Observational Evidence for Intermediate-Mass Black Holes in

Evidence is mounting that some Ultra-luminous X-ray sources (ULXs) may contain accreting intermediate-mass black holes (IMBHs). We review the current observational evidence for IMBH-ULXs. While low-luminosity ULXs with L_X <~ 10^39.5 erg/s (assuming isotro

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aFebruary2,20089:38WSPC/TrimSize:9.75inx6.5inforProceedingsmainmod2OBSERVATIONALEVIDENCEFORINTERMEDIATE-MASSBLACKHOLESINULTRA-LUMINOUSX-RAYSOURCESE.J.M.COLBERT JohnsHopkinsUniversity,CenterforAstrophysicalSciences,DepartmentofPhysicsandAstronomy,3400NorthCharlesStreet,Baltimore,MD21218,USAE-mail:ed@http://www.77cn.com.cnM..C.MILLERUniveristyofMaryland,DepartmentofAstronomy,CollegePark,MD20742,USAE-mail:miller@astro.umd.eduEvidenceismountingthatsomeUltra-luminousX-raysources(ULXs)maycontainac-cretingintermediate-massblackholes(IMBHs).WereviewthecurrentobservationalevidenceforIMBH-ULXs.Whilelow-luminosityULXswithLX<39.5ergs 1(as-emission)areconsistentwithmildlyX-raybeamed～10sumingisotropichigh-massX-raybinaries,thereareaconsiderablenumberofULXswithlargerX-rayluminositiesthatarenoteasilyexplainedbythesemodels.Recenthigh-S/NXMMX-rayspectraare

showinganincreasingnumberofULXswith“cooldisks”–accretiondiskswithmulti-colorblackbodyinnerdisktemperatureskTin～0.1 0.2keV,consistentwithaccretingIMBHs.Opticalemission-linestudiesofULXnebulaeprovideusefulmeasurementsofX-rayenergetics,andcanthusdetermineiftheX-raysareemittedisotropically.AnalysisofanopticalspectrumoftheHoIIULXnebulaeimpliesanX-rayenergysourcewith～1040ergs 1ispresent,suggestinganisotropically-emittingIMBH.Thespatialcoin-cidenceofULXswithdensestarclusters(youngclustersandglobularclusters)suggeststhatIMBHSformedintheseclusterscouldbethecompactobjectsintheassociatedULXs.Quasi-periodicoscillationsandfrequencybreaksinXMMpower-densityspec-traofULXsalsosuggestthattheblackholemassesaremoreconsistentwithIMBHsthanstellar-massblackholes.SincealloftheseULXswithevidenceforIMBHsarehigh-luminosityULXs,i.e.,LX>～1040ergs 1,wesuggestthatthisclassofULXsisgenerallypoweredbyaccretingIMBHs.

Evidence is mounting that some Ultra-luminous X-ray sources (ULXs) may contain accreting intermediate-mass black holes (IMBHs). We review the current observational evidence for IMBH-ULXs. While low-luminosity ULXs with L_X <~ 10^39.5 erg/s (assuming isotro

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1.Introduction

Ithaslongbeensuspectedthatblackholesofmasses～102 104M⊙mayformin,forexample,thecentersofdensestellarclusters(e.g.,Wyller1970;Bahcall&Ostriker1975;Frank&Rees1976;Lightman&Shapiro1977;Marchant&Shapiro1980;Quinlan&Shapiro1987;PortegiesZwartetal.1999;Ebisuzakietal.2001).However,formanyyearstherewasnoobservationalevidenceforsuchamassrange.Inroughlythelastdecade,X-rayandopticalobservationshaverevivedthispos-sibility.Ifsuchblackholesexist,especiallyindensestellarclusters,theyhaveahostofimplications,especiallyforclusterdynamicalevolutionandthegenerationofgravitationalwaves.

Inthisarticle,wediscusstheevidenceforintermediate-massblackholes

(IMBHs)inUltra-LuminousX-raysources(ULXs).ULXsareextra-nuclearpointsourcesthathaveX-ray uxesmanytimestheangle-averaged uxofaM>～20M⊙blackholeaccretingattheEddingtonlimit.EvidenceforIMBHsinglobularclustersandotherastrophysicalobjectshasbeendiscussedinseveralrecentreviewarticles,suchasvanderMarel(2003),andMiller&Colbert(2004).WefocushereontheULXs,whichweregardashavingthemostconvincingevidenceforIMBHs.

2.ABriefHistoryofULXs

Ifweconsiderstellar-massblackholes(BHs)tohaveamaximummassof≈20M⊙(e.g.Fryer&Kalogera2001),thentheEddingtonluminosity(thelimitingbolomet-

39 1ricluminosity;seeEqn.2)ofan“intermediate-mass”BHis>～3×10ergs.

ThelimitingX-rayluminosityinthe2 10keVband,forexample,willbeafac-torofafew 10timessmaller,anditwillbedependentonthemetallicityaswell.ThelowerlimittotheX-rayluminosityforaULXisde nedtobe1039.0ergs 1.

39.0ergs 1)Inpractice,thislimitdistinguishesthe“normal”BHXRBs(LX<～10

foundinourGalaxyfromtheintriguinglymoreluminousULXsfoundinsomenearbygalaxies.TheupperlimitforLXforULXsisnotspeci ed,butusuallyobjectshaveLX<1040.5ergs 1,andmostofthemhaveLX<1040.0ergs 1.Quasars,supernovae,andothergalaxynucleiareusuallyomitted,althoughsomeworkers(e.g.Robertsetal.2002a)includeX-rayluminoussupernovae.

ULXswerenamedassuchbyseveralJapaneseworkerswhoanalyzedspectra

fromtheJapaneseX-raysatelliteASCA(Mizunoetal.1999,Makishimaetal.2000).Here,“ultra-luminous”isgaugedwithrespectto“normal”X-raybinaries.Anothertermthatisusedis“Intermediate-luminosityX-rayObjects,”(IXOs),whichsimplyindicatesthattheirX-rayluminositiesareintermediatebetweenthoseof“normal”stellar-massBHXRBs,andAGNs.

ULXswereobservedasearlyasthe1980s,whenextensiveX-rayobservations

ofexternalgalaxieswere rstperformedwiththeEinsteinsatellite.ManyofthenearbyAGNsinSeyfertgalaxieswereexpectedtohaveveryluminousX-raynuclei,butitwasasurpriseto ndthatmany“normal”spiralgalaxiesalsohadcentralX-raysources(seeFabbiano1989forareview).TheseX-raysourceshadX-ray

Evidence is mounting that some Ultra-luminous X-ray sources (ULXs) may contain accreting intermediate-mass black holes (IMBHs). We review the current observational evidence for IMBH-ULXs. While low-luminosity ULXs with L_X <~ 10^39.5 erg/s (assuming isotro

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39 1luminosities>～10ergs,wellabovetheEddingtonvalueforasingleneutron

starorastellar-massblackhole.ThespatialresolutionofthemostwidelyusedinstrumentonEinstein(theImagingProportionalCounter,ortheIPC,FWHM>～1′)is>～1kpcfortypicalgalaxydistances～4Mpc,soitwasnotclearwhetherth …… 此处隐藏：35504字，全部文档内容请下载后查看。喜欢就下载吧 ……