Chandra observations of five ultraluminous X-ray sources in nearby galaxies

0.55 MB·English

by  T. P. Roberts

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed2February2008 (MNLATEXstylefilev2.2)
Chandra observations of five ultraluminous X-ray sources in nearby
galaxies
T.P. Roberts1,∗, R.S. Warwick1, M.J. Ward1 & M.R. Goad2
1X-rayandObservationalAstronomyGroup,Dept.ofPhysics&Astronomy,UniversityofLeicester,UniversityRoad,Leicester,LE17RH
4 2Dept.ofPhysics&Astronomy,UniversityofSouthampton,Highfield,Southampton,Hants.,SO171BJ
0 ∗E-mail:[email protected]
0
2

n
a
J
ABSTRACT
5 Wereporttheresultsofaprogrammeofdual-epochChandraACIS-Sobservationsoffiveul-
1
traluminousX-raysources(ULXs)innearbyspiralgalaxies.AllfiveULXsaredetectedasun-
resolved,point-likeX-raysourcesbyChandra,thoughtwohavefadedbelowthe1039ergs−1
1
luminositythresholdusedtofirstdesignatethesesourcesasULXs.Usingthissamecriterion,
v
wedetectthreefurtherULXswithintheimagedregionsofthegalaxies.TheULXsappearto
6
0 berelatedtothestarformingregionsofthegalaxies,indicatingthatevenin“normal”spiral
3 galaxiesthe ULX populationis predominantlyassociated with youngstellar populations.A
1 detailedstudyof theChandraACIS-Sspectraof six ofthe ULXsshowsthatfive arebetter
0 describedbyapowerlawcontinuumthanamulti-colourdiscblackbodymodel,thoughthere
4 is evidenceforadditionalverysoftcomponentsto two of the powerlawcontinua.Themea-
0 suredphotonindicesinfouroutoffivecasesareconsistentwiththelow/hardstate inblack
/
h holebinaries,contrarytothesuggestionthatpowerlaw-dominatedspectraofULXsoriginate
p intheveryhighstate.Asimpleinterpretationofthisisthatweareobservingaccretiononto
- intermediate-massblackholes,thoughwemightalsobeobservingaspectralstateuniqueto
o
veryhighmassaccretionratesinstellar-massblackholesystems.Short-termfluxvariability
r
t isonlydetectedinoneoftwoepochsfortwooftheULXs,withthelackofthischaracteristic
s
arguingthattheX-rayemissionofthissampleofULXsisnotdominatedbyrelativistically-
a
: beamedjets. TheobservationalcharacteristicsofthissmallsamplesuggestthatULXsarea
v
distinctlyheterogeneoussourceclass.
i
X
Keywords: X-rays:galaxies-X-rays:binaries-Blackholephysics
r
a
1 INTRODUCTION lationisongoing,sinceiftheseareaccretion-poweredsourcestheir
X-ray luminosities match, and in many cases greatly exceed, the
Ultraluminous X-ray Sources (ULXs) can be broadly defined as Eddingtonlimitforatypical stellar-mass(∼ 10M⊙)blackhole.
the most luminous point-like extra-nuclear X-ray sources located Suggestions for their physical composition currently focus upon
withinnearbygalaxies,displayingX-rayluminositiesinexcessof four possibilities. The first is that many ULXs are accreting ex-
1039ergs−1. These sources were first observed in Einstein ob- amplesofanewclassof102 −105 M⊙ intermediate-massblack
servations of nearby galaxies (e.g. Fabbiano & Trinchieri 1987), holes(IMBH;e.g.Colbert&Mushotzky1999).Theformationof
and more than one hundred were subsequently catalogued in suchobjectsremainsamatterofsomedebate;someintermediate-
ROSAThigh-resolution imager observations (Roberts& Warwick massblackholesmaybetheremnantsofprimordialPopulationIII
2000;Colbert&Ptak2002).WhilstafractionofthisobservedULX stars(e.g. Madau & Rees 2001), whilst others may be formed in
population is associated with recent supernovae (e.g. SN 1986J, dense globular clusters prior to being deposited in a galaxy disc
Bregman & Pildis 1992; SN 1979C, Immler, Pietsch & Aschen- (Miller & Hamilton 2002). An alternative scenario is that IMBH
bach1998),ASCAstudieshaveshownthatmanyULXsappearto areformed bytherunaway mergerof stellarobjectsat thecentre
displaythecharacteristicsofaccretingblackholes(e.g.Makishima of young, dense stellar clusters (Ebisuzaki et al. 2001; Portegies
etal.2000; Mizuno, Kubota&Makishima2001). Crucially,high Zwart & McMillan 2002). The existence of IMBH may be sup-
spatial resolution Chandra observations (e.g. Kaaret et al. 2001; portedbytherecentinferrenceof3×103 and2×104 M⊙ mas-
Strickland et al. 2001), have failed to resolve most ULX targets. sivedarkobjectsinthecoresoftheglobularclustersM15andG1
Whencombinedwiththeobservedsignificantfluxvariability,this respectively(vanderMareletal.2002;Gerssenetal.2002;Geb-
suggeststhepresenceofasingle,luminoussourceofX-raysrather hardt et al. 2002), though thepresence of such an object in M15
thanagroupingoflessluminoussources. isfar fromproven (Baumgardt et al. 2002). Further evidence has
However,debateoverthenatureofthebulkoftheULXpopu- recentlyemergedwiththediscoveryoftheX-rayspectralsignature
2 T.P.Robertset al.
of“cool”accretiondiscs,consistentwiththepresenceofIMBH,in
Table2.Thehostgalaxies.
severalULXs(e.g.Milleretal.2003,Roberts&Colbert2003).
The remaining models focus upon interpreting ULXs as ex- Galaxy Hubbletypea db ib NHc
treme examples of “ordinary” stellar-mass (i.e. ∼ 10 M⊙) black (Mpc) (◦) (×1020cm−2)
holeX-raybinaries.ThesecondphysicalmodelisthatmanyULXs
IC342 SAB(rs)cd 3.9 20 30.3
areordinaryX-raybinariesinanunusuallyhighaccretionmode,in NGC3628 SAbpecsp 7.7 87 2.0
whichtheiraccretiondiscbecomesradiationpressure-dominated, NGC4136 SAB(r)c 9.7 0d 1.6
producingphotonbubbleinstabilitiesthatallowthedisctoradiate NGC4559 SAB(rs)cd 9.7 69 1.5
at a truly super-Eddington X-ray flux (Begelman 2002). Thirdly, NGC5204 SA(s)m 4.8 53 1.5
many ULXs may only appear to exceed the Eddington limit, but
Notes:
couldinfactbeX-raybinariesemittinganisotropically(Kingetal. aDatafromtheNASA/IPACextragalacticdatabase(NED).
2001),withonlyamildbeamingfactorb∼0.1(whereb=Ω/4π, bDistance(d)andinclination(i)datafromTully(1988).
and Ω is the solid angle of the X-ray emission) required in most c Foregroundabsorption,interpolated atthepositionofeachgalaxyfrom
casestoreducetheenergyrequirementsbelowtheEddingtonlimit theHImapsofStarketal.(1992).
for a conventional stellar-mass black hole. The fourth model is d There is no recorded inclination, so a face-on aspect is adopted after
a variation on the third scenario, suggested by Reynolds et al. inspectionofDigitisedSkySurveyimages.
(1997) andmorerecentlyKo¨rding,Falcke&Markoff (2001) and
Georganopoulos,Aharonian&Kirk(2002),inwhichULXsaremi- mined from WilliamHerschel Telescope/INTEGRAL IFU obser-
croquasarsinnearbygalaxiesthatweareobservingdirectlydown vations,willbedetailedinafuturepaper(Robertsetal.,inprep.).
thebeamoftheirrelativisticjet(“microblazars”,c.f.Mirabel&Ro- Thelayoutofthispaperisasfollows.Inthenextsectionwe
driguez1999). Observational support for thislastscenario comes brieflyoutlinethedetailsoftheChandraobservationsandthedata
fromthedetectionofradioemission,potentiallythesignatureofa reduction.Thisisfollowedbyadiscussionofthedetectionofthese
relativistically-beamedjet,emanatingfromanULXinNGC5408 (andthreeother)ULXs,andtheirlocationswithintheirhostgalax-
(Kaaretetal.2003). ies.Insection4wedetailthespatial,temporalandspectralprop-
erties of each source, before discussing the implications of these
Current observations do not completely rule out any of the
resultsforpossiblephysicalmodelsofULXsinsection5.Ourfind-
above scenarios. However, they do suggest that there may be at
ingsandconclusionsaresummarisedinsection6.
least two separate underlying populations of ULXs, since alarge
numberareseencoincidentwithactivestarformationregions(e.g.
Zezas & Fabbiano 2002; Roberts et al. 2002) and hence are pre-
sumablyassociatedwithnascentstellarpopulations,whereassome 2 CHANDRAOBSERVATIONSANDDATAREDUCTION
ULXsare found in elliptical galaxies (e.g. Irwin, Athey & Breg-
The details of the ten Chandra observations of ULXs that form
man 2003) and so must be associated with an older stellar pop-
thebasisofthispaperarelistedinTable1.Theobservationswere
ulation. King (2002) suggests that the population associated with
pointedattheROSATHRIpositionsoftheULXs,afteracorrection
star formation are high-mass X-ray binaries (HMXBs) undergo-
totheastrometrycalculatedfromX-ray/opticalmatchesintheHRI
ing an episode of thermal-timescale mass transfer (see also King
field-of-view. All coordinates listed in the Table, and throughout
etal.2001),whilsttheolderpopulationmaybelong-lastingtran-
thispaper,areepochJ2000.Wetabulatesomebasicparametersfor
sientoutburstsinlow-massX-raybinaries.Localexamplesofeach
thehostgalaxiesinTable2.Theobservationswereperformedbe-
suggestedclassareSS433andGRS1915+105respectively.This
tween2001January9and2002August26,andrangeinexposure
heterogeneityissupportedbythefirstreportedopticalstellarcoun-
timefrom9.7to22.5 ks. Eachtargetwas observed ontwoocca-
terpartstoULXs.Robertsetal.(2001)reportthedetectionofablue
sions,separatedby3–5months.Inordertomitigatetheanticipated
continuumsourcecoincidentwithNGC5204X-1,whichHSTre-
effectsofdetectorpile-upintheACIS-SS3chip,severaloftheob-
solvedintothreeseparatesourceswithcoloursthatareconsistent
servationswereperformedinasub-arraymode;thesearelistedin
with young, compact stellar clusters in NGC 5204 (Goad et al.
Table1.Thechoiceofsub-arraywasgovernedbytheanticipated
2002). HST observations also show an ULX in M81 (NGC 3031
countrates,modelledfromthepreviousROSATHRIobservations
X-11) to have a possible O-star counterpart (Liu et al. 2002). In
oftheseobjects(RW2000;Lira,Lawrence&Johnson2000),with
contrast, ULXs have been found with potential globular cluster
theaimoflimitingthepile-upfractionto10%orlessineachobser-
counterparts inboth NGC 4565 (Wuet al. 2002) and NGC 1399
vation.Onthisbasis,sub-arraysweredeemednecessaryforthree
(Angelini,Loewenstein&Mushotzky2001),suggestingthatthese
ofthefivetargetsintheprogramme.
ULXsareassociated withtheolder stellarpopulation, orperhaps
DatareductionwasperformedusingtheCIAOsoftwaresuite,
massivecentralblackholes,oftheseglobularclusters.
versions 2.1 and 2.2. The reduction started in each case with the
Inthispaper,wepresenttheresultsofdual-epochChandraob- leveltwoeventfile,fromwhicheventswithenergiesoutsidethe0.3
servationsoffivedifferentULXslocatedinnearby(d < 10Mpc) –10keVrangewererejected.Alldatasetsweresearchedforperi-
galaxies, awarded Chandra time in AO-2 & AO-3. These ULXs odsofhighbackgroundflaring,butnonewereobserved,allowing
are listed in Table 1. They were selected from the catalogues of thefullscienceexposuretobeutilisedineachcase.Furthersteps
ROSATHRIpoint-likeX-raysourcedetectionsinnearbygalaxies intheanalysisofthedataareoutlinedinthefollowingsections.
presented by Roberts & Warwick (2000; hereafter RW2000) and
Lira, Lawrence & Johnson (2000) on the basis of their high X-
rayluminosities(LX > 1039ergs−1 intheROSATHRI)andthe 3 ULXDETECTIONSANDLOCATIONS
lackofpreviousChandraobservations.Adiscussionoftheobser-
vationalhistoryoftheseULXsispresentedinAppendixA.Further A0.3–10keVimageofthecentral8.1′×8.1′regionofeachfield
discussion of the environment of each of these ULXs, as deter- wasconstructedfromthecorrespondingcleanedeventfile.Thisim-
Chandraobservationsoffive ultraluminousX-raysources innearbygalaxies 3
Table1.DetailsofthetenChandraobservations.
Target Observationaimpoint Chandrasequencenumber Observationdate ACIS-S3subarray Exposure
Rightascension Declination (yyyy-mm-dd) (ks)
IC342X-1 03h45m55.2s +68◦04′55′′ 600253 2002-04-29 1 9.9
8
600254 2002-08-26 9.9
NGC3628X-2 11h20m37.5s +13◦34′28′′ 600255 2002-04-06 full 22.3
600256 2002-07-04 22.5
NGC4136X-1 12h09m22.6s +29◦55′49′′ 600257 2002-03-07 full 18.8
600258 2002-06-08 19.7
NGC4559X-1 12h35m52.0s +27◦56′01′′ 600160 2001-01-14 1 9.7
4
600161 2001-06-04 11.1
NGC5204X-1 13h29m39.2s +58◦25′01′′ 600162 2001-01-09 1 10.1
8
600163 2001-05-02 9.5
agewassearchedforpointsourcesusingWAVDETECT,awavelet- populations,wemightexpecttofindthemco-locatedwiththere-
based source detection algorithm available in the CIAO package, gionsofthegalaxymostpronetohostingstarformation,suchasthe
andtheX-raysourcedetectionscoincidentwiththeopticalextent spiralarms,whereasiftheyareassociatedwiththeolderstarsthen
ofeachgalaxy(orthatpartofitwithintheACIS-S3field-of-view) their distribution is more likely to be centrally peaked, or spread
were catalogued. The target ULXs were detected in every obser- evenlythroughoutthedisc.Table2showsthatthemajorityofthe
vation, and their Chandra nomenclature1, including their refined ULXs(fiveoutofeight)arelocatedmorethanhalfwayoutfromthe
position,andobservedcountratesarelistedinTable3.Countrates nucleustowardstheedgeofthegalaxy.Fouroutofthesefiveappear
wereconvertedtoapproximatefluxesbasedonasimplepowerlaw coincidentwithspiralarms;thefifth(CXOUJ112037.3+133429)is
continuummodelwithastandardphotonindex(Γ)of2,subjectto locatedinanedge-onsystemwherewecannotdistinguishwhether
a foreground absorption column appropriate for each galaxy (see itisinanarm,oraninter-armregion.
Table2).Inadditiontothetargets,threeothersourceswithlumi- TheidentificationoffouroftheULXswithspiralarmsisrel-
nositiespotentiallyintheULXregimewerefound;oneisaprevi- atively robust, as inboth IC 342 and NGC 4136 the galaxies are
ouslycataloguedsource(NGC4559X-4inRW2000),buttheother face-onandsoline-of-sightconfusion through thegalaxyismin-
twoarenew ULXidentifications.Thedetailsofthesenew ULXs imised. This is also true for CXOU J123551.7+275604 in NGC
arealsogiveninTable3. 4559,asthisislocatedinafaintouterspiralarmclearlyseparated
One excellent capability of the Chandra observatory is that in projection from the main body of the galaxy. The co-location
it provides an absolute astrometry solution toan accuracy of one of the ULXswith spiral arms itself argues that they could be as-
arcsecond or better. This positional accuracy facilitates detailed sociated with a young stellar population. If they are ordinary X-
follow-upoftheChandraX-raysourcedetectionsthroughthechar- raybinaries,withtypicalkicksimpartedfromtheformationofthe
acterisation of their multi-wavelength counterparts. This avenue compactprimaryinasupernovaexplosionof∼ 100kms−1 (c.f.
has already borne fruit inthestudy of ULXs,asdiscussed inthe recentresultsforGRO1655-40, Mirabeletal.2002),thenin107
introduction.However,herewelimitourselvestoabriefdiscussion yearsthiswouldmovethepositionoftheULXrelativetoitsbirth
oftheenvironmentoftheChandraULXdetectionsonthebasisof placebyamaximumof1kpc,consistentwithanobservedposition
PalomarDigitisedSkySurvey(DSS)data. inornearaspiralarmifbornthere.However,iftheULXareas-
Thepositionsof theULXsrelativetotheirhost galaxiesare sociatedwithanolderpopulationthenintheir>108yearlifetime
illustratedinFigures1&2,whereweoverlaytheChandraX-ray theycouldbedisplacedby10kpcormore,clearlyleavingnore-
emissioncontoursontoDSS-2(blue)imagesoftheequivalentfield- quirement for anassociationwiththespiralarms. Thearguments
of-view.IneachcasethetargetULXisatthecentreofthefield-of- infavouroftheseULXsbeingrelatedtoyoungstellarpopulations
view.ThepositionofeachULXwithrespecttothenucleusofthe aresupportedbyfollow-upobservationsoftheimmediateenviron-
parent galaxy is quantified in Table 3. The observed offset from ments of three of these “outer” ULXs, with HII regions detected
thepositionofthegalaxynucleus,takenineachcasefromFalcoet closeto(within200pcof)CXOUJ123551.7+275604 andcoinci-
al.(1999),issimplytheprojecteddistancemeasuredinarcseconds; dentwithCXOUJ120922.6+295551(Robertsetal.,inprep.),and
wecorrectthistoadeprojectedradius,assumingthateachsourceis thedetectionofapossiblesupernovaremnantencirclingtheposi-
intheplaneofitshostgalaxy,usingthehost’sinclinationasgiven tionofCXOUJ034555.7+680455(Robertsetal.2003).However,
inTable2andapositionangletakenfromtheRC3catalogue(de the kick argument above requires that either the ULXs are very
Vaucouleursetal.1991).Thisdeprojectedradiusisalsoshownas young, or possess a small kick velocity and/or a velocity almost
f(R25), the fraction of the distance between the nucleus and the entirelyprojectedalongourline-of-sightforthesetobetruephysi-
edgeofthegalaxy,definedbythesemi-majoraxisofthe25mag calassociationsratherthanline-of-sightcoincidenceswiththestar
arcsec−2isophotalellipse,atwhichtheULXisfound.Finally,we formation regions we might expect to observe within a galaxy’s
also give a qualitative description of the region of the galaxy in spiralarms.
whichtheULXisfound.
Line-of-sight confusion is a much bigger issue for the three
IftheULXsinthesegalaxiesareassociatedwithyoungstellar
ULXs that are located closer to the centre of their respec-
tive host galaxies. Despite this, detailed follow-up of CXOU
J132938.6+582506 reveals it to be associated with young stars
1 WerefertotheULXsbytheirChandranamesthroughoutthispaper.The (Goad et al. 2002). The remaining two sources are located close
cross-identificationwithpreviousnamesisshownwherenecessary. to the centre of NGC 4559. However, the nucleus of this galaxy
4 T.P.Robertset al.
Table3.TheULXdetections.
CXOUJ Previous Countrate(ctks−1) Offsetfromgalaxynucleus Location
designation Epoch1 Epoch2 observed(′′) deprojected(kpc) f(R25)
TargettedULX
034555.7+680455 IC342X-1 212±5 226±5 302 6.1 0.68 Spiralarm
112037.3+133429 NGC3628X-2 13±1 13±1 292 10.8 0.66 Outerdisc
120922.6+295551 NGC4136X-1 3±1 4±1 65 3.0 0.54 Spiralarm
123551.7+275604 NGC4559X-1 153±4 192±4 123 15.2 0.96 Faintouterspiralarm
132938.6+582506 NGC5204X-1 411±6 159±4 17 0.7 0.21 Innerdisc
FieldULX
120922.2+295600 - 26±1 18±1 62 2.9 0.52 Spiralarm
123557.8+275807 - 11±1 21±1 31 2.3 0.15 Innerdisc
123558.6+275742 NGC4559X-4 62±3 119±3 12 1.7 0.11 Innerdisc/bulge
isknowntohostactivestarformation(Ho,Filippenko&Sargent 4.1 Spatialextent
1997),implyingthatitispossible(thoughbynomeanscertain)that
A primary goal of this programme was to use the unique 0.5-
theseULXsmayalsoberelatedtothepresenceofyoungstars.It
arcsecond spatial resolution of Chandra to investigate whether
isentirely plausible, then, that all the ULXsinthis small sample
these ULXsareresolved into complexes of many X-ray emitting
maybe associatedwithyoung stellar populations. Thisisconsis-
sources, or whether they remain a single, point-like object at the
tentwithpreviousstudiesfindingcomparativelylargepopulations
highest available X-ray resolution. Hence, the target ULXs were
of ULX in the most active star forming galaxies (e.g. Fabbiano,
placedattheon-axisaimpointoftheACIS-S3chipineachobser-
Zezas & Murray 2001; Lira et al. 2002; Zezas, Ward & Murray
vationtoprovidethebestpossiblespatialdata.
2003),indicatingarelationshipbetweenmanyULXandactivestar
Toinvestigatethequestionofspatialextension,thetwoobser-
formation,thoughinterestinglyinthiscaseweco-locatethemwith
vationsofeachfieldwerecombinedtoenhancethesignal-to-noise
thestarformingregionsofotherwiserelativelynormalgalaxies.
ratioof the images. Theexcellent spatial precision of thedata (1
Toputthisincontext,itisworthnotingthatthehostgalaxies
pixel ≡ 0.492 arcsecond) was maintained by lining-up the peak
inoursamplearealltypeSborlater,indicatingthattheyaredisc-
inintensityofeachtargetsource.Thismethodwasverifiedbyex-
dominated, and so do not possess large bulges dominated by old
amining the peaks in fainter sources within the field, which also
stars.WecaninvestigatetheULX–youngstellarpopulationlink
lined-upaccurately.WethenderivedtheradialprofileofeachULX
furtherbyreferencetotheULXsurveyofColbert&Ptak(2002).
andfititscorewithaGaussianfunction,followingtheprocedure
Theydetectmorethan30candidateULXscoincidentwith,orinthe
describedinsection4.2.1ofRobertsetal.(2002)2.Theresulting
haloesof,ellipticalgalaxies,whichmustbedominatedbyanold
fitsgaveFWHMofbetween1.8–2.2pixelsforallon-axissources,
stellarpopulation.Averagingoverthefourteenellipticalgalaxiesin
and∼2.4pixelsforthe2-arcminoff-axisULXsinNGC4559,all
whichULXsarelocatedwithrespecttotheblueluminosityofeach
consistentwiththenominalChandrapoint-spreadfunctionatthose
galaxy,LB,gives0.8ULXper1010 LB.Thisisverymuchanup- positions.Noevidencewasfoundforafaint,extendedcomponent
perlimitonthenumberofULXfromanoldstellarpopulation,as
surrounding the position of any of the ULXs. This demonstrates
Colbert&Ptaksearchouttoaradiusof2R25,potentiallyallowing
thattheULXsarepoint-likeatthe0.5-arcsecondspatialresolution
amuchgreatercontaminationfrombackground objectsthansim-
ofChandra,corresponding tomaximumphysicalsizesfortheX-
plyfocussingwithinthestandarddefinitionofthegalaxysize,the
25magarcsec−2 isophotalellipse.Also,sincemanyoftheircan- rayemittingregionsof∼ 9–23parsecsinthehostgalaxies.The
spatialdataisalso,ofcourse,consistentwithalltheobjectsbeing
didateULXsarepossiblehaloobjects,thesemaythereforeconsti-
single,point-likeX-raysources.
tuteaphysicallyseparatepopulationtothatassociatedwiththeold
stellarpopulation.Nevertheless,bycomparingtheiraverageULX
toL ratiowiththe blueluminositiesof our targetgalaxies (and
B
correctingforthecoverageofeachgalaxyintheChandraobserva- 4.2 Spectralproperties
tions),wecanestablishaconservativeupperlimitonthenumberof
Spectrawereextractedinaneightarcseconddiameteraperturecen-
ourULXsassociatedwiththeolderstellarpopulations.Thisturns
tred on the position of each ULX using the PSEXTRACT script,
outtobeanupperlimitoftwooutoftheeightULXsweobserve.
which also retrieves the appropriate response matrices and ancil-
Wethereforeconfirmthatoursampleisverylikelytobedominated
laryresponsefilesforeachobservation.Asource-freelocalback-
byULXsassociatedwithayoungstellarpopulation.
groundregionofequivalentsizewasusedineachcase,thoughits
impact was of littlesignificance since it typically contained only
0.1−1%ofthecountsaccumulatedfromthesource.Theancillary
responsefileswerecorrectedforthegradualin-orbitdegradationin
thequantumefficiencyoftheACISdetectorsusingtheCORRARF
4 X-RAYCHARACTERISTICS
Inthissection weinvestigatethedetailed X-rayproperties of the 2 Someadjustmentstothesizeandpositionofthebackgroundannuliwere
sources.Unlessspecificallystated,weperformtheanalysesonall requiredtoeliminatenearbyX-raysourcesinseveralfields.Anexampleis
fivetargetplusallthreefieldULXs.Discussionofthesecharacter- theNGC4136field,inwhichthetwoULXsareseparatedbyonly∼ 22
isticsintermsofphysicalmodelsisdeferreduntillatersections. pixels.
Chandraobservationsoffive ultraluminousX-raysources innearbygalaxies 5
Figure1.ThelocationsoffourofthefivetargetULXs.Eachfigureshowsan8.1′×8.1′DSS-2blueimagecentredonthepositionoftheULX,rotatedslightly
tomatchtheChandraprojection.CoordinatesareshowninJ2000.TheChandraX-rayemissioncontoursareoverlayedinredtohighlightthepositionsofthe
X-raysourcesineachfield-of-view.TheChandradatahasbeensmoothedbyaHWHM3pixelGaussianmasktoaidvisibility,andthecontoursareplotted
at0.3and10countpixel−1.ThetargetULXsarehighlightedbytheverticalarrows,andthefieldULXsbyhorizontalarrows.Thespatialcoverageofthe
Chandrasub-arraysusedforobservationsoftheIC342X-1andNGC4559X-1fieldsarehighlightedinblueforeachrespectivefield,withthefirstepoch
coverageshownbythedashedlines,andthesecondepochbythedottedlines.
tool3.Thespectraweregroupedtoaminimumof20countsperbin, tolimitthedistortingeffectofdetectorpile-upontheobservedX-
andthenanalysedinXSPECv.11.2,excludingdatabelow 0.5keV rayspectrumofeachULX.AsstatedinSection2,theROSATHRI
duetotheuncertaintyinthecalibrationattheseenergies.Inthefol- countratesofeachULXwereusedtodeterminetheoptimumsub-
lowinganalysis,allquotederrorsarethe90%confidenceerrorsfor array size to lower the anticipated pile-up fraction below 10%.
oneinterestingparameter. However, this did not guarantee that each source would have an
Amajorconsiderationinthedesignoftheseobservationswas acceptablelevelofpile-up,duetothelong-termvariablenatureof
theULXsthemselves(seebelow).Wecanderivealowerlimiton
the observed pile-up fraction of each observation by reference to
3 Seehttp://asc.harvard.edu/cal/Acis/Calprods/qeDeg.
6 T.P.Robertset al.
Figure2.Asperfigure1,fortheNGC5204X-1field.
Figure6.24oftheChandra Proposer’sObservatoryGuide(v.4)4, improved χ2 was the 2001 January observation of CXOU
which shows pile-up fraction as a function of counts per readout J132938.6+582506,with∆χ2=16.3foroneextradegreeoffree-
frame (wecalculate a lower limitsince our input isthe observed dom,andastatisticallysignificantsofteningofthespectralslope.
countsperframe,whichalreadyincludespiled-upevents).Thisis Wediscussthiscasefurtherbelow.
very much a first order approximation, since it does not include,
TheULXobservations inwhichlessthan250sourcecounts
for example, the effect of variations in the source spectra on the
wereaccumulatedwerenotconsideredforspectralanalysis,which
degreeofpile-up.Onlyoneobservation,the2001Januaryobserva-
ruled out both observations of CXOU J120922.6+295551 and
tionofCXOUJ132938.6+582506, turnsouttohavealowerlimit
CXOUJ123557.8+275807.Theremainingspectrawereinitiallyfit
inexcessof10%,andthisisonlymarginalat11%.Allotherobser-
withsimpleabsorbedsinglecomponentspectralmodels,namelya
vationshavelimitsof8%pile-uporless,implyingthatthepolicy
powerlawcontinuum, athermalbremsstrahlungmodel,themulti-
ofusingsub-arrayswassuccessful.
colourdiscblackbodymodel(hereafterMCDBB)usedtodescribe
We investigated the presence of residual pile-up effects us-
anaccretiondiscaroundablackholeinitshigh(soft)state(Mit-
ing the XSPEC v.11.2 parameterisation of the CCD event pile- sudaetal.1984), aMEKALopticallythinthermal plasmamodel,
up model of Davis (2001). A simple absorbed powerlaw con-
and a conventional blackbody spectrum. The best-fit parameters
tinuum model, both with and without the pile-up model, was
to the powerlaw continuum and MCDBB models are listed in
fit to the observed spectrum of the brightest ULXs (namely
Table 4, and individual cases are discussed below. The thermal
CXOUJ132938.6+582506,CXOUJ123551.7+275604andCXOU
bremsstrahlung model provided an adequate fit in most cases,
J034555.7+680455inbothepochs,andCXOUJ123558.6+275742
though never asstatisticallyacceptable asapowerlaw continuum
initssecondobservationalepoch).Inthepile-upmodelweallowed
(orMCDBBinthecaseofCXOUJ123558.6+275742).Weomitit
α,thegrademorphingparameter,tovaryfreely,andsettheframe
from the table in favour of the two models that have more gen-
timeto0.7secondsand1.1secondsforthe 1 and 1 sub-arraysre-
8 4 erally been used to describe ULX spectra in past analyses. The
spectively,withtheotherparametersfixedasdefault.Insixofthe
spectra were generally too hard to provide meaningful fits to the
seven casesthe changes tothespectral fit whenthepile-up algo-
MEKALmodel,whoseparameterstendedtowardsthoseofthether-
rithmwasappliedwereminimal(∆χ2 <3foroneextradegreeof
malbremsstrahlungfit.Similarly,simpleblackbodymodelsdidnot
freedom).Thoughinmostcasesthepowerlawphotonindicesbe-
provideagoodfittoanyofthespectra.
cameslightlysofter,asexpectedsinceCCDpile-upactstoharden
AsshowninTable4,mostdatasetsareadequatelydescribed
theobservedsourcespectra,thischangewasoffsetbymuchworse
byasingle(absorbed)spectralmodelcomponent,withareduced-
constraintsontheparameters.Hence,theabsorptioncolumnsand
χ2 value at, or below, unity. However, in several instances the
powerlawphotonindiceswereconsistentbothwithandwithoutthe
reduced-χ2 isstillconsiderablyaboveone.Wehaveattemptedto
inclusionofthepile-upmodel,withinthederivederrors,inallsix
fitthesedatasetswithmorecomplex two-component models(see
cases.Noneofthesesixfitswereabletoplacestrongconstraints
Table5).Allthreedatasetsforwhichthespectralfitwasimproved
onα.Wethereforeconsiderpile-upeffectstobenegligibleinthe
bytheadditionofasecondspectralcomponent(excludingthepile-
X-rayspectraofallbutoneoftheULXs.
upcorrectiontoCXOUJ132938.6+582506) requiredthepresence
The only case in which the pile-up model gave a much
ofaverysoftspectralcomponent. Thesearediscussedonacase-
by-casebasisinthefollowingsub-sections, wherewediscussthe
4 Seehttp://asc.harvard.edu/proposer/POG/index.html. spectralfitstoeachindividual ULX.Wedemonstratethespectral
Chandraobservationsoffive ultraluminousX-raysources innearbygalaxies 7
Table4.Bestfitsoftwosimplemodelstothetwo-epochULXX-rayspectra.
ULX(CXOUJ) Epoch WA*POa WA*DISKBBa LXb
NHc Γd χ2/dof NHc kTine χ2/dof
034555.7+680455 2002-04-29 0.52±0.07 1.63+0.13 78.8/81 0.30+0.05 1.81+0.22 97.4/81 4.4(5.9)
−0.12 −0.04 −0.18
2002-08-26 0.61±0.08 1.70+0.12 92.0/87 0.36±0.05 1.76+0.19 105.7/87 4.4(6.4)
−0.13 −0.16
112037.3+133429 2002-04-06 0.022f 1.57±0.24 8.4/10 0.022f 0.95+0.3 12.3/10 0.7(0.8)
−0.2
2002-07-04 0.16+0.09 2.20+0.34 4.4/10 0.022f 0.86+0.18 6.4/11 0.6(0.7)
−0.13 −0.19 −0.14
120922.2+295600 2002-03-07 0.13+0.08 1.68+0.27 30.1/19 0.016f 1.46+0.27 37.7/20 2.3(2.6)
−0.07 −0.22 −0.25
2002-06-08 <0.21 1.55+0.30 13.9/13 0.016f 1.43+0.44 18.4/14 1.7(1.9)
−0.33 −0.24
123551.7+275604 2001-01-14 0.015f 1.91±0.09 69.9/52 0.015f [∼0.84]g 169/52 9.8(10.0)
2001-06-04 0.04±0.03 2.16+0.15 100/72 0.015f [∼0.66]g 222/73 11.6(12.5)
−0.14
123558.6+275742 2001-01-14 0.17+0.08 1.98+0.23 26.3/22 0.015f 1.14+0.15 24.6/23 4.2(4.3)
−0.07 −0.24 −0.13
2001-06-04 0.16±0.06 1.82+0.17 63.6/51 0.015f 1.30+0.12 49.2/52 8.9(9.1)
−0.15 −0.10
132938.6+582506 2001-01-09 0.10±0.02 2.38+0.10 141/109 0.014f [∼0.7]g 260/110 5.7(6.9)
−0.11
2001-05-02 0.10+0.05 2.96+0.25 41.1/46 0.014f 0.44+0.04 86.5/47 1.8(2.4)
−0.04 −0.21 −0.03
Notes:aSpectralmodelcomponentsareshownaspertheXSPECsyntax,with“WA”representingacoldabsorptionmodel,“PO”a
powerlawcontinuumand“DISKBB”theMCDBBmodel.bObservedluminosityinthe0.5–8keVbandinunitsof1039ergs−1.
Figuresinparenthesesgivetheintrinsic(unabsorbed)luminosity.Wecalculatethesevaluesusingthebest-fittingmodelhighlighted
byshowingitsχ2/dofinbold.cAbsorptioncolumn,inunitsof1022atomscm−2.dPowerlawphotonindex.eInneraccretiondisc
temperatureinkeV.fValuefixedattheforegroundGalacticabsorptioncolumn(seeTable2).gParametervaluenotconstrainedby
modelfit.
Table5.Two-componentfitstotheULXX-rayspectra.
ULX(CXOUJ) Epoch Modela α NH kT/kTin Γ χ2/dof ∆χ2b
120922.2+295600 2002-03-07 WA*(DISKBB+PO) - 0.81+0.37 0.12+0.05 1.77+0.36 16.7/17 13.4
−0.34 −0.04 −0.34
123551.7+275604 2001-01-14 WA*(DISKBB+PO) - <0.25 0.20+0.15 1.71+0.23 65.0/49 4.9
−0.07 −0.16
2001-06-04 WA*(DISKBB+PO) - 0.19+0.08 0.15+0.10 2.10+0.18 92.4/70 7.6
−0.14 −0.03 −0.26
WA*(MEKAL+PO) - 0.41+0.13 0.18+0.05 2.34+0.10 76.7/70 23.3
−0.19 −0.02 −0.14
132938.6+582506 2001-01-09 PILEUP*WA*PO 0.57+0.32 0.14±0.03 - 2.79+0.16 124/108 16.3
−0.18 −0.14
Note:aThemodelcomponentsandparametersareasperTable4,exceptfortheMEKALcomponentwhichisathermalplasma
modelwithitsmetallicityfixedatsolarabundance,andPILEUPwhichisasdescribedinthetext.bImprovementinthe
χ2statisticoverthesinglecomponentmodelbest-fit,fortwoextradegreesoffreedom(oneextrainthepile-upmodel).
qualityofeachobservationofeachsourceinFigures3and4.These at∼5×1039ergs−1(versus6×1039ergs−1inASCA)whenex-
showthedataandthebest-fitsinglecomponentmodelintheupper trapolatedtothe0.5–10keVband.HenceitappearsthatthisULX
windowofeachpanel,andtheresidualswhenthedataisdivided hasbeeninaconstantspectralstateinthreeseparateobservations
bythebest-fittingmodelinthelowerpanel. over2.5years.
4.2.1 CXOUJ034555.7+680455(IC342X-1)
This ULX showed a fairly constant X-ray spectrum between the
two observations, with an absorbed powerlaw continuum provid-
ing a good fit to the data from both epochs. In each case the ab-
4.2.2 CXOUJ112037.3+133429(NGC3628X-2)
sorption was roughly twice the foreground value (c.f. Table 2),
withtheadditionalabsorptionof2–3×1021atomscm−2inex- ThisULXhadfadedtoaluminosityofonly∼ 7×1038ergs−1
cess of the integrated column through IC 342 at the ULX posi- in our Chandra observations, hence the spectra are of low qual-
tion(∼ 8×1020atomscm−2;Crosthwaite,Turner&Ho2000). ity,andthespectralmodelswerenotstronglyconstrained.Apow-
This implies a source of additional absorption intrinsic to, or in erlaw continuum was again the preferred model in both epochs,
the environment of, CXOU J034555.7+680455. This may origi- though the MCDBB also gave a statistically-acceptable fit in the
natein thenebula surrounding thisULX described by Roberts et second epoch. The source flux appeared to change littlebetween
al. (2003). The Chandra spectra are consistent with the low/hard the epochs, perhaps fading slightly in the three months between
spectral stateobserved by ASCAin2000 February24 - March1, observations.However,despitethelowquality,theintrinsicX-ray
describedbyanabsorbedpowerlawcontinuumwithNH=0.64± spectrumshowedchangesbetweentheepochs,withthefirstobser-
0.07 ×1022atomscm−2 and Γ = 1.73 ±0.06 (Kubota et al. vation showing an intrinsically hard (Γ ∼ 1.57) but unobscured
2001). This state has also been interpreted as an anomalous/very powerlaw continuum, whereas the second observation showed a
high state(Kubota, Done &Makishima 2002), whichwe discuss softer intrinsic slope (Γ ∼ 2.2) with a low-energy turnover due
furtherinSection5.1.Theluminosityalsoappearslittlechanged, toabsorption.
8 T.P.Robertset al.
Figure3.TheChandraACIS-S3spectraoftheULXs.Weshowthedataandbest-fitsinglecomponentmodelinthetopwindowofeachpanel,andtheratio
ofthedatatothemodelinthelowerwindow.WeplotthefirstandsecondepochdataforeachULXintheleftandrightcolumnsrespectively,usingthesame
normalisationscalingforeachpairofspectratoaidtheirdirectcomparison.Allspectraareshownusingthesameenergyscale,andtheratiovaluesareall
displayedoverthesamerange.
4.2.3 CXOUJ120922.2+295600 fittothedata.Thepowerlawcontinuumslopeisconsistent(within
thelargeerrors)inbothepochs,thoughtheinferredabsorptioncol-
ThisnewULXhasaspectrumwhichisbest-fitbyapowerlawcon- umnismuchlowerintheabsenceofaverysoftcomponentinthe
tinuum model in both epochs. However, the powerlaw fit is only secondobservation.
statistically-acceptableinthesecondepoch.Inordertoimprovethe
fittothefirstepochdatawetriedavarietyoftwo-componentspec-
4.2.4 CXOUJ123551.7+275604(NGC4559X-1)
tralfitstothedata.Thebestfitscamefrommodelswithahighly
absorbed,verysoftcomponentinadditiontoahardpowerlawcon- This is the most luminous ULX in the sample, with its observed
tinuum. We show such a fit, with the very soft component mod- luminosityat,orabove,1040ergs−1.ItsX-rayspectrumwasob-
elledbyaMCDBB,inTable5.Thisfitisstatisticallyacceptable served to soften between the observations, with the X-ray emis-
forthedata,andanimprovementoverthesinglepowerlawcontin- sion in thesecond, more X-ray luminous epoch represented by a
uummodelat99.97%(> 3σ)confidence.Note,however,thatthe significantlysofter powerlaw photon indexthan thefirst(∼ 2.16
additionofaclassicalblackbodyemissionmodel(withkT = 0.1 against ∼ 1.91). However, its X-ray spectrum is not well-fit by
keV)ratherthanaMCDBBmodelprovidesanequallyacceptable eithersimplemodel,withtheMCDBBmodelrejectedathighsta-
Chandraobservationsoffive ultraluminousX-raysources innearbygalaxies 9
Figure4.AsforFigure3,exceptforthefirstepochspectrumofCXOUJ132938.6+582506.Hereweplotthepile-upcorrectedspectralmodelovertheactual
data,todemonstratethedegreeofpile-upintheobservedX-rayspectrum.
tisticalsignificance. Two-component modelsincorporating avery best-fitwasobtained,withthecontributionoftheputativethermal
softMCDBBplusapowerlawcontinuumofferedimprovementsto componentlimitedtonomorethan3%ofthe0.5–8keVflux.
thefit,asshowninTable5,thoughonlyatthe69%and94%signif- However,theMEKALfitreliesheavilyonthefactthatablend
icancelevelsinthefirstandsecondepochsrespectively.Aclassic oflinesbetween0.5–0.75keV(predominantlyOVIIandOVIII),
blackbody component offeredasuperior improvement inthesec- whencombinedwithafactor∼2increaseinthemodelNH,gives
ondepoch,atthe96%significancelevelaccordingtotheF-statistic. agoodmatchtotheadditionalsoftemissionapparentinthesecond-
epochspectrum.Isthisthermalcomponentrealorsimplyanarte-
Curiously, asubstantial improvement inthefittothesecond factoffittingafairlycomplexmodeltodatawithlimitedspectral
epoch data was obtained when the MCDBB component was re- resolutionandpoorstatistics(N.B.∼600countsoutof∼2000in
placedwithaMEKALsolarabundancethermalplasmawithkT ∼ thefullspectrumoriginatefromtheMEKALcomponent)?Clearly
0.18keV(Table5).Fortwoadditionalfreeparameters,thereduc- thereisnodirectevidenceinthesecond-epochspectrumforindi-
tioninχ2of23.3impliesa99.99%significancelevel(i.e.>3.5σ) viduallines(thoughthisisperhapsconsistentwithlowtemperature
intermsoftheF-test.Figure5showstheresultingbest-fitspectrum oftheplasmaandthe>100eVspectralresolutionoftheACIS-S
andthecontributionoftheMEKALcomponent.Forcomparison,a detectorbelow1keV).Ontheotherhandwhenweallowtheabun-
similar model (powerlaw plus kT ∼ 0.18 keV MEKAL) was fit- dance of O, Ne and Fe (which produce the most significant line
ted to the first-epoch data and no significant improvement to the features in the kT ∼ 0.18 keV plasma) to vary in the VMEKAL
10 T.P.Robertsetal.
apile-upfractionofinexcessof10%ofallevents(seeabove).The
XSPECv.11.2parameterisationoftheDavis(2001)pile-upcorrec-
tionmodelwasusedtocorrectforthiseffect,theresultsofwhich
are shown in Table 5. This resulted in a much improved fit to a
powerlawcontinuumspectrum,withtheinferredphotonindexsoft-
eningconsiderably.Hence,whilstthetwouncorrectedspectraap-
peartohaveverydifferenthardnesses,afterthecorrectionforpile-
uptheyarebothrevealedtobeintrinsicallyverysoftX-rayspec-
tra.Thephotonindicesofthepowerlawcontinuumfitsareconsis-
tentwithintheerrorsafterpile-upcorrection,albeitwithaslightly
hardervalueinthefarmoreluminousfirstobservation.
4.3 Temporalproperties
Figure 5. The second epoch count rate spectrum of CXOU
J123551.7+275604. Thebest-fit ofthe powerlaw plus MEKALmodelto 4.3.1 Short-termvariability
theoverallspectrumisshownasthedottedlinewiththecontributionofthe
We derived short-term lightcurves for seven of the eight ULXs
MEKALcomponenthighlighted(solidline).
in both of their observation epochs, using the CIAO routine
LIGHTCURVE. The exception was CXOU J120922.6+295551,
model, weobtain abest-fitof 1.3 solar and a90%lower-limitof which was too faint for this analysis with only 59 and 90 counts
0.33solar,implyingthatthereisastrongpreferenceforasubstan- detectedintotal perepoch. Eachlightcurve wasbinned toanav-
tialcontributionfromemissionlines.However,thisisonthebasis erage(overtheobservation)of25countsperbin,givingtemporal
of a spectral model which may not be correct. We conclude that resolutionsrangingfrom∼60secondsinthebestcase,to∼2500
aninterpretationof theadditional softfluxpresentinthesecond- seconds.Theresultingdatawastestedforgrossvariabilityusinga
epochintermsoftheemergenceofabrightthermalcomponentis χ2 testagainstthehypothesisofaconstantcountrate.Fiveofthe
potentiallyveryinteresting(seeSection5.3),albeithighlyspecula- ULXshowednostrongevidenceforshort-termtemporalvariabil-
tive. ityineitherepoch,withareduced-χ2statisticof1.25orless.How-
Finally,wenotethatasthispaper wasundergoing therefer- ever,twoULXshowedstrongevidenceforshort-termvariabilityin
eeingprocesstheseChandraspectrawerepublishedbyCropperet oneoftheirtwoobservationepochs.Theselightcurvesareplotted
al.(2003)aspartofanXMM-Newtonstudyofthissource.Theydo in Figure 6. The appropriate χ2/degrees of freedom statistics are
not reportthepossibledetectionof aMEKALcomponent, though 39.5/14forthe2002JuneobservationofCXOJ120922.2+295600
theydidnotexplicitlylookforit.Insteadtheyfitaparticularmodel (which had a 1407 second resolution), and 190/24 for CXOU
totheChandradata,basedonfitstothesuperiorquality(andlater J123558.6+275742 in its 2001 January observation (at 404 sec-
epoch) XMM-Newton spectra, comprising a sub-solar abundance ondresolution).Theotherobservationsofbothsourceswerecon-
absorber (TBVARABS, set at 0.31 times solar) applied to pow- sistentwithaconstant flux,implyingthattheshort-termvariabil-
erlaw plusblackbody emission components. Wehaveapplied the ity states are themselves a transient phenomenon. These highly-
Cropperetal.model,plusvariantsincorporatingsoftMCDBBand variablestatesbothoccurwhentheULXinquestionis(onaverage)
MEKAL components, to the second-epoch spectrum and do get a atthelowerofitstwoobservedX-rayfluxes.
marginalimprovementineachcasewithrespecttoouroriginalfits WealsoperformedaKolmogorov-Smirnovtestcomparingthe
(∆χ2 ∼2−4);neverthelessthepowerlawplusMEKALmodelstill cumulativephotonarrivaltimeswiththeexpectedarrivaltimes,as-
clearlyprovidesthebestresultintermsoftheminimumχ2. suming each source flux was invariant with time. This provides
a separate indicator of short-term variability, free of any possi-
ble gross binning effects, and sensitive to lower-amplitude (10 –
4.2.5 CXOUJ123558.6+275742(NGC4559X-4) 20%) sustained changes inthe ULXcount ratesthan theχ2 test.
We performed this test on all eight ULXs. It confirmed that the
ThisobjectwastheonlyULXinthesamplewithanX-rayspec-
2002 June observation of CXO J120922.2+295600 was variable
trumclearlybest-fitwithaMCDBBmodel.Theinferredinnerac-
at > 99% probability. Curiously, the 2001 January observation
cretiondisctemperatureswerebothgreaterthan1keV,consistent of CXOU J123558.6+275742 was only variable at the ∼ 90%
withpreviousASCA,andsomeChandraobservationsofULXwell-
level according to the Kolmogorov-Smirnov test. Again, none of
fitbythismodel(e.g.Colbert&Mushotzky1999;Makishimaetal.
theotherULXappearedvariableaccordingtothistest.
2000;Robertsetal.2002).TheluminosityoftheULXmorethan
IntheJanuary2001lightcurveofCXOUJ123558.6+275742
doublesinthefivemonthsbetweenobservations,andthisisassoci-
therearefour“peaks”intheX-rayemission,separatedbyregular
atedwithaslighthardeningoftheX-rayspectrum,withthebest-fit
intervalsof6–7bins,duringwhichtheX-rayfluxis4–5times
temperatureoftheinneraccretiondiscrisingfrom∼1.1to∼1.3
brighterthaninthelowestfluxbins.Thishintsatapossiblechar-
keV. acteristictimescale for the variability of ∼ 2500 seconds for the
peaks.Itisalsonoticeablethatthelightcurvebehavessimilarlybe-
tweenthepeaks(thisquasi-repetitivebehaviouristhelikelyreason
4.2.6 CXOUJ132938.6+582506(NGC5204X-1)
for the lower probability of this source being variable according
TheX-rayspectrumofthisULXwasfarbetterfitbyapowerlaw totheKolmogorov-Smirnovtest).Similarregularvariabilityinthe
continuum model than a MCDBB in both epochs, with it consti- formof“quasi-periodicflaring”hasrecentlybeenrevealedinthree
tuting a statistically-acceptable fit in the second epoch. It was a separateobservationsofanextremelyvariableULXinM74,CXO
poorerfitinthefirstepoch,thoughthisobservationwasaffectedby J013651.1+154547(Kraussetal.2003).However,thecharacteris-