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MNRAS000,1–??(2002) Preprint23January2017 CompiledusingMNRASLATEXstylefilev3.0 Confirmation of six Be X-ray binaries in the Small ⋆ Magellanic Cloud V.A. McBride1,2†, A. González-Galán3, A.J. Bird4, M.J. Coe4, E.S. Bartlett1,5, R. Dorda3, F. Haberl5, A. Marco3, I. Negueruela3, M.P.E. Schurch1, R. Sturm5, D.A.H. Buckley2 and A. Udalski6 7 1 1Department of Astronomy, Universityof Cape Town, Private Bag X3, Rondebosch, 7701, South Africa 0 2South African Astronomical Observatory, PO Box 9, Observatory, 7935, South Africa 2 3Departamento de F´ısica, Ingenier´ıa de Sistemas y Teor´ıa de la Senãl, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain n 4School of Physics and Astronomy, Universityof Southampton, Highfield, Southampton SO17 1BJ, UnitedKingdom a 5ESO - European Southern Observatory, Alonso de Co´rdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile J 6Max-Planck-Institut fu¨r extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany 0 7Warsaw UniversityObservatory, Aleje Ujazdowskie 4, 00-478, Warsaw, Poland 2 ] Accepted 2017January19.Received2017January18;inoriginalform2016July22 R S . h ABSTRACT p The X-ray binary population of the Small Magellanic Cloud (SMC) contains a large - numberofmassiveX-raybinariesandthe recentsurveyofthe SMCbyXMM-Newton o hasresultedinalmost50moretentativeHighMassX-rayBinary(HMXB)candidates. r t Using probability parameters from Haberl & Sturm (2016) together with the optical s spectra and timing in this work, we confirm six new massive X-ray binaries in the a [ SMC. We also report two very probable binary periods; of 36.4d in XMM1859 and of 72.2d in XMM 2300. These Be X-ray binaries are likely part of the general SMC 1 population which rarely undergoes an X-ray outburst. v 5 Key words: Magellanic Clouds – X-rays:binaries – stars:emission-line, Be. 6 7 5 0 . 1 INTRODUCTION 5×1033ergs−1 (assumingadistanceof 60kpctotheSMC, 1 Hilditch et al. 2005), with wide coverage across the SMC 0 TheSmallMagellanicCloud(SMC)hasauniquepopulation (see Fig. 3 in Haberl et al. 2012). Through the primary 7 ofX-raybinaries.ItisentirelydominatedbymassiveX-ray 1 analysis of the point source X-ray data from this survey, binaries,allofwhich haveneutronstarcompanions.Thisis v: afactorof∼50moreX-raybinariesthanwouldbeexpected, Sturm et al. (2013) have generated a list of candidate high mass X-ray binaries (HMXBs) which is presented in Table i basedonscalingbymasstheMilkyWayX-raybinarypopu- X 5oftheirpaper.ObjectsareclassifiedascandidateHMXBs lation.Thisexcessisthoughttobeduetothehigherstarfor- on the basis of their X-ray and optical colours. In par- r mationrateintheSMC,butpossiblyalsoinfluencedbythe a ticular, hard X-ray sources are classified as HMXBs when lower metallicity compared to the Milky Way (Dray 2006). they have an optical counterpart in the magnitude range The massive X-ray binary populations in the Milky Way, 13.5 < V < 17 with optical colours characteristic of an Large Magellanic Cloud (LMC) and SMC all have donor early-typestar,andarenotalreadyidentifiedwithaknown stars with spectral types predominantly earlier than B3 background AGN.Forty-fiveX-ray point sources are classi- (Negueruela 1998; Negueruela & Coe 2002; McBride et al. fied as candidate HMXBs bySturm et al. (2013). 2008; Antoniou et al. 2009; Antoniou & Zezas 2016). Our currentunderstandingofthislimited massrangeisthrough InthispaperweselectedsevenofthemostlikelyHMXB angular momentum loss during evolution of the binary candidates from the X-ray classification by Sturm et al. (Portegies Zwart 1995). (2013) for follow-up optical spectral and temporal analysis. With a view to characterising the X-ray population FromthisanalysisweconfirmthesecandidatesasBeX-ray of the SMC, Haberl et al. (2012) have undertaken an X- binariesintheSMC.Insection2wepresentthephotometric ray survey of the SMC down to a limiting luminosity of andspectroscopicdatausedintheanalysis,whileinsection 3 we discuss the selection criteria and analysis techniques. ⋆ BasedonESOdatafrom079.D-0371,088.D-0352 Ourresultsarepresentedinsection4,whileconclusionsare † E-mail:[email protected] (VAM) discussed in section 5. (cid:13)c 2002TheAuthors 2 V.A. McBride et al. 2 DATA 2.3 Photometry from OGLE 2.1 Spectroscopy from ESO The OGLE project2 (see e.g. Udalski et al. 1997, Udalskiet al. 2015) provides long term I-band pho- Spectra were taken between 2011 December 9 and 10 with tometrywithroughlydailysampling.Thesedatawereused theESOFaintObjectSpectrograph(EFOSC2)mountedat to study the time variability of the optical counterparts to theNasmythBfocusofthe3.58mNTT.Aslitwidthof1.5′′ the eight candidate HMXBs. All of the HMXB candidates was employed, together with a grating ruled at 600lmm−1 have been observed with OGLE III & IV. The OGLE thatyielded1˚A/pixeldispersionoverawavelengthrangeof identifications and lightcurve characteristics are presented λλ3095−5085˚A.Spectrawererecordedwithexposuretimes in Table 2 and theOGLE IV lightcurves in Fig. 1. between 300s and 800s depending on source brightness, at a spectral resolution of ∼10˚A. Wealsoutilisedarchivalspectraoftwoobjectsobserved on 2007 September 20 with EFOSC2 mounted on the ESO 3 ANALYSIS 3.6m telescope at La Silla. In this instance a slit width of 1.0′′ was employed with all other instrument parameters 3.1 Criteria for identification as a massive X-ray the same as above. Typical exposure times ranged between binary 1000s – 1500s. Startingwith thelist ofmassive X-raybinarycandidatesin All data were reduced using the standard packages Table 5 of Sturm et al. (2013), and excluding any objects available in the Image Reduction and Analysis Facility (IRAF1).WavelengthcalibrationwasachievedwithHelium previously confirmed as X-ray binary systems, we applied thefollowing criteria: and Argon arc lamps. (i) the X-ray source must belong to confidence classes 2 or 3 – description below (ii) the optical counterpart must be a spectroscopically 2.2 Spectroscopy from AAO confirmed early-typestar The AAT spectra were obtained with the fibre-fed The confidenceclasses listed above were introduced by dual-beam AAOmega spectrograph on the 3.9m Anglo- Haberl & Sturm(2016)intheirclassification ofX-raybina- Australian Telescope (AAT) at the Australian Astronom- ries in the SMC. Class 2 corresponds to an X-ray object ical Observatory on 2012, July 7–8. The Two Degree Field showing significant X-ray variability (> a factor of 30) or a (“2dF”)multi-objectsystemwasutilised.Lightfromanop- tical fibre of diameter 2′.′1 on the sky is fed into two arms hard power law spectrum, but no X-ray pulsations. Class 3 corresponds to an X-ray source with an error circle small via a dichroic beam-splitter with crossover at 5700˚A. Each enoughtoidentifyitunambiguouslywithanearlytypestar armoftheAAOmegasystemisequippedwitha2k×4kE2V showing emission lines. CCD detectorand anAAO2CCD controller. Thebluearm Though we do not require a detected period in the op- CCD is thinned for improved blue response. Because of the tical lightcurve to classify an object as an HMXB, some atmospheric diffraction, our targets did not produce useful systems do exhibit strong evidence for binary modulation. spectra on the red arm, which was observing around the However, we also note that many HMXB systems do not infrared CaII triplet for a different programme. We used exhibit this modulation (Bird et al. 2012). As a result of grating 580V, giving R = 1300 over ∼ 2100˚A. The central theaboveselection criteria wepresentheresixnewHMXB wavelength was set at 4500˚A. systems. We used thestandard reduction pipeline 2dfdr as pro- vided by the AATat the time, with wavelength calibration by observing arc lamps before each target exposure. The arc lamps provide lines of He+CuAr+FeAr+ThAr+CuNe, 3.2 Spectral classification andonly thoselines actually detected inthecalibration ex- Themotivationforandmethodofspectralclassificationhas posures were input into the data reduction pipeline. The beenoutlinedinsection4ofMcBride et al.(2008).Weshow wavelength calibration was excellent, with rms consistently in Fig. 2 the spectra we classified in this work. Spectral <0.1 pixel. classifications can befound in Table 3. Sky subtraction was carried out by means of a mean sky spectrum, obtained by averaging the spectra of 30 fi- bres positioned at known blank locations. The sky lines in 3.3 Timing analysis eachspectrumareevaluatedandusedtoscalethemeansky spectrum prior tosubtraction. The OGLE data sets used in this analysis are listed in Ta- The journal of the spectroscopic observations is pre- ble2 and provideI-bandphotometry over many years. sented in Table 1. The optical lightcurves were subject to detrending and periodsearchingfollowing themethodexplainedinsections 2and3ofBird et al.(2012).Theresultsofthistiminganal- ysis are presented in Table 3. 1 IRAF is distributed by the National Optical Astronomy Ob- servatory, which is operated by the Association of Universities forResearchinAstronomy(AURA)undercooperativeagreement withtheNationalScienceFoundation. 2 http://ogle.astrouw.edu.pl MNRAS000,1–??(2002) BeXRBs in SMC 3 Table1.Summaryofobservations.Thefirstcolumn‘Campaign’isusedtoidentifythesetupforeachindividualsourcelistedinTable3. Campaign Telescope Dates Wavelength range Resolution (˚A˚A) (˚A) eso07 ESO3.6m,LaSilla,Chile 2007Sep19–20 3082–5087 4 eso11 ESONTT,LaSilla,Chile 2011Dec9–10 3095–5085 10 aat AAT,SidingSpring,Australia 2012Jul7–8 4025–4775 1.2 14 XMM12 XMM337 XMM1400 16 XMM1859 XMM2208 18 XMM2300 XMM3285 20 5500 6000 6500 7000 Figure 1.OGLEIVlightcurvesofoursixHMXBcandidates andXMM2300toillustratetheextent ofvariabilityinthelightcurves. MNRAS000,1–??(2002) 4 V.A. McBride et al. Table 2.OGLEIII&IVidentifications andvariabilityparameters.δIisthemeanerroronthedatapoints,whereas∆Irepresentsthe total rangeofthedataset. XMM Iband δI ∆I OGLEIII OGLEIV ID averagemag (mmag) (mmag) ID ID 12 15.53 5 105 SMC121.855 SMC733.202992 337 14.42 3 682 smc105.6.39454 SMC726.3226912 1400 14.90 4 955 smc100.4.62974 smc719.03.43645 1859 14.82 3 867 smc103.7.6594 SMC720.1113342 2208 16.92 7 273 SMC108.58790 SMC719.2620124 2300 14.45 3 178 SMC105.537304 SMC719.187 3285 15.70 5 1246 SMC110.59503 SMC726.2823178 Table 3. Results from optical spectroscopy and OGLE data timing analysis. Note there are two periods seen insource XMM 12, and thattheperiodicityseeninsourceNo.3285isonlyvisibleinoneyear’sworthofdatacoveringtheperiodMJD55650–56100. XMM RA Dec Vmag Campaign SpType Period Detrending Previouslyreferenced ID (J2000) (J2000) MCPS thiswork (days) (days) 12 011938.94 -733011.4 15.8 eso07 B1.5–B2III-Ve 0.5522541(18) 81 [SG05]28, [MA93]1867, 5.18331(24) 81 [HS00]60 337 005614.65 -723755.8 14.6 eso07 B3IIIe - 81 [SG05]22, [MA93]922, orearlier [E04]1135 1400 005341.76 -725310.1 14.7 aat B0.5–B1IIIe - 81 Lambetal.(2013) 1859 004855.55 -734946.4 14.9 eso11 B0IV-Ve 36.432(9) 81 [E04]705, Lambetal.(2016) Coeetal.(2016) 2208 005605.48 -720011.1 16.7 eso11 B2–B3Ve - 81 Novaraetal.(2011) 2300† 005613.87 -722959.7 14.5 eso11+aat B0.5IVe 72.231(35) 101 Evans etal.(2006) 3285 010429.42 -723136.5 15.8 eso11+aat B1V 29.75(18) 41 Rajoelimananaetal.(2011), Schmidtke etal.(2013) MCPS:MagellanicCloudsPhotometricSurveyZaritskyetal.2002,SG05:Shtykovskiy &Gilfanov2005,MA93: Meyssonnier&Azzopardi 1993,HS00:Haberl&Sasaki2000E04:Evansetal.2004†:ThissourceisapreviouslyknownHMXB (Sturmetal.2013)andwereporthereourspectralclassificationandperiodicity. 4 DISCUSSION 4.2 Timing results 4.1 Spectroscopy results Bird et al. 2012 show that there can be confusion between short period (61day) non-radialpulsationsand longer pe- In Table 3 we present spectral classifications of the opti- riod(>10days)orbitalmodulationwhentheapproximately cal counterparts to seven X-ray sources. Six of the candi- daily sampling period beats with the non-radial pulsation dateX-raybinariesarenowconfirmedHMXBs,whileXMM (NRP) period and results in long period peaks in the pe- 2300isidentifiedalreadyinSturmet al.(2013)asanX-ray riodogram. A case in point is XMM 3285, where a 37.15d source corresponding to a Be star(Evans et al. 2006). In all period reported by Rajoelimanana et al. (2011) was associ- cases the counterpart is classified as a B star, confirming ated with aliasing of a possible shorter period modulation the HMXB nature of the system and validating the X-ray (Schmidtkeet al. 2013). andopticalselectioncriteriaasusedbySturm et al.(2013). The spectra are illustrated in Fig. 2. Our spectral type of An effective discriminant between aliased non-radial XMM337isconfirmedbyEvanset al.(2004),butthesame pulsationsandorbitalmodulationistheshapeofthefolded authors find a slightly earlier spectral type (O9.5V, their lightcurve.The orbital modulation, probably caused by the object #705) for XMM 1859, while Lamb et al. (2016) find neutron star disturbing the Be star circumstellar disk typ- a spectral type of O9IIIe for XMM 1859. For XMM 1400, ically shows a fast rise with an exponential decay, whereas Lamb et al.(2013)findaspectraltypeofB0Ve.Ourclassi- non-radial pulsations have more sinusoidal profiles in the fication of XMM 2300 confirmsthat of Evans et al. (2006). lightcurves. These characteristics can be reflected by two Two spectra, XMM 2208 and XMM 3285 do not show metrics:thephaseasymmetryofthefoldedlightcurves,and emission in the Hβ line. XMM 2208 has shown strong Hα the phase FWHM (Bird et al. 2012) with the parameter in emission in previous observations (Novaraet al. 2011). space shown in Fig. 3. Be stars are known to undergo disk loss phases, where the From this plot it can be seen that only sources XMM Balmer lines appear entirely in absorption, so despite the 1859&XMM2300showstrongevidenceforthepresenceof lack of emission lines, the optical period, V magnitude and a modulation that seems incompatible with the sinusoidal shallowHβabsorptionline(comparedwiththeotherBalmer behaviourassociatedwithNRPbehaviour.Consequentlywe lines in this source) all point strongly towards a Be X-ray presenttheseresultsasclearevidenceforbinarymodulation. binary nature of XMM 3285. Source XMM 3285 is marginal, whereas XMM 12 is a clear MNRAS000,1–??(2002) BeXRBs in SMC 5 naries in the SMC. It is likely that they’ve been previously 3.5 0 undetectedas they’veneverbeen observed duringan X-ray 5 3.0 H5 3970 HeI 4026NII 4044 H4 4102SiII 4128 NII 4240 H3 4341 HeI 4387 HeI 4471 CIII+OII 46HeII 4686HeI 4713 H2 4861 bSgerMtighChetr(pEwhviatahsnesw.Seidtweaiflat.rhe2aa0s1c6jou)vs,etarsantgdaerttmhediasyriemugnupelraaorrvtmehdomnseoitnroesriitBnivegitXoyf-rtthaoye- binary systems that haveeluded detection upuntil now. 2.5 nts #1859 u o c d e 2.0 alis #2300 ACKNOWLEDGMENTS m or N 1.5 The AAT observations have been supported by the #1400 OPTICON project (observing proposals 2011A/014 and 2012/A015), which is funded by the European Commission 1.0 #3285 undertheSeventhFramework Programme (FP7). VAMac- knowledges financial support from the National Research 0.5 FoundationofSouthAfrica(GrantRDYR14081189466)and 4000 4200 4400 4600 4800 theWorldUniversitiesNetwork.RD,AM,INfromtheUni- Wavelength (Angstrom) versity of Alicante acknowledge support from the Span- ish Government Ministerio de Econom´ıa y Competitividad 3.0 under grant AYA2015-68012-C2-2-P (MINECO/FEDER). ESBacknowledgessupportfromaClaudeLeonFoundation 2.5 H5 3970 HeI 4026 H4 4102 FeII 4223 HeI 4387 HeI 4471 HeI 4713 fpreeelclaeoniwvCsehdoimpfumnandisdisniogfrnofrm(oFmPth7te-hCeMONaFarUtieiNoCnDau)lr.iSeTcihAeencctOeioGnCLseEnotfprert,ohjPeeocEltauhnrados-, unts 2.0 grant MAESTRO 2014/14/A/ST9/00121 to AU. co #337 d e alis m Nor 1.5 #2208 REFERENCES AntoniouV.,ZezasA.,2016,MNRAS,459,528 1.0 Antoniou V., Hatzidimitriou D., Zezas A., Reig P., 2009, ApJ, #12 61 707,1080 8 2 4 BirdA.J.,CoeM.J.,McBrideV.A.,UdalskiA.,2012,MNRAS, 0.5 H 423,3663 4000 4200 4400 4600 4800 CoeM.J.,McBrideV.,HaberlF.,BirdA.,UdalskiA.,2016,The Wavelength (Angstrom) Astronomer’sTelegram,9198 DrayL.M.,2006,MNRAS,370,2079 EvansC.J.,HowarthI.D.,IrwinM.J.,BurnleyA.W.,Harries Figure 2. The top panel shows those spectra classified as B0 – T.J.,2004,MNRAS,353,601 B1 in this work. The lower panel shows those classified as later EvansC.J.,LennonD.J.,SmarttS.J.,TrundleC.,2006,A&A, thanB1. 456,623 Evans P. A., Kenna J. 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