Unlocking the nature of ultraluminous X-ray sources using their X-ray spectra

0.18 MB·English

by  Jeanette C. Gladstone

Astron.Nachr./ANTBC,No.TBC,789–793(2010)/DOIpleasesetDOI!
Unlocking the nature of ultraluminous X-ray sources using their X-ray
spectra
JeanetteC.Gladstone1,⋆,TimothyP.Roberts2andChrisDone2
1 DepartmentofPhysics,UniversityofAlberta,Edmonton,Alberta,T6G2G7,Canada
2 DepartmentofPhysics,UniversityofDurham,SouthRoad,DurhamDH13LE,UK
1
1 ReceivedTBC,acceptedTBC
0 PublishedonlineTBC
2

n Keywords accretion,accretiondisks-blackholephysics-X-rays:binaries
a
J WeexplorethenatureofultraluminousX-raysourcesthroughdetailedinvestigationsoftheirspectralshapeusingsomeof
thehighestqualitydataavailableintheXMM-Newtonpublicarchives.Phenomenologicalmodelsallowustocharacterise
5
their spectra, while more ‘physically-motivated’ models enable us to explore the physical processes underlying these
2
characteristics.Thesephysicalmodelsimplythepresenceofextreme(probablysuper-Eddington)accretionontostellar

massblackholes.
]
E
H (cid:13)c 2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
.
h
1 Introduction 2 Characterising the spectra ofULXs
p
-
o
r Formorethan30years,bright(LX ∼> 1039ergs−1),extra- Twomainfeatureshavebeenhighlightedbypreviousstud-
t galactic,non-nuclearsourceshavebeenobservedinX-rays, ies ofULX spectra:a softexcess,modelledbya cooldisc
s
a yetthenatureoftheseultraluminousX-raysources(ULXs) withapower-lawtail,andabreakathigherenergies.Glad-
[ is still unknown. Their luminosity precludes the possibil- stone,Roberts&Done(2009;GRD09hereafter)usedsome
1 ityofstandardstellarmassblackholes(StMBHs)accreting of the highest quality ULX data (∼> 10,000 EPIC counts;
v isotropically below the Eddington limit, whilst their loca- based on SRW06) currentlyavailable in the XMM-Newton
9 tionwithinhostgalaxies-alongwithdynamicalfrictionar- ScienceArchive(XSA1)(asampleof12sources,withLX
5 guments - rules out super-massive black holes (SMBHs). ∼1039–afew1040ergs−1)toseeifeachofthesefeatures
8
4 The simplest explanationis that these objectsare interme- arepresentinthemajorityofULXspectra.
. diate in both mass and luminosity between the black hole Tolookforasoftexcess,eachspectrumwasfitwithan
1
0 categoriesmentionedabove,intermediatemassblackholes absorbedpower-law.Adisccomponent(DISKBBinXSPEC;
1 (IMBH;Colbert&Mushotzky1999)thatareaccretingina Mitsudaetal.1984)wasthenaddedtothemodeland∆χ2
1 knownstate.AnalternativeisthattheyareStMBHsthatare found.Animprovement(∆χ2>30)wasfoundin11spec-
: either exceedingthe Eddingtonlimit, by undergoingsome tra,anexampleofwhichcanbeseeninFigure1(a)(seealso
v
i formofsuper-Eddingtonaccretion(e.g.Begelman2002),or tables 4 & 5 in GRD09 for results). Seven of the sample
X circumventingitviageometricorrelativisticbeaming(e.g. showed cooldiscs (kTin < 0.5 keV), while the remaining
r Kingetal.2001;Ko¨rding,Falcke&Markoff2002). sampledisplayedamorestandardStMBHtemperature.
a
Simple phenomenologicalmodelsrevealed warm disc- The possible break at higher energies was investigated
likespectrainASCAdata,suggestingpossiblerapidlyspin- by comparingpower-lawand broken power-lawfits above
ning(Kerr)StMBHs (Makishimaetal. 2000).Increasesin 2keV(seeFigure1(b)).Thisprovidedclearevidencefora
spectral quality allowed fitting of two component models break in the energyspectrumabove 3 keV in 11 of the 12
(multi-coloureddisc +power-law),indicatingthepresence sources(>98percentstatisticalimprovementusingF-test;
ofasoftexcessatlowerenergies,wellfitbyacooldisc.The seealsotable6ofGRD09).
cooldisc(kT ∼0.2keV)indicatedthepossiblepresence A soft excess and a break above ∼ 3 keV appear to
in
of an IMBH (∼ 1000M , Miller et al. 2003). However, be almostubiquitousin thissample, althoughtheirspectra
⊙
analysisofhigherqualitydatashowedevidenceofaspectral can be split into two distinct categories using these mod-
breakat a fewkeV (e.g.Stobbart,Roberts& Wilms 2006, els. The first displays a warm disc and a break (kTin ∼>
SRW06 hereafter), curvature not seen in standard states. 0.8 keV; E ∼ 4 keV). On closer inspection it is ev-
break
ThustheaccretionflowsinULXsmaynotsimplybescaled ident that each component is modelling the same spectral
upversionsofthoseseeninGalacticblackholebinaries. feature.Inthiscasewefindthatthepower-lawcontributes
to emission at both higher and lower energies, broadening
⋆ Correspondingauthor:e-mail:[email protected]
1 Seehttp://xmm.esac.esa.int/xsa/
(cid:13)c 2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 J.Gladstone,T.Roberts,C.Done:UnlockingthenatureofULXs
10−3
10−3 10−4
E FE E FE
10−5
5×10−4
2 5 10 10−60.2 0.5 1 2 5 10
Energy (keV) Energy (keV)
Fig.1 XMM-NewtonEPICpndatafor(a)NGC5408X-1,(b)NGC1313X-2and(c)M81X-6.Ineachcase onlypn
dataisshownandtheyhavebeenrebinnedforclarity.Disccomponentsareplottedinred,power-lawsareplottedingreen
andthecombinedmodelinblue.Plots(a)and(c)demonstratethetwotypesoffitreceivedwhencombininganabsorbed
disc and power-law model. NGC 5408 X-1 (a) has kT ∼ 0.19 kev, Γ ∼ 2.7, while M81 X-6 displays a warmer disc
in
(kT ∼1.42kev)withΓ∼2.6.Thefirstmayindicatethatthesourceisinthenewultraluminousstatewhilstthesecond
in
spectrum is of a source describedby a modified accretiondisc (perhapsin the high, veryhighstate). Plot (b)shows the
clearpresenceofabreakintheenergyspectrumabove2keV.
thediscmodel(seeFigure1(c)).Thisisprobablyduetothe unrealistic.Thisappearstoindicatethatthesimplifiedslim
DISKBB providinga poor description of the broader spec- discmodelisaninadequatedescriptionofhighqualityULX
trum expected from more realistic disc models (Done & data. A more complex version of the slim disc (e.g. rela-
Davis2008;Hui&Krolik2008).Thesecondcategoryex- tivisticslimdiscinSadowskietal.2010)shouldinsteadbe
hibitsacooldisc(kTin ∼<0.8keV)andbreakathigheren- appliedtoinvestigatethistheoryfurther.
ergies(4∼< Ebreak ∼<7keV),thataretwodistinctspectral
features.Thisnewcombinationofobservationalcharacter- 3.2 Comptonisationmodels
isticsindicatethatthesesystemsareresidinginanaccretion
statenotcommonlyseeninGalacticX-raybinaries,an“ul- ComptonisationmodelshavebeenappliedtoGalacticblack
traluminousstate”.Toexplorethenatureofthisnewspec- holebinariesformanyyearswithgreatsuccess.GRD09ap-
tralstate,wediscussmorephysicallymotivatedmodels. plied them to their sample as a starting point for further
study. Two separate Comptonisation models were applied
(COMPTT, Titarchuk 1994; EQPAIR, Coppi 1999), each in
3 Testing physicallymotivatedmodels combinationwithadisc.
The majority of the GRD09 sample were found to be
3.1 Slimdiscs parameterisedbyacoolaccretiondiscwithacool,optically
thickcorona(τ ∼>6),irrespectiveofComptonisingmodel.
The slim disc model suggests that the accretion disc can Thisisverydifferentfromstandardblackholestates.Only
become so optically thick that energyreleased in the mid- sources in the very high state (VHS) have parameters that
plane does nothave time to diffuseto the photospherebe- areevenclosetothoseobservedforULXs(lowstate-τ ∼<
fore being lost over the event horizon (Abramowicz et al. 2, kT ∼ 50 keV; VHS - τ ∼ 3 and kT ∼ 20 keV), and
e e
1988). This causes the disc to swell, creating a geomet- systems residing in this state are accreting at some of the
rically ‘slim’ disc. The change in disc structure causes a highestknownaccretionrates.Hereweobservegreaterop-
change in temperature gradient, represented by rp. Some ticaldepthsandlowerelectrontemperatures.Thisindicates
authorshavelookedforevidenceofthischangingdiscstruc- thatwemaybeobservingULXstatesthataremoreextreme
ture with ‘p-free’ (DISKPBB in XSPEC; e.g. Watarai et al. than the VHS, and possibly super-Eddington accretion on
2001), searching for a shift from the standard value of p to StMBHs. However, we must note that the discs remain
(≃0.75),toavalueof∼0.5(e.g.Vierdayantietal.2006). cool,thatcouldalsobearguedassupporttoIMBHs.
GRD09 appliedthissimplified versionof the slim disc Thereareintrinsicissueswiththeaccretionstructurede-
model to their sample. Initial results, at first glance, ap- scribed above. The model assumed that an optically thick
peared to be in agreement with the slim disc model, with coronadoes notinterceptthe line of sightto inner regions
p∼0.4–0.6(seeGRD09table7).However,closerinspec- oftheaccretiondisc,andthattheunderlyingdiscspectrum
tionrevealedthatwhiletheinner-disctemperaturesofsome isindependentofthepresenceofacorona(Kubota&Done
sources range from 1–3 keV, which can be expected for 2004).Eachassumptionisflawed.Anopticallythickcorona
super-Eddington accretion, 4 of the sample exhibit higher could easily mask the inner regionsof the disc, while two
temperatures(6–13keV),temperaturesthatarephysically opticallythick media in such close proximityrequiresthat
(cid:13)c 2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2010) 791
the energeticsofthesystemmustbeconsidered.Bothma- tures have been noted in some StMBHs (e.g. Miller 2007)
terialandenergywouldberemovedfromtheaccretiondisc & SMBHs (e.g. Tanaka et al. 1995). Caballero-Garc´ıa &
in order to launch (and feed) the corona,depleting the ac- Fabian (2010) chose to apply this model to some of the
cretiondiscandalteringitsemissionspectrum. highestqualitydataavailableinXSA.Theauthorsproposed
thatULXspectracouldbeexplainedbyanX-rayirradiated
discaroundarapidlyspinningblackhole.Heretheauthors
3.3 Energeticallycoupleddiscandcorona
usedREFLIONX(Ross&Fabian2005)incombinationwith
In order to correctthe flawed assumptionsoutlined above, theKerrkernelKDBLUR(Laor1991)orKERRCONV(Bren-
GRD09 applied DKBBFTH (Done & Kubota 2006), incor- neman&Reynolds2006).Theturnoverathigherenergies
poratingtheenergeticdisc-coronacouplingmodelofSvens- wouldbeexplainedbyarelativistically-broadenedironline,
son&Zdziarski(1994).Itassumesthat,astheaccretionrate whilst an absorbed power-law models the underlyingcon-
increases, material and energy are fed into the corona. By tinuum, representing the emission source illuminating the
conservingenergy,whilstcalculatingthefraction(f)dissi- disc. This implies that both the soft excess and the high
patedinthecorona,thismodelisabletoretrievedetailsof energybreakarepossiblesignaturesofanX-rayirradiated
theunderlyingdiscincludingitsresultingluminosity. accretiondisc.The authorsalso notethatnostatistical im-
Figure2(left) showsHoIX X-1,whichclearlydemon- provements are found with the addition of a disc compo-
stratesthatneitherspectralfeaturegivesdirectinformation nent,theyarethereforeunableto commentonthe massof
on the inner-disc temperature. The high energy turn over theblackholeresidingwithinULXs,orontheimpliedac-
is described by the corona, whilst the soft excess provides cretionrateofthesesystems.
the transitional radius(the outermostradiusof the corona, TheresultsofthisstudyindicatethatULXsfallintotwo
beyond which the disc is clearly visible). The inner disc majorgroups.Somespectraappeartobedominatedbyare-
(dashedred line) is notvisible to the observer.Figure 8 in flectioncomponent,whilstothersareinapower-lawdomi-
GRD09 shows fits for the entire sample over-plotted with natedstate.Itisarguedthatthesecorrespondtotwoofthe
therecovereddisccomponent.Therecovereddiscspectrum regimesdiscussedinMiniutti&Fabian(2004),inwhichthe
isthespectrumthatwouldbeobservedifthecorona(andits heightoftheprimarysourceofX-raysisvaried.Thisallows
effects)couldberemovedfromthe system,ascan beseen theobservertoseedifferingamountsofthecontinuumand
overlaidinthreeofthesampleinFigure2(right). reflectioncomponents.
If we consider those systems initially characterised by
a warm disc (plus power-law), we find that the recovered
4 Spectral variability
disc(fromDKBBFTH)isverysimilartotheactualspectrum
(see Figure 2(right-i)). These disc-like spectra are similar
in shape to those of Galactic StMBHs residing in the high Recent work attempted the test the findingsof Section 3.3
andveryhighstate(whenband-passisconsidered;seealso bystudyingaseriesofreasonablequalityspectraofHolm-
figure9inDoneetal.2007).Thiswouldindicatethatthey berg IX X-1 (Vierdayantiet al. 2010a). XMM-Newton and
arepossiblyresidinginthehighorVHS.Oftheremaining Swiftdata(groupedbycountrate)werecombinedtostudy
spectra,wefindtheytwocategories,thosewithatempera- spectralvariationsoverlongtimescales,withthesourcelu-
turethatwouldindicatethepresenceofaStMBH(seeFig- minosity varying by a factor of 3–4. When the data is fit
ure2(right-ii)),andthosewith a cooldisc moreindicative with a combineddisc andComptonisingcoronamodelthe
ofan IMBH(see Figure2(right-iii)).Thefirstgroup’spa- resultsagreewiththoseinSection3.2,thespectraarewell
rameters suggest that we are observing extreme StMBHs fitbyacoolopticallythickcorona.Bycomparingresultant
residing in the ultraluminous state, probably accreting at fits the authorsnotethatthe temperatureof the coronaap-
super-Eddingtonrates. In the second case, we may be ob- pearedtodecrease,withacorrespondingincreaseinoptical
serving IMBHs, or we may be observing something else. depth,astheluminosityofthesystemincreased.Pintore&
Theorypredictsthata photosphereandwindare keycom- Zampieri (2010) also report on similar work within these
ponentsofsuper-Eddingtonaccretion(e.g.Begelmanetal. proceedings, results that appear to be in agreement with
2006; Poutanen et al. 2007) and this new spectral struc- thepredictionsofsuper-Eddingtonaccretion,andtheideas
ture could provide observational evidence for such a pho- outlined by GRD09 (see the authors’ conference proceed-
tosphere.Inthis case, bothmassand energywouldbe lost ingsforfurtherdetails).However,theresultsofVierdayanti
from the system whilst launching the wind. If this is the et al. (2010a)also show further complexity in the spectral
case, these sources are residing in an extreme ultralumi- variability of these sources, with differences appearing in
nous state. In order to pursue this further, new models are thespectralshapeofthesesystemswhenthesamelevelof
requiredtofullyexplainthephysicsofsuchasystem. fluxatdifferenttimes.Inordertoexplorethesedifferences
in more detail, more in-depth studies are requiredinto the
spectralvariabilityofthesesystemswithhighqualitydata.
3.4 Reflectionmodels
Another work published this year adds further support
X-ray reflectionmodelshave also been proposedas a way to GRD09, namely a study of the Galactic black hole bi-
to unlock the nature of ULXs, as X-ray reflection signa- naryGRS1915+105,whichhasbeenobservedtoaccreteat
www.an-journal.org (cid:13)c 2010WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 J.Gladstone,T.Roberts,C.Done:UnlockingthenatureofULXs
Fig.2 (Left) XMM-Newton EPIC pn data from Ho IX X-1 (plotted in black and rebinned for clarity), fitted with an
absorbedultraluminousmodel(DKBBFTH,in blue).Variouscomponentsoftheaccretionsystemdescribedbythemodel
areoverlaid.Disccomponentsareshowninredwhilstthecoronaisingreen.Thevisibleregionsoftheouterdiscandthe
opticallythickcoronaareplottedwithasolidline,whilstthecalculatedmaskedemissionfromthecooled,energetically-
coupled inner disc is plotted wited a dotted line. (Right panels) XMM-Newton EPIC-pn X-ray spectra for three ULXs
fromGRD09.Thespectraare de-absorbed&unfoldedandshowonespectrumfromeachsuggestedspectralregime:(i)
the modified disc (M33 X-8); (ii) ultraluminous (Ho IX X-1); (iii) and extreme ultraluminous (possibly photosphere-
dominated;HoIIX-1)states.Theintrinsicdiscspectrum,recoveredwhenthecoronaisaccountedfor,isinred.
super-Eddingtonrates(givenbestestimatesofmassanddis- ysis methods,such as spectralor temporalvariability with
tance).Thespectralevolutionofthissourceshowssimilar- highqualitydata.Theearlyresultsofsuchstudiesappearto
itiestothatofULXs,andthefindingsofSection3.3(Vier- supportthepredictionsoftheenergeticallycoupleddiscand
dayanti et al. 2010b).When fitting extreme spectra of this coronamodel,butlargerstudiesofsuchhighqualityX-ray
sourcewithadiscplusComptonisationmodel,theauthors dataarerequiredtoconfirmthis.
foundevidenceof increasingopticaldepthwithincreasing
luminosity.Thisprovidesstrongsupportfortheideaofex-
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