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 …… 此处隐藏：23216字，全部文档内容请下载后查看。喜欢就下载吧 ……