A Radio Nebula Surrounding the Ultra-luminous X-ray Source i

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

submittedtoApJ

ARadioNebulaSurroundingtheUltraluminousX-RaySourcein

NGC5408

arXiv:0705.1978v1 [astro-ph] 14 May http://ng,PhilipKaaretDepartmentofPhysicsandAstronomyUniversityofIowa,VanAllenHall,IowaCity,IA52242,USAcornelia-lang@uiowa.eduSt´ephaneCorbelAIM Unit´eMixtedeRechercheCEA CNRSUniversit´eParisVII UMR7158,CEASaclay,Serviced’Astrophysique,F 491191GifsurYvette,France.andAllisonMercerDepartmentofPhysicsandAstronomyUniversityofIowa,VanAllenHall,IowaCity,IA52242,USAABSTRACTNewradioobservationsofthecounterpartoftheultraluminousX-raysourceinNGC5408showforthe rsttimethattheradioemissionisresolvedwithanangularsizeof1.5′′to2.0′′.Thiscorrespondstoaphysicalsizeof35–46pc,and

rulesoutinterpretationoftheradioemissionasbeamedemissionfromarelativis-ticjet.Inaddition,theradiospectralindexofthecounterpartiswelldeterminedfromthreefrequenciesandfoundtobeα= 0.8±0.2.Theradioemissionislikelytobeoptically-thinsynchrotronemissionfromanebulasurroundingtheX-raysource.Theradioluminosityofthecounterpartis3.8×1034ergs 1andthemini-mumenergyrequiredtopowerthenebulais～1×1049erg.ThesevaluesaretwoordersofmagnitudelargerthaninanyGalacticnebulapoweredbyanaccretingcompactobject.

Subjectheadings:blackholephysics–galaxies:individual:NGC5408galaxies:stellarcontent–X-rays:galaxies–X-rays:blackholes

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

1.Introduction

Ultraluminousx-raysources(ULXs)arenon-nuclearx-raysourcesinexternalgalaxieswithapparentluminositiesabovetheEddingtonlimitfora20M⊙blackhole,themaximummassofanydynamically-measuredblackholemassintheGalaxy(Remillard&McClintock2006);forareview,seeFabbiano(2006).ULXswithstrongvariabilityarelikelyaccretingobjectsandmayeitherbe“intermediatemass”blackholes(Colbert&Mushotzky1999;Makishimaetal.2000;Kaaretetal.2001)ornormal(stellar-mass)blackholeswithbeamedorsuperEddingtonradiation(Kingetal.2001;K¨ordingetal.2002;Begelman2002).TheULXinthedwarfirregulargalaxyNGC5408(NGC5408X-1)isvariableandisoneofthefewULXswithaknownradiocounterpart(Kaaretetal.2003).Theradioemissioncouldarisedirectlyfromarelativisticjetbeamedtowardourlineofsight,inwhichcaseastellar-massblackholewouldsu cetoproducetheradioandX-rayemission,orfromanebulasurroundingthecompactobject.RecentradioobservationspresentedbySoriaetal.(2006)suggestthatthesourcehasasteepspectrum(α< 1),butshownoindicationofradio uxdensityvariability.

InordertobetterunderstandthenatureoftheradioemissionfromNGC5408X-1,weobtainednewjointobservationsintheradiousingtheVeryLargeArray(VLA)oftheNationalRadioAstronomyObservatory1andintheX-rayusingtheChandraX-RayObservatory.Inaddition,becausethetargetisalow-declinationsource(δ～ 41 ),wepresentnewandarchivalradioobservationsfromtheAustraliaTelescopeCompactArray(ATCA).Finally,wealsoreanalyzedarchivalopticalobservationsmadeusingtheHubbleSpaceTelescope(HST).Wedescribetheobservationsanddatareductionin§2,presentourresultsin§3,anddiscusstheinterpretationin§4.

2.ObservationsandAnalysis

2.1.VLAObservations

VLAobservationsoftheULXsourceinNGC5408weremadeat4.9GHz(6cm)withfourdi erentarraycon gurationsbetweenDecember2003andJanuary2005:A,BnA,B,andCnB(seeTable1).Eachobservationwasapproximately3.5hourslongandcorrespondstoajointChandraX-rayobservation.Standardprocedureswerecarriedoutfor uxandphasecalibrationusingtheAstronomicalImagingProcessingSystem(AIPS)ofthe

NRAO.

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

Weusedthe uxdensitycalibrator,J1331+305,andthephasecalibrator,J1353-441,asourcewhichislocatedwithin3 ofthetarget.Forallobservations,fastswitchingbetweenthetargetsourceandcalibratorwasusedwithacycletimeforthecalibrator-sourcepairof

2.5to3minutes(with45secondsonthecalibratorand90to120secondsonthesource).

TheVLAimageresolutionsvarybetween1.01′′×0.28′′(A-array)and6.01′′×3.30′′(CnB-array).BecauseoftheproximityofthebrightstarburstregioninNGC5408andtheweakradioemissionassociatedwiththeULX,obtaininganaccurate uxdensityforthissourceisdi cult.However,withthehighresolutionoftheVLAA,BnAandB-arrays,itispossibletodistinguishtheradioemissionassociatedwiththeULXfromtheextendedemissionfromthestarburstregioninmostofourimages.

Infact,inattemptingtodeterminetheaccurate uxdensityandsourcestructure,theextendedemissionfromthisstarburstregionturnsouttobeakeyissue.Onewaytoexaminethecontributionoftheextendedemissionistoadjusttheimagingweightingfunction.TheBriggs’ROBUSTparametercontrolsthedataweightsinthe(u,v)plane.PositivevaluesoftheROBUSTparameter(1to5)bringoutthemoreextendedstructurebyweightingtheinnerpartofthe(u,v)planemoreheavily(“naturalweighting”).Suchvalueswillincreasethesignaltonoiseonextendedfeaturesbutlowerthespatialresolution.NegativevaluesoftheROBUSTparameter( 1to 5)givemoreequalweighttoallpointsinthe(u,v)plane(“uniformweighting”)whichtendstomaximizespatialresolutionbutatthecostofsignaltonoise.AROBUST=0isbalancedbetweentheseextremes.AsmanyfourimagesweremadefromdataforanindividualarrayusingvaryingROBUSTparameters.

Inaddition,amorequantitativewaytoremoveextended uxistolimittherangeof(u,v)baselinesusedforimaging.IncaseswheretheemissionfromtheULXiscontaminatedbyextended uxfromthestarburst,a(u,v)cuto of5kλwasapplied.Thiscuto limits uxonangularsizescaleslargerthan～6′′.FortheVLA-BandVLA-CnBarrayimages,whereextended uxispresent,suchcuto shavebeenapplied.

2.2.ATCAObservations

WealsomadecontinuumradioobservationsofNGC5408usingtheAustraliaTelescopeCompactArray(ATCA)locatedinNarrabri,NewSouthWales,Australia.TheATCAconsistsof ve22-mantennaspositionedalonganE-WtrackwithashortN-Sspurandasixthantennaata xedlocation.WeobtainedthreenewATCAobservationsof12hourseachonconsecutivedayswiththe6kmcon guration6.0A(6A).ThisprovidesthebestangularresolutionpossiblewithATCA(～1′′at4.8GHz).Forthe rsttwoobservations,

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

weswitchedthefeed-hornsatroughlyonehourintervalsobservingineitherthe6cmband(4.8GHz)orthe13and20cmbands(1.4and2.4GHz).Thedutycycleoftheswitchingwasarrangedsotheintegrationat6cmwasfourtimesaslongasthoseat13and20cm.Onthelastday,weobservedwiththe5cmband(6.1GHz)insteadofthe6cmband(4.8GHz).

J1934-638andJ1349-439wereusedfor uxandphasecalibration.Editing,calibration,imagingandanalysiswereperformedusingstandardroutinesintheMIRIADsoftwarepack-age(Sault&Killeen1998).AradiocounterparttotheULXwasdetectedatallfrequencies.BecauseofthelowdeclinationofNGC5408,theATCAformsamoresymmetricsynthesizedbeamthantheVLA.TheextendedemissionassociatedwiththestarburstinNGC5408ispresent,butintheATCAimages,theradiocounterparttotheULXisclearlydistinguished,especiallyinthe6Acon gurationdataat4.8,6.2GHz.However,atthelowerfrequencies(1.4and2.3GHz)thereisstillcontaminationfromthestarburst.Duringimaging,avarietyofROBUSTparameterswerealsousedtodownweighttheextendedemission,similartotheprocedureintheVLAimaging.Inaddition,weanalyzedarchivalATCAobservationsofNGC5408atarangeoffrequencies,observedduring2000-2004(listedinTable2).IntheseveraloftheATCAdatasets,(e.g.,the6D-array4.9GHzdata(Kaaretetal.(2003),andthe2.3and1.4GHzarchivaldatasets)a(u,v)cuto rangeof3kλwasusedtoremove uxonextendedscaleslargerthan11′′.

2.3.X-rayObservations

NGC5408wasobservedwiththeChandraX-RayObservatory(Weisskopf1988) vetimesusingtheACISspectroscopicarray(ACIS-S;Bautzetal.1998)inimagingmodeandtheHigh-ResolutionMirrorAssembly(HRMA;vanSpeybroecketal.1997).Theobserva-tionsbeganin2002andendedin2005,seeTable1.BecauseahighX-raycountratewasexpected,onlytheS3chipwasoperatedwitha1/4sub-arraymodeforthe rstobservationanda1/8sub-arrayfortheotherobservations.Thisgaveanexposuretimeof0.8sforthe rstobservationand0.4sfortheothers.Foreachobservation,weconstructedanimageusingallvalideventsontheS3chipandusedthewavdetecttoolwhichispartoftheCIAOdataanalysispackagetosearchforX-raysources.TypicallyonlytheULXwasdetectedineachobservation.ThepositionoftheULXwaswithin0.23′′ofRA=14h03m19.63s±0.01s,DEC-41d22’58.65”±0.2′′(J2000)inallobservations.

We ttheX-rayspectrumoftheULXforeachobservationusingtheXSPECspectral ttingtoolwhichispartoftheLHEASOFTX-raydataanalysispackageandresponsematricescalculatedusingCIAO.AspreviouslyreportedbyKaaretetal.(2003),wefound

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

thatabsorbedsinglepower-lawmodelsdidnotprovideanadequate ts(exceptforthelastobservation),whilemodelsconsistingofeitheranabsorbedbrokenpower-lawortheabsorbedsumofamulticolordiskblackbodyplusapower-lawdidprovideadequate ts.Therewerenopronouncedchangesinspectrashapebetweenthedi erentobservations.The tparametersweresimilartothosequotedinKaaretetal.(2003).The uxforeachobservationcalculatedusingthe ttedmulticolordiskblackbodypluspower-lawmodelaregiveninTable1.Wealsodidnotobserveanysigni cantchangesintheX-ray uxlevelbetweenobservations.

2.4.HSTObservations

TwoWFPC2observationsofNGC5408arepresentintheHubbleSpaceTelescope(HST)archive.Theobservationswereobtainedaspartofasnapshotsurveyofnearbydwarfgalaxycandidates(GO-8601,PISeitzer)andconsistof600sWFPC2exposureswiththeF606WandF814W ltersobtainedon4July2000.AfterextractingthedatafromtheHSTarchive,weusedIRAFtomosaicandcleantheimages.WecorrectedtheabsoluteastrometryoftheHSTimageusingstarsfromthe2MASScatalog(Skrutskieetal.2006)usingtheGraphicalAstronomyandImageAnalysisTool(GAIA).Weusedonly2MASSsourcesontheWF3chipswheretheULXislocatedinordertoprecludeissuesregardingtherelativepositioningoftheWFPC2chips.Weestimatethattheastrometricuncertaintyis0.2′′.WeusedtheHSTphotstellarphotometrypackagetoobtainphotometry(Dolphin2000).Weremovedbadpixels,cosmicrays,andhotpixelsandthenobtainedsimultaneousphotometryfortheF606WandF814Wimages.

3.

3.1.ResultsRadioCounterpart

The uxdensityoftheradioemissionassociatedwiththeULXintheVLAandATCAimageswasmeasuredby ttinga2-DGaussianpointsourcetothepeakradioemissionineachofthevariousimages.Inallcases,thesourceappearstobepoint-likewithslightlyextendedemission,butnothingwhichwecancon dentlyresolve.Theradiosourcepo-sitionfromourVLAB-arrayimage(resolutionof3.17′′×0.92′′)isRA,DEC(J2000)=14h03m19.63s±0.01s, 41d22′58.7′′±0.2′′,whichiswithin0.1′′oftheULXpositioncal-culatedfromtheChandraobservations(Kaaretetal.2003)http://ingtheATCA4.8GHzand6.1GHzdata,thepositionoftheradiosourceisRA,DEC(J2000)=14h03m19.61s±0.02s, 41d22’58.5”±0.2”.

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

3.1.1.4.9GHzEmission

Table3liststhemeasured4.9GHz uxdensityandcorrespondingROBUSTweighting, arraycon guration,geometricalbeamsize(

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

3.1.2.SourceSizeandStructure

Previously,Soriaetal.(2006)hadmadesomeofthehighestresolutionobservationsoftheULXinNGC5408usingtheATCA.Theymeasuredthe uxdensityinimageswithbeamsizesof1.5′′to3.5′′andfoundanessentially atdistributionof uxdensitywithbeamsize.However,theirobservationsdonotresolvethesourceandthereforeareonlyconsistentwithemissionfromapointsource(Soriaetal.2006).TheVLAdatapresentedhereforthe rsttimeprobeevenhigherangularresolutions(0.5′′to1.5′′).4showsthattheVLAdataarecrucialinstudyingthe uxdensityversusgeometricbeamsizeforscales<

1.5′′.Thereisacleartrendofdecreasing uxdensityatsmallerbeamsizesbelowageometricbeamsizeof1.5′′.Thelinearriseof uxdensitywithbeamsizeshowninFigure4indicatesthatthatthereisextendedemissionassociatedwiththissourceonbeamsizescalesbetween～0.5′′and～1.5′′.Forbeamsizesgreaterthan～1.5′′,the uxdensitymeasurementsarerelativelyconstant(withintheerrors),suggestingthesourcehasanangularextentintherangeof1.5′′to2.0′′.

3.1.3.Variability

BecausetheradiosourceassociatedwiththeULXislikelytobesomewhatextended,di erentcon gurationsoftheVLAradiotelescopewillbesensitivetoemissionondi eringangularsizescales.Therefore,itisnotpossibletolookfor uxvariabilityinourhigh-resolutionVLAobservations(wherethecon gurationrangesfromA-arraytoCnB-array).However,ATCAobservationsoveranumberofepochs(e.g.,Kaaretetal.(2003),Soriaetal.(2006),andthispaper)haveshownthatthe uxdensityat4.9GHzdoesnotappeartovarysigni cantlyandhasanaverage,overallvaluenear0.20mJy.

3.1.4.MultifrequencyDataandSpectralIndex

Multi-frequencyobservationsat1.4,2.3,4.9and6.1GHzweremadeaspartofthenewATCAdatapresentedhere.Ateachfrequency,allexistingATCAdatawerecombined(seeTable2)andapointsourcewas ttotheradioemissionassociatedwiththeULX.We nd uxdensitiesof0.62±0.10mJyat1.4GHz(geometricbeamsizeof6.7′′;ROBUST= 5;(u,v)cuto <5kλ),0.37±0.08mJyat2.4GHz(geometricbeamsizeof3.3′′;ROBUST= 5),0.20±0.03mJyat4.9GHz(geometricbeamsize=1.5′′,ROBUST= 5)and0.17±0.03at

6.1GHz.At1.4GHz,thecontributionfromtheextendedbackgroundmaybepresenteventhoughweuseduniformweightingandlimitedtheshortest(u,v)data;thebeamsizeis

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

large.At2.3GHz,theimageismadewithuniformweightingandthecontributionfromtheextendedstarburstregionisnotapparent.Therefore,wedeterminethespectralindexbasedonthe2.4,4.9and6.1GHzmeasurementsonlyandobtainα= 0.8±0.2,whereSν∝να.Figure5showsthe uxdensityversusfrequencyandincludesthe1.4GHzmeasurementaswellasthe8.5GHzATCAupperlimitfromKaaretetal.(2003).Althoughnotincludedinthe t,the1.4and8.5GHzmeasurementsareconsistentwiththe ttedspectralindex.

3.2.Opticalcounterpart

Figure6showstheHST/WFPC2imageintheF606W lterfortheareaneartheULX.Thecircledrawnonthe gureiscenteredattheVLAsourcepositionquotedaboveandhasaradiusof0.28′′,whichrepresentstherelativepositionuncertaintyincludingboththeradiopositionuncertainandtheopticalastrometryuncertainty.Onlyoneobjectlieswithintheerrorcircle.ItislocatedatapositionofRA,DEC(J2000)=14h03m19.62s, 41d22’58.54(J2000)whichis0.17′′fromtheradiopositionandwellinsidetheerrorcircle.ThenextclosestHSTsourceis0.43′′fromtheVLApositionandisoutsidetheerrorcircle.WeidentifytheHSTsourcewithintheerrorcircleasthelikelyopticalcounterpartoftheULX.

Theopticalcounterparthasmagnitudes,intheWFPC2 ightphotometricsystem,of22.387±0.021intheF606W lterand22.396±0.043intheF814W lter(Dolphin2000).TheequivalentJohnson-CousinsmagnitudesareV=22.4andI=22.4.Wecor-rectedforreddeningusinganextinctionE(B V)=0.068basedonthedustmapsofSchlegel,Finkbeiner,&Davis(1998)andusinganRV=3.1extinctioncurve.Thedered-denedmagnitudesareV0=22.2andI0=22.3andthecolorisV0 I0= 0.1±0.1.

4.Discussion

Theradiocounterpartsizeof1.5′′to2.0′′correspondstoaphysicaldiameterof35–46pcatthedistanceof4.8MpctoNGC5408,asdeterminedbyfromthetipoftheredgiantbranchKarachentsevetal.(2002).Therefore,wecanruleouttheinterpretationoftheradioemissionassociatedwiththeNGC5408ULXasarelativisticallybeamedjet(Kaaretetal.2003).

TheradiosurfacebrightnessofNGC5408at4.9GHzis～4×10 20Wm 2Hz 1sterrad 1,assumingasourcesizeof1.5′′.Takingaspectralindexof 0.8,wecancalculatethesurfacebrightnessat1.4GHzinordertocomparetovaluesforknownsupernovaremnantsofsimilarsizes(Green2004).Thevalueof～1×10 19Wm 2Hz 1sterrad 1issigni cantly

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

higherthanthatofknownsupernovaremnantsofsimilarsize.ItisthereforemorelikelythattheradioemissionassociatedwithNGC5408X-1arisesinsteadfromanextendedradiolobe.Theradiospectrum(α= 0.8)ofthissourceisconsistentwithoptically-thinsynchrotronemission.Forcomparison,theradionebulaW50surroundstherelativisticjetsourceSS433(Margon1984)andisthoughttobepoweredbytherelativisticout ow.SS433hasbeensug-gestedasapossibleGalacticanalogtotheULXs(Fabrika&Mescheryakov2001;Begelman2006).TheradionebulainW50isinterestingbecausethenebularradioemissionisunlikelytobebeamed.ThephysicalsizeofW50isroughly50pcandissimilartowhatweestimatefortheradionebulasurroundingNGC5408X-1.TheradiospectralindexesofthevariouscomponentsofW50rangefrom 0.5to 0.8,alsoconsistentwiththespectralindexoftheradioemissioninNGC5408X-1(α～ 0.8).Acomparisonoftheradiobrightnesscanbemadeforthetwosources.W50hasanintegrated uxdensityat1.4GHzof～70Jyandadistanceof～5kpc(Dubneretal.1998).Assumingaspectralindexof 0.5,thattranslatestoa4.9GHz uxdensityof40Jy,andifitwereatthedistanceofNGC5408(4.8Mpc),its uxdensitywouldbe～40µJy.Theintegrated uxdensityofNGC5408X-1at4.9GHzis～200µJy,soitsradiobrightnessismorethanafactorof5greaterthanthatofW50.

Weinvestigatetheenergeticsoftheradiolobeassumingradiationviasynchrotronemis-sion,equipartitionbetweenparticlesand elds,andequalenergyinelectronsandbaryons.Weuseaspectralindexα= 0.8(see§3.1.4),alowerfrequencycuto of1.3GHz,andanup-perfrequencycuto of6.2GHz.Thetotalradioluminosityofthesourceis3.8×1034ergs 1.Forasourcediameterof46pcanda llingfactorofunity,we ndthatthetotalenergyre-quiredis3.6×1049erg,themagnetic eldis16µG,andthesynchrotronlifetimeis～20Myr.Forcontrast,foradiameterof35pcanda llingfactorof0.1,thetotalenergyrequiredis9×1048erg,themagnetic eldis39µG,andthelifetimeis～5Myr.WenotethattheestimatesofSoriaetal.(2006)fortheenergycontentofasynchrotronnebulasurroundingNGC5408X-1areaboutanorderofmagnitudelarger.ThisisprimarilybecausetheyassumeanelectronenergydistributionthatextendsdowntoaLorentzfactorof1,whilewe,conservatively,assumethattheelectronenergydistributionextendsonlyovertherangeneededtoproducetheobservedradioemission(1.3to6.2GHz).

ThetotalenergycontentinrelativisticelectronsinW50isintherange(0.5 7)×1046erg(Dubneretal.1998).ThetotalenergycontentinrelativisticelectronsintheradionebulasurroundingNGC5408X-1isatleasttwoordersofmagnitudelarger.Therefore,iftwonebulaaresimilar,thenthejetpoweringthenebulasurroundingNGC5408X-1mustbeatleasttwoordersofmagnitudemorepowerfulthanthatfromSS433poweringW50.Theradiolobespoweredbythepersistentaccretingstellar-massblackholeGRS1758-258

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

havearadioluminosityof3×1030ergs 1andrequireanenergycontentof2×1045erg(Rodr´ guez,Mirabel,Mart´ 1992)andareevenlesspowerfulthanW50.WenotethattheULXHolmbergIIX-1hasanassociatedradionebulawithatotalenergysimilartothatoftheNGC5408X-1nebula(Milleretal.2005).

AtthedistancetoNGC5408,theabsolutemagnitudeoftheopticalcounterpartwouldbeMV= 6.2.Ifthelightarisesonlyfromthestellarcompanion,thenthemagnitudeandcolorexcludemainsequenceandgiantstarsandrequireasupergiantstar.Theabsolutemagnitudeandcolorareconsistentwithclassi cationasaBorearlyAsupergiant.Theluminosityofsuchastarwouldbeintherangeof1 5×1038ergs 1.However,thestellarclassi cationissuspectsincelightmayarisefromreprocessingofX-raysfromthecompactobjectandsincetheX-rayluminosity(～1040ergs 1iftheemissionisisotropic)exceedstheexpectedstellarluminositybyafactorofatleast20andmaystronglya ectthephysicalstateofthestar.

TheX-raytooptical uxratio,de nedfollowingvanParadijs&McClintock(1995)asξ=B0+2.5logFXwhereFXistheX-ray uxdensityat2keVinµJyandweapproximateB0=V0duetolackofaB-bandimage,isξ=20.0.Ifweusethe uxdensityat1keV,thisrisestoξ=21.1.Thesevaluesarehigherthanthoseofanyhigh-massX-raybinary(HMXB),otherthanLMCX-3,andareintherangetypicallyfoundforlow-massX-raybinaries(LMXBs).Thus,itispossiblethatthecompanionstarcontributeslittletotheobservedopticalemission,e.g.,itisnotsupergiant,andthatmostoftheopticallightarisesfromreprocessingofX-rays,asoccursinLMXBs(Kaaretetal.2005).

Acknowledgments

PKacknowledgespartialsupportfromChandragrantCXCGO4-5035AandaFacultyScholarAwardfromtheUniversityofIowa.STSDASandPyRAFareproductsoftheSpaceTelescopeScienceInstitute,whichisoperatedbyAURAforNASA.TheauthorsalsothankMichaelRupenforplanningandcarryingouttheVLAobservations.

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New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

Table1.ChandraandVLAObservationsofNGC5408

Date/TimeX-RayFluxVLAarray

Note.—ThetableliststheChandra/VLAob-

servationsofNGC5408andincludesthedateand

UTtimeofthestartoftheChandraobservation(the

VLAobservationswereroughlysimultaneous),the

ULXX-ray uxinthe0.3-8keVbandinunitsof

10 12ergcm 2s 1duringtheobservation,andthe

VLAarraycon guration.

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

Table2.ATCAObservationsofNGC5408

Date/TimeArrayFrequency(GHz)Reference

Note.—ThetableliststheATCAobservationsofNGC5408andincludes

theobservationdate,thefrequencyorfrequenciesofobservation,andthe

measured uxdensity.

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

Table3.4.9GHzFluxDensityofNGC5408

Telescope

ArrayRobustWeightFluxDensity(mJy)GeometricBeamsize(u,v)cuto

VLA-CnB

VLA-CnB

VLA-CnB

VLA-CnB

ATCA-6D0 5 5 150.51±0.030.46±0.040.32±0.040.47±0.030.43±0.044.453.773.714.083.46<5kλ

New radio observations of the counterpart of the ultraluminous X-ray source in NGC 5408 show for the first time that the radio emission is resolved with an angular size of 1.5 to 2.0 arcseconds. This corresponds to a physical size of 35-46 pc, and rules ou

-41 22 40

45

DECLINATION (J2000)50

55

23 00

14 03 20.019.519.018.5RIGHT ASCENSION (J2000)18.017.5

Fig.1.—VLA4.9GHzBnA-arrayimageoftheradioemissioninNGC5408.ThecompactradiosourceassociatedwiththeULXislocatedinthelowerleft(SE)oftheplot.ThecrossrepresentsthepositionoftheULX.Thespatialresolutionofthisimageis1.94′′×

1.20′′,PA=8 ,andtheimagehasbeenmadewithROBUST=1weighting(slightlynaturallyweighted).Thecontourlevelsrepresentradioemissionat-3,3,5,7,9,10,15,20,25,50,75,100,150,and200timesthermslevelof20µJybeam 1.