Observations of ultraluminous X-ray sources

0.18 MB·

by  Jeanette Claire Gladstone

Observations of ultraluminous X-ray sources
Jeanette C. Gladstone
DepartmentofPhysics,UniversityofAlberta,Edmonton,Alberta,T6G2G7,Canada
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Abstract.
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UltraluminousX-raysources(ULXs),firstobserved∼30yearsago,havebeenarguedasextremestellarmassblackhole
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binariesoranew classintermediatemassblackhole. Inorder tosettlethisdebate,scientistshaveutilisedawiderangeof

n telescopes,exploitinglargesectionsoftheelectro-magneticspectrum.Herewereviewsomeoftheinsightgainedfromthese
a observationalstudies,collatinganoverviewofourcurrentpositioninULXresearch.
J
Keywords: accretion,accretiondisks-blackholephysics-X-rays:binaries-galaxies-stars-ISM:jetsandoutflows,
7 PACS: 95;98
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]
E INTRODUCTION
H
UltraluminousX-raySources(ULXs)arepointlike,extragalactic,non-nuclearobjectswithinferredX-rayluminosi-
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h tiesinexcessoftheEddingtonlimit(LEdd≃1039ergs−1)forastellarmassblackhole(StMBH,3–20M⊙),radiating
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isotropically.Thesimplestwaytoexplainthisistoassumealargermasscompactobject,withproposedmassesrang-
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o ingfrom102 –104M⊙ (e.g.Colberts&Mushotzky1999),accretingatsub-Eddingtonrates.Blackholesofthissize
r providean attractiveopportunitytoexplainthe ‘missinglink’inthemassscale of blackholes,andpresentpossible
t
s formation routes for super-massive black holes (SMBHs) and the formation of galaxies (e.g. Madau & Rees 2001;
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Ebisuzakietal.2001).
[
AnalternatetheoryisthatweareinsteadobservingStMBHs1thatappeartobeemittingabovetheEddingtonlimit,

1 circumventingtheEddingtonlimitviarelativisticorgeometricbeaming.Relativisticbeamingoccursasaconsequence
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of the observerlookingdownthe funnelof a jet (Kördinget al. 2002),resulting in Dopplerboostedemission. This
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occursineitherthelow/hard(LHS)or(occasionally)veryhighstate(VHS),whenjetemissionispresent.Geometric
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beamingoccursathighermassaccretionratesasthediscapproachestheEddingtonlimitandwhenradiationpressure
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5 becomessignificant(e.g.Kingetal.2001;King2008;2009).Theinflowingmaterial,beingfedthroughtheaccretion
. disc reachesthe Eddingtonlimit (locally)at a pointknownas the spherizationor trappingradius.To keepthe local
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radiationenergyreleaseclosetoEddington,theexcessenergyandmatterislostviaastrongoutflow.Thiscollimation
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1 causesabeamingeffectthatbooststheapparentemissionfromthesourceforanobserveralongtheconeofemission.
1 The third option is that we are in fact observing super-Eddington accretion onto a StMBH, forcing a change in
: the accretiongeometry.Thereare manysuggestionsforpossible geometriesat suchhighaccretionrates.One is the
v
slim disc model,where an increase in mass accretionrate causes the disc to becomeso opticallythick thatphotons
i
X areadvectedradiallyleadingtoadditionalcooling(Abramowiczetal.1988).Analternativetothiswasproposedby
r Poutanenetal.(2007),inwhichitispredictedthatwewouldobserveacoolaccretiondiscandaphotosphere(radiating
a
ablackbody-likespectrum)emittingacool(∼1keV)thermalcomponentalongwithapossibleoutflow/wind.
AbovewehaveoutlinedsomeofthecurrenttheoriesproposedtoexplaintheemissionfromULXs.Throughoutthis
paperwewilldiscussmulti-wavelengthobservationsofthesesources,focussingontheinsightsthatcanbeobtained
withregardtotheunderlyingcompactobject,inanattempttounlockthenatureofthesesystems.
EARLY ULX SURVEYS
ThefirstcataloguesofULXswerecreatedusingROSAT(e.g.Colbert&Mushotzky1999;Roberts&Warwick2000;
<
Colbert&Ptak2002).Thesesurveysshowedthatnon-nuclearluminousX-raysourcesappearedin∼20percentthe
1 HerewedefineStMBHstobetheproductoftheendpointofasinglestellarevolution(∼<100M⊙),andusethisthroughoutthearticle.
ObservationsofultraluminousX-raysources January28,2011 1
galaxiessampled(Miller&Colbert2004)andcoveredawiderrangeofluminosities(∼<afew×1040ergs−1).
Althoughthesesourcesarerare,over-densitieshavebeenobservedin somegalaxiessuchastheAntennae,orthe
>
Cartwheel. In a survey of the Antennae, it was found that ∼ 10 sources emitting in the ultraluminousregime (e.g.
Fabbianoetal.2003).ThemassoftheseobjectswasinitiallyestimatedbyassumingtheywereX-raybinarysystems
containing a black hole emitting at the Eddington limit. The luminosity of these sources suggested that we were
observingblackholesofmassMBH∼>10–(few×)100M⊙(assumingnobeaming;Miller&Colbert2004),however
theiraccretionrateisunknown.
OPTICALSTUDIES
Starting from the largest size scales and moving inwards, ULXs have been found preferentially in actively star-
forming galaxies (Kaaret 2005), along with starburst, interacting/merging and dwarf galaxies (Swartz et al. 2008)
preferablywithlowmetallicities(Pakull&Mirioni2002;Zampieri&Roberts2009;Mapellietal.2009),intherange
16Z⊙<Z< 21Z⊙ (Ripamontietal2010).SuchlowmetallicitiesareimportantintheformationoflargeStMBHs.The
lowmetallicityreducesthelossofmaterialviaawindduringastarslife,enhancingthemaximumpossibleblackhole
mass(e.g.Mapellietal2010),italsoenhancestheprobabilityofformingHighMassX-rayBinaries(HMXBs;Linden
etal2010).
An observedcorrelationbetween the numberof ULXs and the globalstar-formationrate in spirals hasalso been
noted (Swartz et al. 2004; Liu et al. 2006). The sheer number of sources observed in starburst galaxies together
with their shortlifetimes (implied by their location in regionsof active star formation)meansthat, if these systems
werealltocontainIMBHs,anunrealisticallylargeunderlyingpopulationmustbepresent(seeFigure1(Left);King
2004).InsteaditismuchmorelikelythatthebulkofthepopulationarethemostextremeexamplesofHMXBs,that
somehowexceedorcircumventtheEddingtonlimit.Thesehighmasscompanionsprovideanaturaloriginforthehigh
masstransferratesrequiredtopowertheobservedluminosities(Rappaportetal.2005).
Anassociationwithstar-formingregions(Fabbiano,Zezas&Murray2001;Liraetal.2002;Gaoetal2003)andthe
unbrokenluminosityfunctionconnectingULXstothestandardX-raybinarypopulation(Grimm,Gilfanov&Sunyaev
2003)alsoindicatesthatwemaybeobservingHMXBsystems.
ULXnebulae
Pakull&Mirioni(2002)presentedastudyofULXenvironmentsinwhichsomeULXsappeartoresideinemission
nebulaeofafewhundredparsecsindiameter.Theyreportedevidenceofbothlowandhighionisationemissionlines
withintheirspectra,indicatingthattheycouldbepoweredbyeithershockorphoto-ionisedradiation.Iftheselinesare
signaturesofphoto-ionisation,thesefeaturescanbeusedtoplacelimitsontheX-rayemissionpoweringit.Bydoing
thisPakull&Mirioni(2002)concludedthattheopticalemissionwasconsistentwithanisotropicX-rayemitter.This
wouldindicatethepresenceofanIMBHatitscentre.
ObservationofotherULXnebulae(e.g.HoIIX-1;Kaaretetal.2004)showedevidenceofX-rayionisation,with
highlevelsofexcitationneartheULX.DespitethehigherimpactofX-rayemission,thesesourcesalsoshowalackof
beamedemission,suggestingthatweareobservingeitherIMBHsorsomeformofsuper-Eddingtonaccretion.
Further analysis of some systems revealed evidence of cavities in the nebulae of some systems, possibly cleared
by shocks (e.g. NGC 5204 X-1; Roberts et al. 2002), while explorations of their spectra showed evidence of both
photoionisationandshocks(e.g.Pakull&Mirioni2003).Also,someULXnebulaeshowsignaturesofstrongwinds
and/or jets (Abolmasov et al. 2007). The presence of cavities/bubbles and winds/jets suggests a link to features
observed around both StMBHs and SMBHs. Such comparisons can give us insights into the feedback that occurs
withinthesesystems(Pakulletal.2010)andintothestrengthofoutflows(Russelletal2010).Althoughthisworkis
stillintheearlystagesofdevelopment,resultsindicatethatULXscouldbeaheterogeneoussampleofsources.
Stellarcounterparts
The search for optical counterparts has revealed a number of faint blue potential counterparts (m ∼ 22 – 26;
V
Robertsetal.2008),suggestingOorBtypestars.Wemustalsoconsiderthattheemissioncouldbecontaminatedby
reprocessedX-raysfromthe accretiondisc/stellarsurface(e.g.Copperwheatetal.2005;Madhusudhanetal. 2008).
ObservationsofultraluminousX-raysources January28,2011 2
FIGURE 1. (Left) Cartwheel galaxy: purple - Chandra Observatory; ultraviolet/blue - Galaxy Evolution Explorer satellite;
visible/green-HubbleSpaceTelescope;infrared/red-SpitzerSpaceTelescope.TheinserthighlightsthelargeX-raypopulation
containedwithinthisgalaxy(Imagecomposite:NASA/JPL/Caltech/P.Appletonetal.X-ray:NASA/CXC/A.Wolter&G.Trinchieri
etal.).(Right)PilotspectraoftheeULXstakenbytheGeminiObservatory(Robertsetal.2010),wenotethepresenceofstrong
nebulaemissionlinesinallcasesandtheHeII4686Ålineintwocases(thoughttobeassociatedwiththeaccretiondisc).
Models created to account of such X-ray irradiation can estimate the underlying stellar spectral type and the mass
of the black hole, with the aide of colour-magnitudediagrams. However, studies using such models have provided
conflicting values for black hole mass, Madhusudhan et al. (2008) indicated a slight preference for IMBHs, whilst
anothershowssupportforStMBHs(Copperwheatetal.2007).Ineithercase,theemissionwasimpactedgreatlyby
X-ray irradiation (increase of ∼ 0.5 – 5 magnitudes, depending on assumptions; Copperwheat et al. 2007). Such a
dramaticeffectmustbetakenintoaccountinfuturestudiesofthesesystems.
StudiesofthestellarenvironmentofULXshavealsoprovidedinsightintothenatureofthecompanion.Forexample,
thecolour-magnitudediagramoftheneighbouringstarsofNGC1313X-2showthatthecompanionis∼20Myrsold
with an uppermass limitof 12M⊙ (Grise et a 2008).Thisconfirmsthatthese systems are HMXBs, butit doesnot
providethemassoftheblackhole.AnHSTphotometricstudyofthesameobjectrevealedapossible∼6dayorbital
period(Liuetal.2009,althoughfurtheranalysisisrequiredtoconfirmthis).WhencombinedwiththeresultsofGrise
etal(2008),Patruno&Zampieri(2010)indicatedthatMBH≃50–100M⊙,suggestingaStMBHresidinginaHMXB
system.
Spectroscopic studies of these systems have also recently been undertaken. Such studies are difficult due to the
luminosity of these systems, but are possible with some of the worlds largest telescopes (e.g. Gemini, VLT; for an
example of data received, see Figure 1 (Right). By obtaining a series of observations, it may be possible to obtain
radialvelocitycurvemeasurementsfromemission/absorptionfeaturesassociatedwitheithertheaccretiondiscorthe
companionstar.Thisprovidesadirectmassmeasurementbasedonconstraintsplacedbythebinaryorbit.Onlyasmall
numberofopticalpilotspectrahavebeentakenandpublishedtodate.Robertsetal.(2001)studiedthecounterpartof
NGC5204X-1,findingablue,featurelessspectrum.SimilarresultshavebeenfoundinNGC1313X-2(Zampieriet
al.2004).Pilotspectraofothersourceshaverevealedalackofspectralfeatures,andslopesthatappearnon-stellarin
origin(seeFigure1(Right);Robertsetal.2010;Gladstoneetal.inprep).
Radialvelocitymeasurementsarebeginning,however,initialresultsfromasurveyoftwoULXs(NGC1313X-2
& Ho IX X-1)foundthatradialvelocityvariationsare observed,butthatthey maynotbesinusoidal(Robertsetal.
2010;Gladstoneetal.inprep),howeverfurtheranalysisisrequiredtoconfirmthis.Theanalysisofthesespectraalso
indicatedthattheHeIIemissionline,whichisassociatedwiththeaccretiondisc,appearstobehighlyvariable(Pakull
et al. 2006;Roberts et al. 2010).Such dramatic variationon short timescales implies that, at a minimum,the He II
emission originateswithin 24 light-hoursof the ULX (Robertset al. 2010).Motch et al. (2010)reportedon studies
ofULXP13,inNGC7793,statingthatvariationsintheHe II linearepresent,andsuperimposedonaphotospheric
spectrum.Thisrevealsthe possible presenceof a late B typesupergiantcompanionof between10 – 20M⊙. Radial
velocity variations were also detected for this source, but again further data and analysis is required to confirm its
periodandnature.
ObservationsofultraluminousX-raysources January28,2011 3
RADIOEMISSION
Radio emission is observed regularly from sources residing in the LHS, and occasionally from those in the VHS
(although the type of radio emission would vary depending on state). By combining this information with the
luminosity of these systems, it would indicate the presence of an IMBH if these sources reside in the LHS (steady
stateemission),oralargeStMBH–IMBHresidingintheVHS(morevariedemission).
Todate,veryfewradiodetectionhavebeenmade,butsteadystateemissionhasbeenobservedintwooftheULXs
within M82,and in oneof those locatedin NGC 4736(Kördinget al. 2005).In this case it wouldsuggestthe LHS
thereforethe radioandX-rayemissionare relatedas followsL (cid:181) L1.38M0.81 (Falcke etal. 2004),implyinga mass
X R
ofMBH≃102−105M⊙ (althoughtheremaybesomesourceconfusion,furtherobservationsarerequiredtoconfirm
this).
A study by Miller etal. (2005)identifiedregionof diffuse resolvedradioemission in the area surroundingHo II
X-1,coincidentwithanextendedHIIregion.Theirinvestigationindicatesthatthiscouldnotbepoweredbybeamed
emission,andso wouldneedto bepoweredbyeithersomeformofsuper-Eddingtonaccretionontoa StMBH orby
sub-Eddington accretion onto an IMBH. However, the nebula surrounding NGC 5408 X-1 also shows evidence of
radio emission (Kaaret et al. 2003), although the authors note that this emission could be the result of relativistic
beamingfromajet.Laterworkrevealedthisemissiontobeslightlyextended,withadiameterof35–46pc(Langet
al.2007).Theirresultscouldnotruleoutrelativisticallybeamedemission,buthighlightedthefactthatthatthenature
of the emission appearedto be comparable with SS 433. This would indicate that we are observingradio emission
poweredbyoutflowsfromasuper-Eddingtonsystem,similartotheW50nebulaaroundSS433(Dubneretal.1998)
X-RAY ANALYSIS
ThestudyofULXsbeganwithobservationsintheX-rayband,andsincethentherehavebeenanumerousobservations.
Studieshavefocusedonboththespectraandtimingforthesesystems,aswesearchforinsightsintotheirnature.These
arediscussedinmoredetailbelow,however,foramoredetaileddiscussionofX-rayanalysisofULXs(pre-2007)we
referthereadertoRoberts(2007)andreferencestherein.
X-ray spectral studies
Earlyspectralstudies,withthecurrentgenerationofX-raytelescopeswerefitwithastandardcanonicaldiscplus
power-law continuummodel, a simple modelthat has been used extensively with Galactic X-ray binary systems to
describe accretion in standard states. The resultant ULX fits indicated the presence of a remarkably cool disc (soft
excess;kT∼150eV)temperature,implyingaMBH ∼few×104M⊙ (e.gMilleretal.2003).However,recentresults
indicatedthepresenceofaspectralbreakabove∼3keV(Stobbartetal.2006;Gladstoneetal.2009),afeaturethatis
notpresentinthestandardaccretionstates,andsoshouldnotbepresentifwewereobservingIMBHs.Re-evaluation
ofsomeofthehighestqualityULXspectraalsorevealedthattheycouldbeequallywellexplainedbymodelsthatinfer
extreme accretion rates onto a StMBH (e.g. Stobbart et al. 2006, Goncalves& Soria 2006) and found a potentially
newstate-theultraluminousstateinwhichasourceexhibitthepresenceofbothasoftexcessandabreakathigher
energies(inthe0.3–10keVbandpass;Gladstoneetal.2009).
Gladstone et al. (2011)discussed some of the currentphysicalmodelsfor ULXs; the slim disc, disc plus Comp-
tonisation,the energeticallycoupleddisc-coronaandthe reflectiondominatedmodel.A simplifiedslim disc models
hasbeenappliedtoULXspectraonanumberofoccasions(e.g.Vierdayantietal.2006).However,whenappliedto
thehighestqualitydata,Gladstoneetal.(2009)findsitaninadequatedescriptor.Itisproposed,therefore,thatamore
complexversionoftheslimdisc(e.g.Sadowskietal.2010)shouldinsteadbeappliedtoinvestigatethistheoryfurther.
Comptonisationmodelsrevealedapreferenceforacooldiscwithanopticallythickcorona(t >8,Stobbertetal.
2006; t > 6, Gladstone et al. 2009). This is very different from standard black hole systems (LHS - t ∼< 2, kTe ∼
50 keV; VHS - t ∼ 3, kT ∼ 20 keV), indicating that ULXs may be residing in a state that is more extreme than
e
eventheVHS,andthatwemaybeobservingsuper-Eddingtonaccretion.However,thediscsremaincool,whichcould
alsobeusedassupporttheargumentforIMBHs(Gladstoneetal.2009).Themodelassumedthatanopticallythick
coronadoesnotinterceptthelineofsighttoinnerregionsoftheaccretiondisc,andthattheunderlyingdiscspectrum
is independentof the presence of a corona (Kubota & Done 2004). These flawed assumptions can be overcomeby
ObservationsofultraluminousX-raysources January28,2011 4
FIGURE2. (Left)XMM-NewtonEPIC-pnX-rayspectraforthreeULXsfromGladstoneetal.(2009).Thespectraarede-absorbed
&unfolded and show onespectrumfromeachsuggested spectral regime:thehigh(M33 X-8);ultraluminous(HoIXX-1);and
extremeultraluminous(possiblyphotosphere-dominated;HoIIX-1)states.Theintrinsicdiscspectrum,recoveredwhenthecorona
isaccountedfor,isshowninred(seeGladstoneetal.2009formoredetails).(Right)Comparisonoftheultraluminousstatemodel
andtheX-rayreflectionmodelextendedto100keV.Thetwoshortsimilaritiesbelow10keV,butthedifferenceinfluxlevelabove
10keVcoversmanyordersofmagnitude(seeWaltonetal.2011formoredetails).
applyingthe energeticallycoupleddisc plus coronamodelto the sample. Resulting fits indicate that there are three
possibleregimes(seeFigure2(Left));thehigh/VHSorsomeformofhighmassaccretionratemodifieddisc-dominated
state,theultraluminousstateandtheextremeultraluminousstate(whereweseepossibleevidenceforaphotosphere
and/oroutflow(e.gBegelman,King&Pringle2006;Poutanenetal2007).
AnalternativescenarioistheX-rayreflectionmodel(Caballero-García&Fabian2010).Theauthorschosetoapply
thismodelforsimilarreasonstothoseoutlinedabove,reflectedironlineshavebeenobservedinthespectrumofboth
StMBHs(e.g.Miller2007)&SMBHs(e.g.Tanakaetal.1995).InthiscasetheauthorsproposethatULXspectracan
beexplainedbyanX-rayirradiateddiscarounda rapidlyspinningblackhole.ResultsindicatedthatULXsfallinto
twomajorgroups,areflectiondominatedorpower-lawdominatedstate.Itisarguedthatthesecorrespondtotwoof
the regimesdiscussed in Miniutti & Fabian (2004),in which heightof the primarysource of X-raysis varied.This
allowstheobservertoseedifferingamountsofthecontinuumandreflectioncomponents.
Walton et al. (2011) also compared two of these leading (energetically coupled disc-corona & X-ray reflection)
models.Theauthorsfoundthatthe fitsare statistically similar with moderatequalitydata, highlightingtheneedfor
a wider band-pass to differentiate between models (see Figure 2 (Right)). The ultraluminousstate model continues
fallingoffinflux,whilstthereflectionmodelcontinuesitspower-lawslopeouttolargerenergies.Theabilitytolook
to higherenergieswouldalso settle thedebateoverthemassofthese sources,byclearlydemonstratingthe spectral
stateofthesesystems.Althoughwestillcannotconfirmstatisticallythestructureoftheaccretiongeometry,itseems
that the majority of the current physical models indicate the presence of StMBHs when using the highest quality
spectraldata.
X-rayspectralvariability
Spectral variability of StMBHs have proved invaluable in understanding of accretion states (e.g. McClintock &
Remillard2006),yetitisanavenuethatremainsunder-exploredinULXs.Somesimplesurveyshavebeenperformed,
revealing that the majority of ULXs display a general hardening as the luminosity of the system increases (e.g.
Antennae population,Fabbiano et al. 2003; HombergII X-1, Dewangan et al. in 2004;NGC 5204 X-1, Roberts et
al.2006).Thisistheoppositeofwhatisseenintheevolutionofthestandardaccretionstates,thiscouldsupportthe
ideaofanewaccretionstate.However,recentstudieshavehighlightedaheterogeneityinULXs.AstudyoftheULX
population of NGC 4485 & 4490 (Gladstone & Roberts 2009), along with a survey of some of the most observed
ULXs(Kajava&Poutanen2009),revealedthatnotonlyaL vs.G correlation,butalsoaninversecorrelationinsome
X
sources,withGladstone&Roberts(2009)notingthechangeinbehaviourcoincidedwithapossiblechangeinspectral
shape(andpossiblechangeinstate).
Kajava&Poutanen(2009)alsonotedthatthecooldisc(softexcess)componentvariedasL(cid:181) T−3.5.Thisimplies
thatthe innermostradiusof the disc movesoutand the luminosityincreases, which is strikinglydifferentfrom the
standarddecreasein innerradiusobservedin Galactic StMBH systems(L(cid:181) T4). Thiscouldbedue to materialand
ObservationsofultraluminousX-raysources January28,2011 5
energybeinglostattheseradiiasaresultoflaunchingawind/outflow.
Recent work attempted to test the findings of Gladstone et al. (2009) by studying a series of reasonable quality
spectraof NGC1313X-2 (Pintore& Zampieri2010)andHoIX X-1(Vierdayantietal. 2010).Pintore& Zampieri
(2010)presentedanalysisofmoderatequalityXMM-Newtonobservations,while Vierdayantietal. (2010)presented
XMM-Newton and Swift data (grouped by count rate) to study spectral variations over long timescales. When the
dataisfitwithacombineddiscandComptonisingcoronamodel,theauthorsfoundthatthespectraarewellfitbya
coolopticallythickcorona,whichshoweda decreasein thecoronaltemperatureandincreasingopticaldepthasthe
luminosityofthe system increases. Thisappearsto bein agreementwith thepredictionsofGladstoneet al. (2009).
However,whenKongetal.(2010)fitgroupedSwiftdataofHoIXX-1inasimilarwaytoVierdayantietal.(2010),
they did not see the same trends. They found that all the spectra are well fit by a duel thermal model, which they
arguedindicatessuper-Eddingtonaccretion.Clearlyfurthervariabilitystudiesarerequiredtodisentanglethespectral
evolutionofthesesystems,andprovidefurtherinsightsintotheaccretiongeometryofthesesystems.
X-ray temporal studies
Long-term temporal variability studies have suffered, in part, due to ULXs being too faint to be picked up by
monitoringmissionssuchasRXTE(withtheexceptionofM82X-1,Kaaretetal.2006),andsocanonlybeobserved
duringpointedobservations.Thisleadstolargegapsinthelightcurves,makingitdifficulttotrackthetrendsofthese
systems. Someauthorshavemanagedto find evidenceforvariation(upto ∼ an orderof magnitude)ontime-scales
ofmonthsto years,withthe useofmulti-missiondata.ExamplesincludetheULXsin M81(LaParolaetal. 2001),
HoII(Miyajietal2001),NGC4485&4490(Robertsetal.2002a;Gladstone&Roberts2009)andM51(Terashima
&Wilson2003).Ifweconsidershortertime-scales,ofdaystoweeks,wefindthatfewerstudieshavebeenreported.
Robertsetal.(2006)presentedastudyofNGC5204X-1inwhichfactor5variationswereobservedoveratime-scales
ofdays.However,nosignofvariabilitywereobservedovertimescalesofhoursorless.
Intra-observational analysis for both StMBHs and SMBHs is explored using power spectral densities (PSD). It
hasbeenshownthatmanyfeaturesarecommontobothStMBHsandSMBHs(McHardyetal.2006;McClintock&
Remillard2006;vanderKlis2006).However,ULXsseemtohavesuppressedvariability(Heiletal.2009),withonly
a small number of features observed.These include a break at 2.5 mHz in the PSD of NGC 5408 X-1 (Soria et al.
2004)indicatingamassof∼100M⊙ (seeHeiletal.2009formoredetaileddiscussionsandanalysis).
Quasi-periodicoscillations(QPOs)havealsobeendetectedinonlythreecases;M82X-1(Strohmayer&Mushotzky
2003),anothersourceinM82(Fengetal.2010),&NGC5408X-1(Strohmayeretal.2007;2010).TheseQPOshas
beenusedasevidenceforIMBHs,byusingblackholemass-QPOfrequencyscaling.Recentre-analysisofthedata
ofNGC5408X-1hasshownadifferentinterpretation,however,Middletonetal.(2010)hasshownthatthespectra,
PSDandvariabilityofthissourceisbettermatchtoamodelofsuper-EddingtonaccretionthanthatoftheLHS.This
wouldindicatethattheQPOpresentinthissourceismoreanalogoustothemHzQPOobservedinGRS1915+105,
indicatingthepresenceofaStMBH.
THENATUREOFTHESE SYSTEMS?
Since the adventof XMM-Newton & the Chandra Space telescopes, the study of ULXs had advanceddramatically.
Initial results from these telescopes indicated the presence of massive black holes, more massive than could be
explainedby the end pointof (single) stellar evolution (implyingIMBHs). Early X-ray emission providedtentative
support,but the strongestX-rayevidenceforIMBHs to date comesfromthe observationsof hyper-luminousX-ray
source (HLXs). These are ULXs emitting at ∼>1041 erg s−1. The brightest of these sources is ESO 243-49HLX-1
(Farrelletal.2009;2010)withalowermasslimitof∼500M⊙.Opticalobservationsofthenebulaeassociatedwith
some ULX systems also lentsupportto IMBHs, by indicatingthat the X-rayemission poweringthese systems was
not strongly beamed. There have been few Radio observationsof these source, with many only revealing limits on
possibleemission.InthefewinstanceswhereRadioemissionhasbeenobserved,scalingrelationsindicatethatthese
ULXscontainIMBHs.ThepresenceofQPOsinthreeULXhasbeenpointedtoassomeofthestrongestevidencefor
thepresenceofIMBHsinULXsystems.
Morerecentstudiesinmanyofthesewavebandshaverevealedadifferentscenario.StudiesoftheX-rayspectraof
thesesystemshasrevealedthepresenceofabreakat∼3–7keV,afeaturenotpresentintheLHS,whiletheleading
ObservationsofultraluminousX-raysources January28,2011 6
physicalmodels used to describe these systems indicate that we are observingsuper-Eddingtonaccretion flows. X-
raytimingstudieshavealsorevealedsuppressedvariabilityinmanysources,againafeaturenotseeninthestandard
accretionstates.Thiscombinedwiththerecentre-analysisofoneQPOsource,whichindicatesabettermatchtosuper-
EddingtontheorythantheLHS,suggestthatweareinfactobservingsomeofthemostextremeStMBHX-raybinary
systemsinthe Universe.Itis notjustX-rayanalysisthatsupportsthistheory.Opticalphotometrystudiesrevealthe
presenceofhighmasscompanionstars,andanassociationwithyoung,lowmetallicity,starformingregions(regions
inwhichlargeStMBHsarethoughttoform).Theover-abundanceinsomegalaxieshasalsobeenusedasevidenceto
supportthepresenceofStMBHs.
Although there appears to be mounting evidence for extreme StMBHs powering ULXs, we must note that each
ofthesemassestimatesaredegenerate,relyingonassumptionto gainmassestimates. Tosettle the debate,we must
instead turn to more direct estimates of the mass, and for this we need to gain dynamical mass estimates of these
systems,achallengingtaskthatisstillintheearlystagesatpresent,butmayyetrevealtheunderlyingnatureofthese
systems.
ACKNOWLEDGMENTS
JCG would like to thank the organisersof this meeting for inviting her to present this review. She would also like
to thank her collaborators for some of the work presented here, as well as colleagues and researchers in this and
relatedfieldsfortheirdiscussionsandguidance.JCGwouldalsoliketoapologisetothoseauthorswhoseworkisnot
mentionedhere,duetothelackofspaceandthewealthofinformationavailableonthesefascinatingsources.
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