The Chandra Small Magellanic Cloud Wing Survey - the search for X-ray Binaries

We have detected 523 sources in a survey of the Small Magellanic Cloud (SMC) Wing with Chandra. By cross-correlating the X-ray data with optical and near-infrared catalogues we have found 300 matches. Using a technique that combines X-ray colours and X-ray

Mon.Not.R.Astron.Soc.000,(2007)Printed2February2008

A(MNLTEXstyle lev2.2)

TheChandraSmallMagellanicCloudWingSurvey-the

searchforX-rayBinaries

arXiv:0710.2232v1 [astro-ph] 11 Oct 2007

K.E.McGowan1 ,M.J.Coe1,M.P.E.Schurch1,V.A.McBride1,J.L.Galache2,

1324,5

W.R.T.Edge,R.H.D.Corbet,S.Laycock,D.A.H.Buckley1

SchoolofPhysicsandAstronomy,SouthamptonUniversity,High eld,Southampton,SO171BJ

CenterforAstrophysics,Cambridge,MA02138,USA

3UniversitiesSpaceResearchAssociation,X-rayAstrophysicsLaboratory,MailCode662,NASAGoddardSpaceFlightCenter,Greenbelt,MD20771,USA

4SouthAfricanAstronomicalObservatory,Observatory,7935,CapeTown,SouthAfrica

5SouthernAfricanLargeTelescopeFoundation,Observatory,7935,CapeTown,SouthAfrica

2Harvard-Smithsonian

ABSTRACT

Wehavedetected523sourcesinasurveyoftheSmallMagellanicCloud(SMC)WingwithChandra.Bycross-correlatingtheX-raydatawithopticalandnear-infraredcatalogueswehavefound300matches.UsingatechniquethatcombinesX-raycoloursandX-raytooptical uxratioswehavebeenabletoassignpreliminaryclassi cationsto265oftheobjects.Ouridenti cationsincludefourpulsars,onehigh-massX-raybinary(HMXB)candidate,34starsand185activegalacticnuclei(AGNs).Inaddition,wehaveclassi ed32sourcesas’hard’AGNswhicharelikelyabsorbedbylocalgasanddust,andnine’soft’AGNswhosenatureisstillunclear.ConsideringtheabundanceofHMXBsdiscoveredsofarintheBaroftheSMCthenumberthatwehavedetectedintheWingislow.

Keywords:X-rays:binaries–stars:emission-line,Be–(galaxies:)MagellanicClouds

1INTRODUCTION

Multi-wavelengthstudiesoftheSmallMagellanicCloud(SMC)haveshownthatitcontainsalargenumberofX-raybinarypulsars.FromanalysisofHαmea-surements(Kennicutt1991)andsupernovabirthrates(Filipovicetthestarformationrate(SFR)fortheSMCisestimatedtolieintherange0.04–0.4M⊙yr 1.Shtykovskiy&Gilfanov(2005)usedtheseupperandlowerSFRestimatesandthelinearrelationbetweenthenumberofhigh-massX-raybinaries(HMXBs)andtheSFRofthehostgalaxyfromGrimm,Gilfanov&SunyaevtopredictthenumberofHMXBsexpectedintheSMCwithlumi-nosities 1035ergs 1.Theyfoundthatbetween6and49ofthesesystemsshouldbepresent.Currently～60knownorprobableHMXBshavebeendetectedintheSMC(seee.g.Haberl&Pietsch2004;Coeetal.McGowanetal.2007).

Itisbelievedthattheconsiderablenumberofpulsarscanbeexplainedintermsofadramaticphaseofstarfor-mation,probablyrelatedtothemostrecentclosestap-proachoftheSMCandtheLargeMagellanicCloud(LMC;Gardiner&NoguchiTodatemostoftheX-raystud-

iesoftheSMChaveconcentratedontheBarwhichhasprovedtobeasigni cantsourceofHMXBs.Thesesys-temsnotonlyprovideanhomogeneoussampleforstudy,butalsogivedirectinsightsintothehistoryofourneigh-bouringgalaxyastheyaretracersofstarformationrates.PartofthepuzzleoftheX-raypopulationoftheSMCisthemissingorunderrepresentedcomponents.Inpartic-ular,therearenoknownlow-massX-raybinaries(LMXBs)orblackholebinariesandonlyonecon rmedsupergiantX-raybinarydetectedtodate(seealsoAsurveyoftheX-raybinarypopulationoftheLMCbyNegueruela&revealedasimilardistribution(withinsmallnumberstatistics)tothatinourgalaxy-alltypeswerepresent.Itisthereforeimportanttotryandiden-tifythe“missing”X-raybinarytypesintheSMC.

Werecentlycompletedthe rstX-raysurveyintheSMCWingwithChandra(seeSection2formoredetails).Astudyofthebrightest(>50counts)X-raysourcesuncov-eredtwonewpulsars,anddetectedtwopreviouslyknownpulsars(McGowanetal.Inadditiontothefourpul-sars,thesampleincludedtwoforegroundstars,12probableAGNsand veunclassi edsources.Wefoundthatthepul-sarshadharderspectrathantheotherbrightX-raysources.Inthispaperwereportontheanalysisofthewholesurvey

E-mail:kem@astro.soton.ac.uk

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