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08.06.1;08.14.1;09.09.1RCW86;09.19.2;13.25.5

AnunresolvedX-raysourceinsidethesupernovaremnantRCW86

JaccoVink,FabrizioBocchino,FrancescoDamiani,andJelleS.Kaastra

AstrophysikalischesInstitutPotsdam,AnderSternwarte16,D-14482Potsdam,Germany

AstrophysicsDivision,SpaceScienceDepartmentofESA,ESTECPostbus299,2200AGNoordwijk,TheNetherlandsOsservatorioAstronomicodiPalermo,PiazzadelParlemento1,I-90134Palermo,Italy

SRON,LaboratoryforSpaceResearch,Sorbonnelaan2,NL-3584CAUtrecht,TheNetherlands

Received/Accepted

Abstract.WereportonthediscoveryofanunresolvedX-raysourceinsidethesupernovaremnantG315.4-2.3(RCW86).Thesourceislocated7totheSouthwestofthegeometricalcentreandmaybeclosetotheactualexplosioncentreofthesupernova,whichmakesthisacandidateforthestellarremnantassociatedwithRCW86.However,thepresenceofapossible

opticalcounterpartwith

at3fromtheX-raypositionandevidenceforlongtermvariabilitymeansthatthesourceisprobablyanactivestar.AbetterX-raypositionandbetterX-rayspectroscopyalongwithanidentificationoftheopticalsourceareneededtoexcludetheX-raysourceasaneutronstarcandidate.

Keywords:Stars:flare–Stars:neutron–ISM:individualobjects:RCW86–ISM:supernovaremnants–X-rays:Stars

Sendoffprintrequeststo:J.Vink(jvink@aip.de)

Table1.AsummaryofROSATandEinsteinobservationswhichcoverthepointsource.Datesrefertothestartdateoftheobservation.

4/2/1980h1437s62SWEinsteinHRI87564/9/1992rp500078n00NEROSATPSPC32122/2/1993rp500079n00SWROSATPSPC45673/2/1993rp500078a01NEROSATPSPC222418/8/1993rp500078a02NEROSATPSPC45092/9/1994rh500077a01SWROSATHRI24474/8/1995rh500077a02SWROSATHRI6034

2JaccoVinketal.:AnunresolvedX-raysourceinsidethesupernovaremnantRCW86

05:0010:0015:0020:0025:0030:0035:00-62:40:0045:0046:0044:0042:0014:40:00Fig.1.RCW86asobservedbythePSPC(SWpointing).Thecircleindicatesthepositionofthepointsource.

Table2.Bestfitpositionsandcountratesofthepointsource.ThefirstthreerowslisttheresultofmaximumlikelihoodfitstotheROSATandEinsteinHRIdata.Thestatistichasadistributionwiththreedegreesoffreedom.Positionerrorsdonotincludesystematic

errorsandcorrespond

(90%confidenceregions).ThePSPCpositionsandcountrateswereestimatedwithawaveletanalysismethod(Damianietal.1997)usingtheenergychannels20-200(keV).

h1437s62144151.52-623613.12.60.4

28.6rh500077a01144151.64-623611.93.512.4rh500077a02144151.11-623613.21.955.3

rp500078n00144153.3-62363765rp500079n00144151.4-6236815rp500078a01144149.0-62355259rp500078a02144148.9-62362239

1

TheblackbodymodelinSPEXv1.10containsasmallbugwhichwefixedforthisanalysis.ThethinthermalplasmaorCIE(collisionalionizationequilibration)modelissimilartothemekalmodelinxspec.

JaccoVinketal.:AnunresolvedX-raysourceinsidethesupernovaremnantRCW863

Table3.ResultsofthespectralfitstothePSPCdata.Theluminositiesarenormalisedtoadistanceof1kpc.Foreachmodelthespectrumwasfittedwithcmandwithasafreeparameter.Errorrangescorrespondto,or90%confidencelimits.

BlackbodyPowerLawhotthinplasma

(mekal)idem,

kk

keVkeV

cmcm

1.7

3.61.616.95.11.91.3

8.4/91.1/84.1/91.7/814.7/94.3/810.5/91.0/8

ph/s/keV@1keVph/s/keV@1keV

1.7

kk

keVkeVkeVkeV

cmcmcmcm

1.7

kk

1.7

3.01.9

4JaccoVinketal.:AnunresolvedX-raysourceinsidethesupernovaremnantRCW86

materialshouldbeconsideredasanalternativemodel(Umedaetal.2000,Chakrabartyetal.2000).ThesoftnessoftheX-rayspectrumisapropertysharedwiththeunresolvedsourcesinPuppisA,G296.5+10.0andAXPs(Mereghettietal.1996).Itslongtermvariabilityismoreinlinewiththebehaviourofanactivestar,butawellestablishedneutronstarcandidatelikethepointsourceinRCW103isalsovariableonsimilartimescales(Gotthelfetal.1999).

ThepositionoftheX-raysourceisroughlyatanan-gulardistanceof7fromthegeometricalcenterofthesu-pernovaremnant.Thiscorrespondstoatransversevelocity1200km/s,ifthesupernovaremnantisatadistanceof2.5kpcand5000yrold,orifthedistanceis1kpcandanageof1800yr.Suchakickvelocityisratherhigh,butstillconsistentwithobservationsofotherneutronstars(Lyne&Lorimer1994).Note,however,thattheremnantisfarfromcircularlysymmetric,andthecontrastinemissionbetweentheNortheastandSouthwestoftheremnant(seee.g.Vinketal.1997,Bocchinoetal.2000)suggeststhatintheSouth-westtheshockwaveisencounteringadensermedium,whichcouldmeanthattheactualexplosioncenterwasmoretotheSouthwestofthegeometricalcenterandclosertotheunre-solvedsource.Interestingly,theremnantisquitesymmetricintheNortheast/SouthwestdirectionandtheX-raysourceisroughlylocatedontheaxisofsymmetry.Thepositionofthepointsourceisoneofitssalientproperties.Thewaveletde-tectioncodehasalsodetectedanothersignificantpointsourceinsidetheremnant,7fromtheNorthernshellwithcoordinates

14h43m47.0sand-621929.Thesourceseemsem-beddedinasmall1sizeextendedstructure.However,spectralanalysisofthissourcerulesoutmodelswithnon-thermalorblack-bodyradiationandthespectrumisconsistentwithther-malX-rayemission,characteristicfortheNorthrimasreportedbyBocchinoetal.(2000).Thissuggeststhatthisasmallfrag-mentoftheshellseeninprojection.

ItseemsunlikelythattheunresolvedX-raysourceismuchfurtherawaythanRCW86,astheX-rayabsorptioncolumnistoolowforadistantsourcenearthegalacticplane.ThismeansthatitisunlikelythatitiseitheranAGNoranX-raybinary.ForthesourcetobeatypicalX-raybinarywithaluminosityof

erg/s,itshouldbeatadistanceof300kpc.Thisis

incompatiblewithboththeX-rayabsorptionandwiththesizeofourgalaxy.

Themainargumentagainstanunambiguousidentificationofthesourcewithastellarremnantisthepresenceofacan-didateopticalcounterpart.TheUSNO-A2.0catalogue(Monetetal.1998)containsonesourceatanangulardistanceof3totheunresolvedX-raysource(seeFig.2).Thestarhasmag-nitudesand,whichsuggestaK-star,al-thoughthemagnitudeerrorsandtheunknownextinctionmakealatetypeG-staroraearlytypeMstaralsopossible(Zombeck

,1990).ThetypicalVmagnitudeofthisstarwouldbe

which,foramainsequencestar,impliesadistanceofpc.ForthisdistancetheX-rayluminosityoftheX-raysourceis

.ThisisconsistentwiththeX-rayluminosityof

abrightactivestar(seee.g.Zombeck1990p.88,Agrawalet

al.1986).ThepossibilitythatthisisjustachancealignmentcanbeestimatedusingtheUSNO-A2.0catalogue.Thereareabout0.0015starspersquarearcsecwithinthefieldaroundtheX-raysource,whichmeansthatthechancetofindabrightstarwithinthe95%errorcircle(seeFig.2)is11%.

TheevidenceforlongtermvariabilityandthepresenceofalatetypestarclosetotheX-raypositionfavourtheidentifica-tionoftheX-raysourcewithanactivestar,ratherthanwiththestellarremnantassociatedwithRCW86.However,tofirmlyestablishthisidentification,opticalspectroscopyofthepos-sibleopticalcounterpartandX-rayobservationswithabetterpositionalaccuracyofthesourceareneeded.ImprovedX-rayspectroscopicdatacanalsobeusedtodistinguishbetweenthetypicalopticallythinthermalspectrumofanactivestarandthefeaturelessspectrumofaneutronstar.

Acknowledgements.WethankGuentherHasingerandRickHarn-denfortheirinformationregardingthesystematicpositionerrorsofROSATandEinstein.WethankRichardStromandJohnDickelfordiscussionsontheradiomapofRCW86/G315.4-2.3.

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