Ib) [12]. constituted of supergiant HMXBs with high intrinsic absorption for some of them [13] [8]. Fundamental differencesexist between obscured sources and SFXTs, and one possibility to explain those is to invoke a different geometry of thebinary systems, or a different extension of a wind/cocoon enshrouding either the companion star or the whole system[14]. It is now clear that a careful study of this new population will provide a better understanding of the formationand evolution of such short-living HMXBs of our Galaxy, and will allow in the future stellar population models tobetter take these systems into account, to assess a realistic number of high-energy binary systems in our Galaxy. Ourfinal word is that the GLAST satellite will certainly discover such new and unexpected objects, and that, as for theseobscured high-energy sources, only a multiwavelength study will allow to reveal their nature.
ACKNOWLEDGMENTS
SC would like to thank the organisers for the opportunity to report on these exciting results on newly discovered
INTEGRAL sources and for organizing a press conference on this subject. SC is grateful to Juan-Antonio Zurita Herasfor useful comments on the manuscript, and to Farid Rahoui for making the SEDs of Figure 1. Based on observationscollected at the European Southern Observatory, Chile (proposals ESO N ◦ REFERENCES
1. A. J. Bird et al. ApJ , 765–776 (2006), arXiv:astro-ph/0611493 .2. S. Chaty, and P. Filliatre, ApSS , 235–244 (2005).3. I. Negueruela, D. M. Smith, P. Reig, S. Chaty, and J. M. Torrejón, “Supergiant Fast X-ray Transients: a new class of high massX-ray binaries unveiled by INTEGRAL,” in
ESA Special Publication , edited by A. Wilson, 2006, vol. 604 of
ESA SpecialPublication , pp. 165–170.4. T. J.-L. Courvoisier, R. Walter, J. Rodriguez, L. Bouchet, and A. A. Lutovinov, IAU Circ. , 3–+ (2003).5. G. Matt, and M. Guainazzi, MNRAS , L13–L17 (2003), astro-ph/0303626 .6. P. Filliatre, and S. Chaty, ApJ , 469–484 (2004), astro-ph/0408407 .7. R. Walter, J. Rodriguez, L. Foschini, J. de Plaa, S. Corbel, T. J.-L. Courvoisier, P. R. den Hartog, F. Lebrun, A. N. Parmar,J. A. Tomsick, and P. Ubertini, A&A , L427–L432 (2003), arXiv:astro-ph/0309536 .8. F. Rahoui, S. Chaty, P.-O. Lagage, and E. Pantin, A&A (2007), subm.9. R. A. Sunyaev, S. A. Grebenev, A. A. Lutovinov, J. Rodriguez, S. Mereghetti, D. Gotz, and T. Courvoisier,
The Astronomer’sTelegram , 1–+ (2003).10. R. González-Riestra, T. Oosterbroek, E. Kuulkers, A. Orr, and A. N. Parmar, A&A , 589–594 (2004).11. J. J. M. in’t Zand, A&A , L1–L4 (2005), astro-ph/0508240 .12. L. J. Pellizza, S. Chaty, and I. Negueruela, A&A , 653–658 (2006).13. S. Chaty, C. Foellmi, F. Rahoui, J. Rodriguez, J. A. Tomsick, and R. Walter, A&A (2007), subm.14. S. Chaty, and F. Rahoui, “Optical to Mid-infrared observations revealing the most obscured high-energy sources of theGalaxy,” in