Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL. II. The environment of the companion star
Farid Rahoui, Sylvain Chaty, Pierre-Olivier Lagage, Eric Pantin
aa r X i v : . [ a s t r o - ph ] A p r Astronomy&Astrophysicsmanuscript no. INTEGRAL c (cid:13)
ESO 2018December 5, 2018
Multi-wavelength observations of Galactic hard X-ray sourcesdiscovered by INTEGRAL ⋆ II. The environment of the companion star
F. Rahoui , , S. Chaty , P-O. Lagage , and E. Pantin European Southern Observatory, Alonso de C´ordova 3107, Vitacura, Santiago de Chilee-mail: [email protected] Laboratoire AIM, CEA / DSM - CNRS - Universit´e Paris Diderot, IRFU / Service d’Astrophysique, Bˆat. 709, CEA-Saclay, F-91191 Gif-sur-Yvette C´edex, FranceReceived ; accepted
ABSTRACT
Context.
The
INTEGRAL mission has led to the discovery of a new type of supergiant X-ray binaries (SGXBs), whose physical propertiesdi ff er 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 thesecond 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 thecontribution 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 performbroad-band spectral energy distributions of the companion stars and fitted them with a combination of two black bodies representing the starand 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 sourcesdo not exhibit any MIR excess even when the intrinsic absorption is very high. Indeed, the stellar winds of supergiant stars are not suitablefor dust production, and we show that this behaviour is not changed by the presence of the compact object. Concerning IGR J16318-4848and probably IGR J16358-4726, the MIR excess can be explained by their sgB[e] nature and the presence of an equatorial disk around thesupergiant companion in which dust can be produced. Moreover, our results suggest that some of the supergiant stars in those systems couldexhibit an absorption excess compared to isolated supergiant stars, this excess being possibly partly due to the photoionisation of their stellarwind in the vicinity of their atmosphere. (abridged)
Key words.
Infrared: stars – X-rays: binaries, individuals: IGR J16195-4945, IGR J16207-5129, IGR J16318-4848, IGR J16320-4751,IGR J16358-4726, IGR J16418-4532, IGR J16465-4507, IGR J16479-4514, IGR J17252-3616, IGR J17391-3021, IGR J17544-2619,IGR J19140 +
1. Introduction
High-mass X-ray binaries (HMXBs) are X-ray sources forwhich high-energy emission stems from accretion onto a com-pact object (black hole or neutron star) of material coming froma massive companion star. Until recently, the huge majority ofknown HMXBs were Be / X-ray binaries, i.e a neutron star ac-creting from a disc around a Be star. Most of these sourcesare transient, even if a few are persistent weak X-ray emitters
Send o ff print requests to : F. Rahoui ⋆ Based on observations carried out at the European SouthernObservatory under programmes ID 075.D-0773 and 077.D-0721. (Lx ∼ erg s − ). The other known HMXBs were super-giant X-ray binaries (SGXBs), composed of a compact objectorbiting around an early-type supergiant and fed by accretionfrom the strong radiative wind of the companion. These objectsare persistent sources (Lx ∼ erg s − ), and their relativelow number compared to the population of Be / X-ray binarieswas explained as the consequence of the short lifetime of su-pergiant stars.The launch of the
INTErnational Gammay-RayAstrophysics Laboratory ( INTEGRAL , Winkler et al. 2003)in October 2002 completely changed the situation, as manymore HMXBs whose companion stars are supergiants were
F. Rahoui et al.: MIR observations revealing the obscured
INTEGRAL binary systems discovered during the monitoring of the Galactic centre andthe Galactic plane using the onboard IBIS / ISGRI instruments(Ubertini et al. 2003; Lebrun et al. 2003). Most of thesesources are reported in Bird et al. (2007) and Bodaghee et al.(2007), and their studies have revealed two main features thatwere not present on previously known SGXBs: – first, many of them exhibit a considerable intrinsic absorp-tion, with a column density up to N H ∼ × cm − in the case of IGR J16318-4848 (Matt & Guainazzi 2003),which explains why previous high-energy missions had notdetected them. – second, some of these new sources reveal a transitory na-ture. They are undetectable most of the time and occasion-ally present a fast X-Ray transient activity lasting a fewhours. Moreover, they exhibit a quiescent luminosity ofLx ∼ erg s − , well below the persistent state of otherSGXBs.It then appears that the supergiant HMXBs discovered by INTEGRAL can be classified in two classes: one class of con-siderably obscured persistent sources that we will simply callobscured SGXBs in this paper and another of supergiant fastX-ray transients (SFXTs, Negueruela et al. 2006b).High-energy observations can give some information aboutthe compact object or about the processes that lead to the emis-sion but do not allow study of the companion star. It is there-fore very important to perform multi-wavelength observationsof these sources - from optical-to-MIR wavelength - as this rep-resents the only way to characterise the companion or to detectdust around these highly obscured systems. However, positionsgiven by
INTEGRAL are not accurate enough ( ∼ ′ ) to identifytheir optical counterparts, because of the large number of ob-jects in the error circle. Observations with X-ray telescopes like XMM-Newton or Chandra are therefore crucial because theyallow a localisation with a position accuracy of 4 ′′ or better,which lowers the number of possible optical counterparts.We performed optical-to-MIR wavelength observations ofseveral candidate SGXBs recently discovered with INTEGRAL .Optical and NIR observations were carried out at ESO / NTT us-ing EMMI and SofI instruments and aimed at constraining thespectral type of the companions through accurate astrometry, aswell as the spectroscopy and photometry of the candidate coun-terparts. They are reported in the companion paper (Chaty et al.2008, CHA08 hereafter), and it is shown that most of thesesources are actually supergiant stars.In this paper, we report MIR photometric observations ofthe companions of twelve
INTEGRAL candidate SFXTs andobscured SGXBs that aimed at studying the circumstellar en-vironment of these highly absorbed sources and, more particu-larly, at detecting any MIR excess in their emission that couldbe due to the absorbing material. These sources were cho-sen because they had very accurate positions and confirmed2MASS counterparts.All the sources in our sample are listed in Table 1. Thetotal galactic column density of neutral hydrogen N H (H i ) iscomputed using the web version of the N H FTOOL fromHEASARC. This tool uses the data from Dickey & Lockman (1990), who performed H i observations from the Lyman- α and21 cm lines. Moreover, N H (H i ) is the total galactic column den-sity, which means it is integrated along the line of sight over thewhole Galaxy. Therefore, it is likely to be overestimated com-pared to the real value at the distance of the sources.The total galactic column density of molecular hydrogen N H (H ) is computed using the velocity-integrated map (W CO )and the X-ratio given in Dame, Hartmann, & Thaddeus (2001).It is also likely to be overestimated compared to the real valueat the distance of the sources, because it is integrated alongthe line of sight over the whole Galaxy. In contrast, N Hx , theintrinsic X-ray column density of the source, is computedfrom the fitting of the high-energy spectral energy distribution(SED), so it takes all the absorption into account at the rightdistance of the source.Using these observations, the results reported in the com-panion paper (CHA08), as well as archival photometric datafrom the USNO, 2MASS, and GLIMPSE catalogues whenneeded, we performed the broad-band SEDs of these sourcesand fitted them with a two-component black body model to as-sess the contribution of the star and the enshrouding materialin the emission. The ESO observations, as well as our model,are described in Section 2. In Section 3, results of the fits foreach source are given and these results are discussed in Section4. We conclude in Section 5.
2. Observations
The MIR observations were carried out on 2005 June 20-22and 2006 June 29-30 using VISIR (Lagage et al. 2004), theESO / VLT mid-infrared imager and spectrograph, composed ofan imager and a long-slit spectrometer covering several fil-ters in N and Q bands and mounted on Unit 3 of the VLT(Melipal). The standard ”chopping and nodding” MIR observa-tional technique was used to suppress the background dominat-ing at these wavelengths. Secondary mirror-chopping was per-formed in the north-south direction with an amplitude of 16 ′′ at a frequency of 0.25 Hz. Nodding technique, needed to com-pensate for chopping residuals, was chosen as parallel to thechopping and applied using telescope o ff sets of 16 ′′ . Becauseof the high thermal MIR background for ground-based obser-vations, the detector integration time was set to 16 ms.We performed broad-band photometry in 3 filters, PAH1( λ = ± µ m), PAH2 ( λ = ± µ m), and Q2( λ = ± µ m) using the small field in all bands(19 . ′′ . ′′ . ′′
075 plate scale). All the observations werebracketed with standard star observations for flux calibrationand PSF determination. The weather conditions were good andstable during the observations.Raw data were reduced using the IDL reduction packagewritten by Eric Pantin. The elementary images were co-addedin real-time to obtain chopping-corrected data, then the dif-ferent nodding positions were combined to form the final im-age. The VISIR detector is a ff ected by stripes randomly trig-gered by some abnormal high-gain pixels. A dedicated destrip- . Rahoui et al.: MIR observations revealing the obscured INTEGRAL binary systems 3 ing method was developed (Pantin 2008, in prep.) to suppressthem. The MIR fluxes of all observed sources including the 1 σ errors are listed in Table 2. When we did not have optical-to-MIR data for our sources, wesearched for the counterparts in 3 catalogues: – in the United States Naval Observatory (USNO) cataloguesin B , R , and I for USNO-B1.0, B and R for USNO-A.2.Positions and fluxes accuracies are 0 . ′′
25 and 0.3 magni-tudes in the case of USNO-B.1, 0 . ′′ – in the 2 Micron All Sky Survey (2MASS), in J (1.25 ± µ m), H (1.65 ± µ m) and Ks (2.17 ± µ m) bands.Position accuracy is about 0 . ′′ – in the Spitzer ’s Galactic Legacy Infrared Mid-Plane SurveyExtraordinaire (GLIMPSE, Benjamin et al. 2003), surveyof the Galactic plane ( | b | ≤ ◦ and between l = ◦ and l = ◦ on both sides of the Galactic centre) performed withthe Spitzer Space Telescope , using the IRAC camera in fourbands, 3.6 ± µ m, 4.5 ± µ m, 5.8 ± µ m, and8 ± µ m.All sources had a confirmed 2MASS counterpart and three ofthem (IGR J16195-4945, IGR J16207-5129, and IGR J16318-4848) had a GLIMPSE counterpart given in the literature. Wefound all the other GLIMPSE counterparts using the 2MASSpositions and they are listed in Table 3. We used all thefluxes given in the GLIMPSE catalogue except in the case ofIGR J17252-3616, IGR J17391-3021 and IGR J17544-2619because their fluxes were not present in the catalogue tables.Nevertheless, we measured their fluxes on the archival imagesdirectly with aperture photometry. Uncertainties on the mea-surements were computed in the same way on the error mapsgiven with the data. Absorption at wavelength λ , A λ , is a crucial parameter to fit theSEDs, especially in the MIR. Indeed, inappropriate values canlead to a false assessment of the MIR excess. Visible absorptionAv was a free parameter of the fits. An accurate interstellarabsorption law - i.e. the wavelength dependence of the A λ Av ratioin the line of sight - was then needed to properly fit the SEDs.In the optical bands, we built the function with the ana-lytical expression given in Cardelli et al. (1989) who derivedthe average interstellar extinction law in the direction of theGalactic centre. From 1.25 µ m to 8 µ m, we used the analyticalexpression given in Indebetouw et al. (2005). They derived itfrom the measurements of the mean values of the colour excessratios ( A λ − A K )( A J − A K ) from the colour distributions of observed starsin the direction of the Galactic centre. They used archival datafrom 2MASS and GLIMPSE catalogues, which is relevant inour case as we use GLIMPSE fluxes. Above 8 µ m, where absorption is dominated by the sil-icate features at 9.7 µ m and 18 µ m, we found several ex-tinction laws in the literature (Rieke et al. 1989; Lutz et al.1996; Moneti et al. 2001), which exhibit some di ff erences.Considering the high importance of a good assessment to cor-rectly fit the MIR excess, we decided to assess the ratio A λ Av in2 VISIR bands - PAH1 and PAH2 - from our data in order tobuild the relevant law for our observations.Rieke & Lebofsky (1985) gave the interstellar extinctionlaw up to 13 µ m, and from their results, we derived 0 . ≤ A PAH1 Av ≤ .
074 and 0 . ≤ A PAH2 Av ≤ .
06. To get the bestvalues corresponding to our data in PAH1 and PAH2, we pro-ceeded in 3 steps. – We first selected the sources for which we had VISIR fluxesin PAH1 and / or PAH2 and fitted their SEDs with extinctionlaws given in Cardelli et al. (1989) and Indebetouw et al.(2005) from 0.36 to 8 µ m and half-interval values taken inPAH1 and PAH2. – Then, when we did not need any MIR excess to fit the IRACfluxes, we adjusted the A
PAH1
Av and A
PAH2
Av ratios to improvethe χ of our fits. – We finally averaged all the extinction values obtained forall sources to get what we consider as the right ratios inPAH1 and PAH2 in the direction of the Galactic plane.The resulting values are in good agreement with those given bythe extinction law from Lutz et al. (1996), so we chose their ex-tinction law to fit our SEDs above 8 µ m, the Q2 filter included.The A λ Av values we used in each band are listed in Table 4, andthe overall extinction law is displayed in Fig 1.
With all the archival and observational data from optical-to-MIR wavelength, we built the SEDs for these sources. We fit-ted them (using a χ minimisation) with a model combiningtwo absorbed black bodies, one representing the companionstar emission and a spherical one representing a possible MIRexcess due to the absorbing material enshrouding the compan-ion star: λ F ( λ ) = π hc D ∗ λ − . A λ R ∗ e hc λ kT ∗ − + R D2 e hc λ kT D − in W m − We added to the flux uncertainties systematic errors as fol-lows: – a 2% systematic error in each IRAC band as given in theIRAC manual – comparing the variations of the flux calibration values ob-tained from standards with VISIR during our observationnights, we figured out that systematic errors with VISIRwere about 5% at 10 µ m and 10% at 20 µ m. http: // ssc.spitzer.caltech.edu / documents / som / som8.0.irac.pdf F. Rahoui et al.: MIR observations revealing the obscured INTEGRAL binary systems
The free parameters of the fits were the absorption in theV-band Av, the companion star black body temperature T ∗ andradius to distance ratio R ∗ D ∗ , as well as the additional sphericalcomponent black body temperature and radius T D and R D .The best-fitting parameters for individual sources, as well ascorresponding χ are listed in Table 5 and the fitted SEDsare displayed in Fig 3. Moreover, 90%-confidence ranges ofparameters are listed in Table 6.In Table 5, along with the best-fitting parameters, we alsogive the total galactic extinctions in magnitudes A H i and A H inthe line of sight, as well as the X-ray extinction of the source inmagnitudes Ax. The values of A H i , A H , and Ax are computedfrom N H (H i ), N H (H ), and N Hx given in Table 1 using the re-lation A H = . . × cm − N H (Bohlin, Savage, & Drake 1978;Rieke & Lebofsky 1985).
3. Results
IGR J16195-4945 was detected by
INTEGRAL during obser-vations carried out between 2003 February 27 and October19 (Walter et al. 2004) and corresponds to the ASCA sourceAX J161929-4945 (Sugizaki et al. 2001; Sidoli et al. 2005).Analysing
INTEGRAL public data, Sidoli et al. (2005) derivean average flux level of ∼
17 mCrab (20-40 keV). Performinga follow-up with
INTEGRAL , Sguera et al. (2006) show it be-haves like an SFXT and report a peak-flux of ∼
35 mCrab (20-40 keV).Tomsick et al. (2006) observed the source with
Chandra between 2005 April and July and give its position with 0 . ′′ Γ ∼ . N H ∼ × cm − .Moreover, using their accurate localisation, they found its NIRand MIR counterparts in the 2MASS (2MASS J16193220-4944305) and in the GLIMPSE (G333.5571 + / NTTobservations. They show its spectral type is compatible withan O, B, or A supergiant star. They also found possibleUSNO-A.2 and USNO-B.1 counterparts. Nevertheless, asalready suggested in their paper, the USNO source is a blendedforeground object (Tovmassian et al. 2006).We observed IGR J16195-4945 on 2006 June 30 inPAH1 during 1200 s, but did not detect it. Typical see-ing and airmass were 0 . ′′
88 and 1.07. We nevertheless fit-ted its SED using the NIR and the GLIMPSE flux valuesgiven in Tomsick et al. (2006) and the best-fitting parametersare Av = ∗ = ∗ D ∗ = . × − , T D = D = ∗ , and the reduced χ is 3.9 / = ∗ = ∗ D ∗ = . × − , and the corre-sponding reduced χ is 15.8 /
4. We then found a MIR excess.The additional component is then needed to correctly fitthe SED, as this source exhibits a MIR excess. Nevertheless, asshown in Fig 4, this excess is small, and the lack of data above 8 µ m does not allow to reach definitive conclusions. Moreover,in both cases (with and without dust), the stellar componentis consistent with an O / B supergiant, as already suggested inTomsick et al. (2006).
IGR J16207-5129 is an obscured SGXB that was discoveredby
INTEGRAL during observations carried out between 2003February 27 and October 19 (Walter et al. 2004).Tomsick et al. (2006) observed it with
Chandra duringthe same run as IGR J16195-4945 and give its positionwith 0 . ′′ Γ ∼ . N H ∼ . × cm − . Thanks to their accurate locali-sation, they found its NIR and MIR counterparts in the2MASS (2MASS J16204627-5130060) and in the GLIMPSE(G333.4590 + / NTT observations. They show itstemperature to be > . ′′
72 and 1.09. We did notdetect it in Q2 but in PAH1 and PAH2. The fluxes we derivedare 21.7 ± ± / NTT observations and the GLIMPSE archives found inTomsick et al. (2006), we fitted its SED, and the best-fittingparameters are Av = ∗ = ∗ D ∗ = . × − ,and the reduced χ is 28.5 /
9. Negueruela & Schurch (2007)find the spectral type is earlier than B1I; our parameters aretherefore in good agreement with their results.The best fit with the additional component gives a largerreduced χ of 30 / D <
200 K, which is not significant,as the presence of such cold material marginally enhances theMIR flux. We therefore think IGR J16207-5129 is an O / B mas-sive star whose enshrouding material marginally contributes toits MIR emission.
Main high-energy characteristics of this source can befound in Matt & Guainazzi (2003) and Walter et al. (2003).IGR J16318-4848 was discovered by
INTEGRAL on 2003January 29 (Courvoisier et al. 2003) and was then observedwith
XMM-Newton , which allowed a 4 ′′ localisation. Those ob-servations showed that the source was exhibiting a strong ab-sorption of N H ∼ × cm − , a temperature kT = ∼ / NTT and . Rahoui et al.: MIR observations revealing the obscured
INTEGRAL binary systems 5
Table 1.
Sample of sources studied in this paper. We give their name, their coordinates (J2000 and galactic), the total galacticcolumn density of neutral hydrogen ( N H (H i )) and the total galactic column density of molecular hydrogen ( N H (H )) in the line ofsight, the X-ray column density of the source ( N Hx ), their type (SFXT or OBS - obscured sources) and their spectral type (SpT).Their spectral classifications come from optical / NIR spectroscopy, reported in the following references (Ref): c: Chaty et al.(2008), f: Filliatre & Chaty (2004), i: in’t Zand et al. (2006), n1: Negueruela et al. (2005), n2: Negueruela et al. (2006a), n3:Nespoli et al. (2007), p: Pellizza et al. (2006), t: Tomsick et al. (2006).
Sources α (J2000) δ (J2000) l b N H (H i )(10 ) N H (H )(10 ) N Hx (10 ) Type SpT RefIGR J16195-4945 16 19 32 . −
49 44 30 . .
56 0 .
339 2 . . . / B tIGR J16207-5129 16 20 46 . −
51 30 06 . . − .
050 1 . . . / B tIGR J16318-4848 16 31 48 . −
48 49 00 . . − .
448 2 . . . . −
47 52 27 . .
30 0 .
169 2 . . . / BI cIGR J16358-4726 16 35 53 . −
47 25 41 . . − .
007 2 . . . . −
45 32 25 . .
19 0 .
489 1 . . . / B cIGR J16465-4507 16 46 35 . −
45 07 04 . .
05 0 .
135 2 . . . . −
45 12 08 . . − .
124 2 . . . / BI cIGR J17252-3616 17 25 11 . −
36 16 58 . . − .
354 1 . . . / BI cIGR J17391-3021 17 39 11 . −
30 20 37 . .
07 0 .
445 1 . . . . −
26 19 52 . . − .
336 1 . . . + . +
09 52 58 . . − .
469 1 . . . Table 2.
Summary of VISIR observations of newly discovered
INTEGRAL sources. We give their MIR fluxes (mJy) in the PAH1(8.59 µ m), PAH2 (11.25 µ m) and Q2 (18.72 µ m) filters. When we did not detect a source, we give the upper limit. When no fluxnor upper limit is given, the source was not observed in the considered filter. Sources PAH1 PAH2 Q2IGR J16195-4945 < . < . < . . ± . . ± . < . . ± . . ± . . ± . . ± . . ± . < . < . . ± . < . . ± . . ± . . ± . < . . ± . . ± . . ± . . ± . + . ± . . ± . show that the source presents a significant NIR excess and thatit is strongly absorbed (Av ∼ . ∼ .
5, T D = D = ∗ .We observed IGR J16318-4848 with VISIR twice: – the first time on 2005 June 21 during 300 s in PAH1 andPAH2, and 600 s in Q2. Typical seeing and airmass were0 . ′′
81 and 1.14. We detected the source in all bands, andthe derived fluxes are 409.2 ± ± ± – the second on 2006 June 30 during 600 s in all bands.Typical seeing and airmass were 0 . ′′
68 and 1.09. We de-tected the source in all bands, and the derived fluxes are426.2 ± ± ± F. Rahoui et al.: MIR observations revealing the obscured
INTEGRAL binary systems
Table 3.
List of GLIMPSE counterparts we found for 9 sources. We give their name, their separation from the 2MASS counter-parts and their fluxes in mJy.
Sources GLIMPSE counterpart Separation 3 . µ m 4 . µ m 5 . µ m 8 µ mIGR J16320-4751 G336.3293 + . ′′
17 48 . ± . . ± . . ± . . ± . . ′′
46 5 . ± . . ± . . ± . + . ′′
28 12 . ± . . ± . . ± . . ± . + . ′′
16 45 . ± . . ± . . ± . . ± . . ′′
13 68 . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . . ± . + . ′′
48 185 . ± . . ± . . ± . . ± . Table 4.
Adopted A λ A v values. Filters
U B V R I J H Ks . µ m 4 . µ m 5 . µ m 8 µ m 8 . µ m 11 . µ m 18 . µ mA λ A v .
575 1 .
332 1 0 .
757 0 .
486 0 .
289 0 .
174 0 .
115 0 . . . . . . . A λ / A v λ ( µ m)Cardelli et al. Indebetouw et al. Lutz et al. Fig. 1.
Adopted extinction law. We used the law given in Cardelli et al. (1989) in the optical, the one given in Indebetouw et al.(2005) from 1.25 µ m to 8 µ m, and the law from Lutz et al. (1996) above 8 µ m.SED and the best-fitting parameters are Av =
17, T ∗ = ∗ D ∗ = . × − , T D = D = ∗ , and the reduced χ is6.6 /
6. The best-fitting parameters without the additional com-ponent are Av = ∗ = ∗ D ∗ = . × − , and thecorresponding reduced χ is 425 /
8. We then confirm that theMIR excess is likely due to the presence of warm dust aroundthe system, as already suggested by Filliatre & Chaty (2004)and reported in Kaplan et al. (2006).
IGR J16320-4751 was detected by
INTEGRAL on 2003February (Tomsick et al. 2003) and corresponds to the ASCAsource AX J1631.9-4752. Rodriguez et al. (2003) report ob-servations with
XMM-Newton . They give an accurate localisa- tion (3 ′′ ) and fitted its high-energy spectrum with an absorbedpower law. They derive Γ ∼ . N H ∼ . × cm − .Lutovinov et al. (2005) report the discovery of X-Ray pul-sations (P ∼ / B supergiant, it had to beextremely absorbed.The optical and NIR photometry and spectroscopy of thissource were carried out at ESO / NTT, and results are reported . Rahoui et al.: MIR observations revealing the obscured
INTEGRAL binary systems 7
Table 5.
Summary of best-fitting parameters of the SEDs of the sources. We give the total galactic extinctions in magnitudes A H i and A H , the X-ray extinction of the source in magnitudes Ax and then the parameters themselves: the extinction in the opticalAv, the temperature T ∗ and the R ∗ D ∗ ratio of the companion and the temperature and radius T D and R D (in R ∗ unit) of the dustcomponent when needed. We also add the reduced χ we reach for each fit. Sources A H i A H Ax Av T ∗ (K) R ∗ D ∗ T D (K) R D (R ∗ ) χ / dofIGR J16195-4945 11 . . . . . × − . . / . . . . . × − . / . . . . . × − . . / . . . . . × − . / . . . . . × −
810 10 . . / . . . . . × − . / . . . . . × − . / . . . . . × − . / . . . . . × − . / . . . . . × − . / . . .
70 6 . . × − . / + . . . . . × − . / Table 6.
Ranges of parameters that give acceptable fits (90%-confidence) for each source.
Sources ∆ Av ∆ T ∗ ∆ R ∗ D ∗ ∆ T D ∆ R D IGR J16195-4945 14 . − . − . × − − . × − − . − . . − . − . × − − . × − IGR J16318-4848 16 . − . − . × − − . × − − . − . . − . − . × − − . × − IGR J16358-4726 17 . − . − . × − − . × − − . − . . − . − . × − − . × − IGR J16465-4507 5 . − . − . × − − . × − IGR J16479-4514 18 . − . − . × − − . × − IGR J17252-3616 20 . − . − . × − − . × − IGR J17391-3021 8 . − . − . × − − . × − IGR J17544-2619 6 . − . − . × − − . × − IGR J19140 + . − . − . × − − . × − in CHA08. It is shown that its NIR spectrum is consistentwith an O / B supergiant and that its intrinsic absorption is veryhigh, because it was not detected in any of the visible bands.We searched for the MIR counterpart of IGR J16320-4751in the GLIMPSE archives and found it to be consistent withG336.3293 + . ′′
63 and1.13. We detected it in both filters, and the respective fluxesare 12.1 ± ± / NTT NIRmagnitudes given in CHA08, as well as the GLIMPSE and theVISIR fluxes, we fitted its SED and the best-fitting parametersare Av = ∗ = ∗ D ∗ = . × − , and the reduced χ is 7.7 /
6. This result is in good agreement with an extremelyabsorbed O / B supergiant as reported in CHA08.The best fit with the additional component gives a largerreduced χ of 8 / D <
200 K. We therefore think that IGR J16320-4751 is an O / B supergiant whose enshrouding ma-terial marginally contributes to its MIR emission, even if itsintrinsic absorption is extremely high.
IGR J16358-4726 was detected with
INTEGRAL on 2003March 19 (Revnivtsev et al. 2003) and first observed with
Chandra on 2003 March 24 (Patel et al. 2004). They give itsposition with 0 . ′′ Γ ∼ . N H ∼ . × cm − . They also found a 5880 ±
50 s mod-ulation, which could be either a neutron star pulsation or anorbital modulation. Nevertheless, Patel et al. (2006) performeddetailed spectral and timing analysis of this source using multi-satellite archival observations and identified a 94 s spin up,which points to a neutron star origin. Assuming that this spinup was due to accretion, they estimate the source magnetic fieldis between 10 and 10 G, which could support a magnetarnature for IGR J16358-4726.
F. Rahoui et al.: MIR observations revealing the obscured
INTEGRAL binary systems
Kouveliotou et al. (2003) propose 2MASS J16355369-4725398 as the possible NIR counterpart, and NIR spec-troscopy and photometry of this counterpart was performed atESO / NTT and is reported in CHA08. They show that its spec-trum is consistent with a B supergiant belonging to the samefamily as IGR J16318-4848, the so-called B[e] supergiants.We also found its MIR counterpart in the GLIMPSE archives(G337.0994-00.0062).We observed IGR J16358-4726 with VISIR on 2006 June29 but did not detect it in any filter. Using the NIR magni-tudes given in CHA08 and the GLIMPSE fluxes, we fitted itsSED and the best-fitting parameters are Av = ∗ = ∗ D ∗ = . × − , T D =
810 K, R D = ∗ , and the reduced χ is3.6 /
2. The best-fitting parameters without the additional com-ponent are Av = ∗ = ∗ D ∗ = . × − and thecorresponding reduced χ is 8.8 / µ m, we think this excessis real and stems from warm dust, as it is consistent with thesource being a sgB[e], as reported in CHA08. IGR J16418-4532 was discovered with
INTEGRAL on2003 February 1-5 (Tomsick et al. 2004). Using
INTEGRAL observations, Sguera et al. (2006) report an SFXT be-haviour of this source and a peak-flux of ∼
80 mCrab(20-30 keV). Moreover, using
XMM-Newton and
INTEGRAL observations, Walter et al. (2006) report a pulse period of1246 ±
100 s and derive N H ∼ cm − . They also proposed2MASS J16415078-4532253 as its likely NIR counterpart.The NIR photometry of this counterpart was performed atESO / NTT and is reported in CHA08. We also found the MIRcounterpart in the GLIMPSE archives (G339.1889 + = ∗ = ∗ D ∗ = . × − , and the reduced χ is 1.4 / χ of 3.9 / D <
200 K.Uncertainties on the data are high, which is the reason whythe reduced χ are low. Nevertheless, parameters of the fit, aswell as the 90%-confidence ranges of parameters listed in Table6, are consistent with an O / B massive star nature. The temper-ature of the additional component being insignificant, we con-clude this source is an O / B massive star whose enshroudingmaterial marginally contributes to its MIR emission.
IGR J16465-4507 is a transient source discovered with
INTEGRAL on 2004 September 6-7 (Lutovinov et al. 2004). Observations were carried out on 2004 September 14 with
XMM-Newton , and Zurita Heras & Walter (2004) report aposition with 4 ′′ accuracy, allowing identification of an NIRcounterpart in the 2MASS catalogue (2MASS J16463526-4507045 = USNO-B1.0 0448-00520455). With the ESO / NTT,Negueruela et al. (2005) performed intermediate-resolutionspectroscopy of the source, estimate the spectral type is aB0.5I, and propose that it is an SFXT. Using
XMM-Newton and
INTEGRAL , Walter et al. (2006) find a pulse period of227 ± N H ∼ × cm − . We found its MIRcounterpart in the GLIMPSE archives (G340.0536 + – The first one on 2005 June 20 during 600 s in PAH1.Typical seeing and airmass were 0 . ′′
81 and 1.14. We de-tected the source and the derived flux is 8.7 ± – the second on 2006 June 30 during 1200 s in PAH1 andPAH2. Typical seeing and airmass were 0 . ′′
68 and 1.09. Wedetected the source in PAH1 but not in PAH2. The derivedflux is 6.9 ± = ∗ = ∗ D ∗ = . × − ,and the reduced χ is 13.9 /
7. The best fit with the additionalcomponents gives a larger reduced χ of 20 / D < IGR J16479-4514 was discovered with
INTEGRAL on 2003August 8-9 (Molkov et al. 2003). Sguera et al. (2005) suggestit is a fast transient after they detected recurrent outbursts,and Sguera et al. (2006) report a peak-flux of ∼
120 mCrab(20-60 keV). Walter et al. (2006) observed it with
XMM-Newton and gave its position with 4 ′′ accuracy. Moreover,they derive N H ∼ . × cm − from their observations.They also propose 2MASS J16480656-4512068 = USNO-B1.0 0447-0531332 as its likely NIR counterpart. The NIRspectroscopy and photometry of this counterpart were per-formed at ESO / NTT and are reported in CHA08. It is shownthat its spectrum is consistent with an O / B supergiant. Wealso found the MIR counterpart in the GLIMPSE archive(G339.1889 + . ′′ ± ± . Rahoui et al.: MIR observations revealing the obscured INTEGRAL binary systems 9 given in CHA08, as well as the GLIMPSE and the VISIRfluxes, we fitted its SED, and the best-fitting parameters areAv = ∗ = ∗ D ∗ = . × − , and the reduced χ is7.4 /
6. The best fit with the additional component gives a largerreduced χ of 9 / D <
200 K.We then do not need any additional component to fit theSED, and our result is consistent with IGR J16479-4514 beingan obscured O / B supergiant, in good agreement with CHA08.
IGR J17252-3616 is a heavily-absorbed persistent source dis-covered with
INTEGRAL on 2004 February 9 and reported inWalter et al. (2004). It was observed with
XMM-Newton on2004 March 21, and Zurita Heras et al. (2006) give its posi-tion with 4 ′′ accuracy. Using the XMM-Newton observations, aswell as those carried out with
INTEGRAL , they show the sourcewas a binary X-ray pulsar with a spin period of ∼ ∼ .
72 days, and derive N H ∼ . × cm − .Moreover, they fitted its high-energy spectrum with either anabsorbed compton (kT ∼ τ ∼ .
8) or a flat powerlaw ( Γ ∼ . / NTT and are reported in CHA08,where it is shown that its spectrum is consistent with an O / Bsupergiant. Using the 2MASS position, we searched for itsMIR counterpart in the GLIMPSE catalogue. Unfortunately,we did not find its IRAC fluxes in the database. Nevertheless,we found post-Basic Calibrated Data (post-BCD) images ofthe source in all filters. We then reduced those data and de-rived fluxes directly from the images. They are listed in Table 3.We observed IGR J17252-3616 with VISIR on 2006 June30 in PAH1 and PAH2 and the exposure time was 1200 s ineach filter. Typical seeing and airmass were 0 . ′′
97 and 1.09. Wedetected it in PAH1, and the derived flux is 6.1 ± = ∗ = ∗ D ∗ = . × − , and thereduced χ is 3.8 /
5. The best fit with the additional componentgives a larger reduced χ of 6.9 / D <
200 K. We thendo not need any additional component to fit the SED, and ourresult is consistent with IGR J17252-3616 to be an obscuredO / B supergiant, in good agreement with CHA08.
IGR J17391-3021 is a transient source discovered with
INTEGRAL on 2003 August 26 (Sunyaev et al. 2003) andit corresponds to the
Rossi X-ray Timing Explorer ( RXTE )source XTE J1739-302. Sguera et al. (2005) analysed archival
INTEGRAL data and classified the source as a fast X-ray transient presenting a typical neutron star spectrum.Smith et al. (2006) observed it with
Chandra on 2003 October15 and give its precise position with 1 ′′ accuracy. They also give its optical / NIR counterpart 2MASS J17391155-3020380 = USNO-B1.0 0596-0585865 and classify IGRJ17391-3021 as an SFXT. Negueruela et al. (2006a) performedoptical / NIR photometry and spectroscopy of the companionusing ESO / NTT and find it is a O8Iab(f) star whose distance is ∼ / NTTand confirm the nature of the companion. Using the 2MASSposition, we searched for its MIR counterpart in the GLIMPSEcatalogue and as for IGR J17252-3616, we had to reducepost-BCD data and derive the fluxes directly from the images.The fluxes are listed in Table 3.We observed IGR J17391-3021 with VISIR on 2005 June20 in PAH1 and PAH2, and the exposure time was 600 s in eachfilter. Typical seeing and airmass were 0 . ′′
63 and 1.13. We de-tected it in both filters, and the derived fluxes are 70.2 ± ± = ∗ = ∗ D ∗ = . × − , and the reduced χ is 11.7 /
10. The best fit with the additional component gives alarger reduced χ of 15.3 / D <
200 K.We then do not need any additional component to fit theSED, and the parameters derived from our fit are in good agree-ment with IGR J17391-3021 to be an O8Iab(f) supergiant star,as initially reported in Negueruela et al. (2006a).
IGR J17544-2619 is a transient source discovered with
INTEGRAL on 2003 September 17 (Sunyaev et al. 2003).Gonz´alez-Riestra et al. (2004) observed it with
XMM-Newton and derive N H ∼ × cm − . They also confirm theassociation of the companion with 2MASS J17542527-2619526 = USNO-B1.0 0636-0620933, as proposed inRodriguez (2003). in’t Zand (2005) report on observationsperformed with
Chandra , give its position with 0 . ′′ N H ∼ × cm − , and show that its high-energy spectrumis typical of an accreting neutron star. Moreover, they identifythe counterpart as a blue supergiant. Sguera et al. (2006) reporta peak-flux of ∼
240 mCrab. Using ESO / NTT, Pellizza et al.(2006) performed optical / NIR spectroscopy and photometryof the companion, and give its spectral type as O9Ib at2.1-4.2 kpc. Using the 2MASS position, we searched forits MIR counterpart in the GLIMPSE catalogue, and as forIGR J17252-3616 and IGR J17391-3021, we had to reducepost-BCD data and derived the fluxes directly from the images.The fluxes are listed in Table 3.We observed IGR J17544-2619 with VISIR on 2005 June20 in PAH1 and PAH2, and the exposure time was 600 s inPAH1 and 1200 s in PAH2. Typical seeing and airmass were0 . ′′
64 and 1.13. We detected it in both filters, and the derivedfluxes are 46.1 ± ± INTEGRAL binary systems Av = ∗ = ∗ D ∗ = . × − , and the reduced χ is6.1 /
8. The best fit with the additional component gives a largerreduced χ of 9 / D <
200 K.We then do not need any additional component to fit theSED, and the parameters derived from our fit are in good agree-ment with IGR J17544-2619 as an O9Ib supergiant star, as ini-tially reported in Pellizza et al. (2006).
IGR J19140 + INTEGRAL on 2003 March 6-7 (Hannikainen et al.2003). Observations carried out with
RXTE allowed Γ ∼ . N H ∼ × cm − to be derived (Swank & Markwardt2003). Timing analysis of the RXTE data showed a periodof 13.55 days (Corbet et al. 2004), which shows the binarynature of the source. After a comprehensive analysis of
INTEGRAL and
RXTE data, Rodriguez et al. (2005) showthe source is spending most of its time in a faint state butreport high variations in luminosity and absorption columndensity (up to ∼ cm − ). They also find evidence thatthe compact object is a neutron star rather than a blackhole. Using Chandra observations carried out 2004 May 11,in’t Zand et al. (2006) give its position with 0 . ′′ + µ m. The NIR photometry and spectroscopy of this sourcewere performed at ESO / NTT and results are reported inCHA08. It is shown that its spectrum is consistent with an O / Bmassive star, in good agreement with Nespoli et al. (2007),who show it is a B1I supergiant. Using the 2MASS position,we also found its MIR counterpart in the GLIMPSE archive(G044.2963-00.4688).We observed IGR J19140 + . ′′
12 and 1.17.We detected it in both filters, and the derived fluxes are35.2 ± ± + + = ∗ = ∗ D ∗ = . × − , and the reduced χ is 14.4 / χ of 20.2 / D <
200 K.We do not need any additional component to fit the SED,and the parameters derived from our fit are in good agreementwith IGR J19140 + Fig. 2.
VISIR image of IGR J19140 + µ m).19 . ′′ . ′′ . ′′
075 plate scale. We clearly seethe two sources that were blended with MSX. The MIR coun-terpart of IGR J19140 +
4. Discussion
All SEDs were best-fitted without any dust component (eventhe very absorbed one like IGR J16320-4751), except three ofthem (IGR J16195-4945, IGR J16318-4848, and IGR J16358-4726, see Fig. 3) that exhibit a MIR excess likely due to thepresence of dust in their stellar wind.Blue supergiants are known to exhibit a very strong butsparse stellar wind of high velocity ( ∼ − ).This has been explained through the so-called radiationline-driven CAK model (Castor, Abbott, & Klein 1975) inwhich the wind is driven by absorption in spectral lines. Hotstars emit most of their radiation in the ultraviolet (UV) wheretheir atmosphere has many absorption lines. Photons comingfrom the photosphere of the star with the same wavelength areabsorbed and re-emitted to the expanding medium in a randomdirection with almost the same momentum, which resultsin acceleration of the wind. This process is very e ff ectivebecause the line spectrum of the scattering ions in the windis Doppler-shifted compared to the stellar rest frame, so thescattering atoms are shifted with respect to their neighbours atlower velocities and can interact with an una ff ected part of thestellar spectrum.IGR J16318-4848 was proven to belong to a particularclass of B1 supergiants, the B[e] supergiants or sgB[e](Filliatre & Chaty 2004). A physical definition of B[e] starscan be found in Lamers et al. (1998). We just recall two ofthe characteristics here: the presence of forbidden emissionlines of [Fe ii ] and [O i ] in the NIR spectrum and of a strongMIR excess due to hot circumstellar dust that re-emits theabsorbed stellar radiation through free-free emission. AnsgB[e] is defined by the B[e] phenomenon, the indication ofmass-loss in the optical spectrum (P-cygni profiles), and ahybrid spectrum characterised by the simultaneous presenceof narrow low-excitation lines and broad absorption featuresof high-excitation lines. This hybrid nature was empiricallyexplained by the simultaneous presence of a normal supergianthot polar wind (fast and sparse) and responsible for the broad . Rahoui et al.: MIR observations revealing the obscured INTEGRAL binary systems 11 λ F λ ( W / m ) λ ( µ m) IGR J16195-4945AbsorbedUnabsorbed 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 J16207-5129AbsorbedUnabsorbed 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 J16318-4848AbsorbedUnabsorbed 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 J16320-4751AbsorbedUnabsorbed 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 J16358-4726AbsorbedUnabsorbed 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 J16418-4532AbsorbedUnabsorbed 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 J16465-4507AbsorbedUnabsorbed 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 J16479-4514AbsorbedUnabsorbed 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 J17252-3616AbsorbedUnabsorbed 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 J17391-3021AbsorbedUnabsorbed 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 J17544-2619AbsorbedUnabsorbed 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 J19140+0951AbsorbedUnabsorbed Fig. 3.
Optical-to-MIR absorbed (line) and unabsorbed (dotted-line) SEDs of 12
INTEGRAL sources, including broad-band photometric datafrom ESO / NTT, 2MASS, GLIMPSE, and VISIR. lines and a cool equatorial outflowing disk-like wind (slow anddense) responsible for the narrow lines (Shore & Sanduleak1983; Zickgraf et al. 1985; Shore et al. 1987). This empiricalmodel has received some confirmation from polarimetry(Oudmaijer & Drew 1999).There are a few models that explain the creation of thisoutflowing disk, and all of them consider the star rotation tobe an important parameter in the process. In this paper, wepresent only the most consistent of them, the Rotation InducedBi-stability mechanism (RIB), but a review can be found in Kraus & Miroshnichenko (2006).The lines responsible for the creation of the wind are depen-dent on the ionisation structure, and a change in this structureleads to a change in the radiative flux. This is the bi-stabilityjump found by Lamers & Pauldrach (1991), which appears forB stars with e ff ective temperatures of about 23000 K. Abovethis temperature, the wind tends to be fast and sparse. Below,the mass-loss rate if five times higher and the terminal velocitytwo times slower, which leads to a wind that is ten times denser. INTEGRAL binary systems λ F λ ( W / m ) λ ( µ m)IGR J16195-4945star and duststar onlydust only 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 10 λ F λ ( W / m ) λ ( µ m)IGR J16318-4848star and duststar onlydust only 1e-18 1e-17 1e-16 1e-15 1e-14 1e-13 1e-12 1e-11 10 λ F λ ( W / m ) λ ( µ m)IGR J16358-4726star and duststar onlydust only Fig. 4.
IR SEDs of IGR J16195-4945, IGR J16318-4848, andIGR J16358-4726 in the NIR and the MIR. We show theirSEDs including the contribution of the star and the dust (line),the star only (dashed-line), and the dust only (dotted-line).Cassinelli & Ignace (1997) propose that the same e ff ect isimportant from polar to equatorial regions for rapidly rotat-ing B stars. Indeed, the rapid rotation leads to polar brighten-ing that increases the poles temperature to the hot side of thejump. At the same time, the rotation leading to gravity dark-ening, the equatorial region may be on the cool side of thejump. Consequently, the wind in the equatorial region is denserthan the wind in the polar region. Nevertheless, Pelupessy et al. (2000) show that the rotational velocity of the star should bevery close to its critical value to allow the equatorial wind toreach the density needed to create the disk. However, super-giant stars cannot be close to critical rotational velocity becauseof probable disruption. Additional mechanisms are thereforeneeded to allow the supergiant star to reach its critical velocity(see e.g. Owocki 2006). In the particular case of an sgB[e] starin an X-ray binary system, the spin-up should occur during thesupergiant phase of the companion, which indicates a di ff erentevolutionary stage from other HMXBs.This disk itself cannot explain the strong MIR excess thesgB[e] stars exhibit. Nevertheless, Bjorkman & Cassinelli(1993) have shown the existence of a zone in the disk (about50-60 stellar radii from the star) in which the temperatureis below the temperature of sublimation of the dust (about1500 K) and the density high enough to allow for its creation.IGR J16318-4848 is the source in our sample that exhibitsthe strongest MIR excess, and we believe it is due to the sgB[e]nature of its companion star. Indeed, many other strongly ab-sorbed sources in our sample do not present any MIR excess.Moreover, it is suggested that IGR J16358-4726, the secondsource in our sample that exhibits a MIR excess and whoseSED needs an additional component to be properly fitted is ansgB[e], because its spectrum has all the characteristic featuresof supergiant stars plus the [Fe ii ] feature (CHA08). Our fit istherefore in good agreement with their result, and the only othersource of our sample that definitely exhibits a MIR excess isindeed an sgB[e] star.In the case of IGR J16195-4945, we are more cautiousconcerning the presence of warm dust that could be responsiblefor a MIR excess, as we lack data above 8 µ m. Indeed, Fig 4.shows that these source could exhibit a MIR excess, but onemuch lower than the other two. Nevertheless, if this excesswere to be confirmed, we believe it would be also due to thesgB[e] nature of the companion.We would like to point out that, because the dust is most lo-cated in an equatorial disk in an sgB[e] star, the simple modelwe used to fit the SEDs cannot reproduce the complex distribu-tion of the dust around these stars. Nevertheless, it allows thedetection of a warm MIR excess because of the presence of dustin the stellar winds. Finally, for all the stars in our sample, wecannot exclude the presence of a cold component - responsiblefor their intrinsic absorption - which we cannot detect becauseof the lack of data above 20 µ m. In our sample, six sources are supergiant stars with a knownspectral type - IGR J16318-4848 and IGR J16358-4726 aresgB[e], IGR J16465-4507 is a B0.5I, IGR J17391-3021 is an08Iab(f), IGR J17544-2619 is an O9Ib, and IGR J19140 + / B supergiants whosetemperatures derived from our fits allow an assessment of thespectral types using the classification given in Martins et al.(2005) and Crowther et al. (2006) for O and B galactic super- . Rahoui et al.: MIR observations revealing the obscured
INTEGRAL binary systems 13
Table 7.
Summary of spectral types (SpT) and distances (D ∗ )derived from our fits for confirmed supergiant stars in our sam-ple. ( ∗ ) sources with an accurate spectral type found in the liter-ature ( † ) confirmed supergiant stars whose temperature derivedfrom our fits was used to assess their accurate spectral type.References to the determination of the spectral type and / orspectral class of these sources are found in Table 1. Sources SpT D ∗ (kpc)IGR J16318-4848 ∗ sgB[e] ∼ . † O8I ∼ . ∗ sgB[e] ∼ . ∗ B0.5I ∼ . † O8.5I ∼ . † O8.5I ∼ . ∗ O8I ∼ . ∗ O9I ∼ . + ∗ B1I ∼ . Table 8.
Summary of the distances (D ∗ ) derived from our fitsfor sources with unconfirmed spectral classes. Sources SpT D ∗ (kpc)V III IIGR J16195-4945 B1 ∼ . ∼ . ∼ . ∼ . ∼ . ∼ . ∼ . ∼ . ∼ giants, respectively, given the uncertainties of observationalresults ( ∼ / B massive stars, and we also used theirderived temperatures to assess their spectral type using theclassification given in both papers quoted above. IGR J16195-4945 could be a B1 star, and as already stressed above, it couldbe an sgB[e] due to its MIR excess, IGR J16207-5129 andIGR J16418-4532 could be O7.5-O8.5 stars. Nevertheless,even if the high intrinsic X-ray absorption of their associatedcompact objects points towards a supergiant nature since theaccretion is likely to be wind-fed, the fits themselves do notallow an assessment of their spectral classes. We then considerthey could be either main sequence, giant, or supergiant stars.Martins & Plez (2006) give a UBVJHK synthetic photom-etry of galactic OI, OIII, and OV stars, with which one can getthe expected unabsorbed absolute magnitude in J band M J forstars having a given spectral classification. Using the absorbedapparent magnitudes m J of our sources and the J band absorp-tion we derived from our fits, A J = . × Av, it is then possible to assess the distance of O stars in our sample usingthe standard relation: D ∗ = . m J − A J − M J +
5) in pcWe did not find any synthetic photometry for B supergiants.Nevertheless, expected radii of galactic BI, BIII, and BV starsare given in Vacca et al. (1996), and we divided these values bythe R ∗ D ∗ ratio derived from our fits to get the star distance. Thederived distances for the sources whose spectral class is knownare listed in Table 7, in Table 8 for the others. Except in the case of IGR J17544-2619, X-ray absorptions aresystematically significantly larger than the visible absorptions.This indicates the presence in the system of a two-componentabsorbing material: one around the companion star, responsi-ble for the visible absorption, and a very dense one around thecompact object coming from the stellar winds that accrete ontothe compact object and are responsible for the huge X-ray ab-sorption those sources exhibit.The obscuration of the compact object by the stellarwind is caused by the photoelectric absorption of the X-ray emission by the wind, and this absorption varies alongthe orbit of the compact object. This orbital dependence hasfor instance been observed and modelled on 4U 1700-37(Haberl, White, & Kallman 1989).Moreover, an e ff ect on the X-ray absorption by thephotoionisation of the stellar wind in the vicinity of thecompact object by its X-ray emission was predicted byHatchett & McCray (1977). Indeed, in SGXBs, the compactobject moves through the stellar wind of the companion star,and the X-rays are responsible for the enhancement or thedepletion of the ionised atoms responsible for the accelerationof the wind (e.g. C iv and N v ). This has a direct consequenceon the velocity profile of the wind; when the wind enters intoan ionised zone, it follows a standard CAK law until it reachesa location in which it is enough ionised for no further radiativedriving to take place, and the wind velocity is “frozen” to aconstant value from this point. This results in a lower windvelocity close to the compact object and consequently a higherwind density that leads to a higher obscuration of the compactobject.Most of the sources studied in this work are very absorbedin the high-energy domain. Nevertheless, this absorption maynot be always that high. In the case of very wide eccentric or-bits, the column density of the sources could normally varyalong their orbit and suddenly increase when very close to thecompanion star because of the wind ionisation. In contrast, ifthese objects were to be always very absorbed, it could meanthat their orbit is very close to the companion star and weaklyeccentric. If this e ff ect were to be observed, we think it couldexplain the di ff erence in behaviour between obscured SGXBs(close quasi-circular orbits) and SFXTs (wide eccentric orbits). INTEGRAL binary systems
Table 9.
Sample of parameters we used to fit the SEDs of theisolated supergiants. We give their galactic coordinates, theirspectral types, the interstellar extinction in magnitudes Ai andthen the parameters themselves: the extinction in the opticalAv, the temperature T ∗ and the R ∗ D ∗ ratio of the star. Sources l b
SpT Ai Av T ∗ (K) R ∗ D ∗ HD 144969 333 .
18 2 . .
34 3 . . × − HD 148422 329 . − . .
75 0 . . × − HD 149038 339 .
38 2 .
51 B1Ia 0 .
81 1 24000 5 . × − HD 151804 343 .
62 1 .
94 O8Iaf 0 .
83 1 . . × − HD 152234 343 .
46 1 .
22 B0.5Ia 1 .
17 1 . . × − HD 152235 343 .
31 1 . .
37 3 . . × − HD 152249 343 .
35 1 .
16 O9Ib 1 .
34 1 . . × − HD 156201 351 .
51 1 .
49 B0.5Ia 2 .
68 2 . . × − We were able to fit all but three sources with a simple stellarblack body model. For these three sources, we explained thatthe MIR excess was probably caused by the warm dust createdwithin the stellar wind due to the sgB[e] nature of the compan-ions. Therefore, it seems that the optical-to-MIR wavelengthemission of these SGXBs corresponds to the emission of ab-sorbed blue supergiants or sgB[e].Moreover, the results of the fits listed in the Table 5 showthat it is a priori impossible to di ff erentiate an obscured SGXBand an SFXT from their optical-to-MIR wavelength SEDs, andit then seems that the di ff erence in behaviour between bothkinds of SGXBs only depends on the geometry of the system,i.e. its orbital distance or its orbit eccentricity (Chaty & Rahoui2007).Nevertheless, to assess a possible e ff ect of the compact ob-ject on the companion star, we took a sample of eight isolatedO / B supergiants in the direction of the Galactic centre and fittedtheir optical-to-NIR wavelength SEDs with an absorbed stel-lar black body. The best-fitting parameters are listed in Table9 along to their galactic coordinates, their spectral types andthe interstellar H i absorption (Ai). The distances of these su-pergiants are known, which allowed us to calculate Ai out totheir position using the tool available on the MAST website(Fruscione et al. 1994).We see that their visible absorption is of the same order ofmagnitude as the interstellar H i absorption and well below thelevel of absorption of our sources. This could mean that somesupergiant stars in SGXBs exhibit an excess of absorption dueto a local absorbing component. Unfortunately, the total inter-stellar absorption out to the distance of our sources is unknown,and we cannot compare their visible absorptions derived fromour fits to the total interstellar absorption out to their position.Nevertheless, if this was the case, we think that this ex-cess of absorption could also be caused partly by the photoion-isation of the wind in the vicinity of the companion star bythe high-energy emission of the compact object, as this would make their winds slower than in isolated supergiant stars. Sincethe wind velocity is lower, the medium is denser and suitablefor creating a more absorbant material.Indeed, in the case of persistent sources with very close andquasi-circular orbits, we think that this possible e ff ect couldbe particularly strong, since the wind around the companionstar would be permanently photoionised and would have lowervelocities than in isolated supergiants. This could be the generalscheme of obscured SGXBs.On the other hand, in the case of very wide and eccentricorbits, the compact object would be most of the time far fromthe secondary and its X-ray emission would not photoionise thewind close to the companion star, which would not exhibit anyvisible absorption excess until the compact object got closer.This could be the general scheme of SFXTs.At last, in both cases, it would be possible to observe avariation in the P-Cygni profiles of the companion star (i.e.a variation in the wind velocity) with the phase angle of thecompact object along its orbit.As a possible confirmation of this general behaviour, wepoint out that the visible absorptions derived from our fitsfor the companion stars of the only sources in our samplethat surely exhibit the SFXT behaviour (IGR J16465-4945,IGR J17391-3021, and IGR J17544-2619) are far smaller thanthe visible absorptions of the others. Moreover, concerning ob-scured SGXBs, the wind velocity of IGR J16318-4848 wasfound to be ∼
410 km s − (Filliatre & Chaty 2004), far lowerthan the expected wind velocity for O / B supergiants ( ∼ − ).
5. Conclusions
In this paper, we presented results of observations performed atESO / VLT with VISIR, which aimed at studying the MIR emis-sion of twelve
INTEGRAL obscured HMXBs, whose compan-ions are confirmed or candidate supergiants. Moreover, usingthe observations performed at ESO / NTT and reported in thecompanion paper (CHA08), previous optical / NIR observationsfound in the literature and archival data from USNO, 2MASS,and GLIMPSE, we fitted the broad-band SEDs of these sourcesusing a simple two-component black body model in order toobtain their visible absorptions and temperatures, and to assessthe contribution of their enshrouding material in their emission.We confirmed that all these sources were likely O / B su-pergiant stars and that, for most of them, the enshroudingmaterial marginally contributed to the emission. Moreover, inthe case of IGR J16318-4848, IGR J16358-4726, and perhapsIGR J16195-4945, the MIR excess could be explained by thesgB[e] nature of the companion stars.By comparing the optical and high-energy character-istics of these sources, we showed that the distinctionSFXTs / obscured SGXBs does not seem to exist from optical-to-MIR wavelength. Nevertheless, most of the sources inour sample are significantly absorbed in the optical, and wethink that the wind can be denser around some supergiantsin SGXBs, which could be due to the photoionisation by the . Rahoui et al.: MIR observations revealing the obscured INTEGRAL binary systems 15 high-energy emission of the compact object.Several improvements in our study are needed to allowdefinitive conclusions. Indeed, the data used to perform theSEDs were not taken simultaneously, which can for instancelead to an incorrect assessment of the MIR excess in theemission. Moreover, the lack of optical magnitudes for severalsources could have led to an incorrect fitting of their intrinsicvisible absorption Av. Finally, the absence of an accuratemeasurement of the total interstellar absorption out to thedistance of these sources does not allow us to say whetherthe presence of the compact object can lead to a stellar winddenser in some supergiants belonging to SGXBs than inisolated supergiants.We then recommend further optical investigations of thesesources to study any possible variation in their P-cygni profilewith the phase of the compact object. We also think that themeasurement of the distance of these sources is crucial to al-low a good assessment of the real interstellar absorption up totheir distance, in order to detect any local absorbing componentaround companion stars. Finally, we recommend X-ray moni-toring so as to study the dependence of their column densityon orbital phase angle, which could help for understanding thedi ff erence between obscured SGXBs and SFXTs. Acknowledgements.
We are pleased to thank J´erˆome Rodriguez forhis very useful website in which all the
INTEGRAL sources are refer-enced ( http: // isdc.unige.ch / ∼ rodrigue / html / igrsources.html).Based on observations carried out at the European SouthernObservatory, Chile (through programmes ID. 075.D-0773 and 077.D-0721). This research has made use of NASA’s Astrophysics DataSystem, of the SIMBAD and VizieR databases operated at the CDS,Strasbourg, France, of products from the US Naval Observatory cata-logues, of products from the Two Micron All Sky Survey, which isa joint project of the University of Massachusetts and the InfraredProcessing and Analysis Center / California Institute of Technology,funded by the National Aeronautics and Space Administration and theNational Science Foundation as well as products from the GalacticLegacy Infrared Mid-Plane Survey Extraordinaire, which is a
SpitzerSpace Telescope
Legacy Science Program.
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