The Ultra Luminous X-ray sources in the High Velocity System of NGC 1275

0.41 MB·English

by  O. Gonzalez-Martin

Mon.Not.R.Astron.Soc.000,000–000(0000) Printed5February2008 (MNLATEXstylefilev2.2)
The Ultra Luminous X-ray sources in the High Velocity System of
NGC1275
O. Gonza´lez-Mart´in1,2⋆, A.C. Fabian2 and J.S. Sanders2
1InstitutodeAstrofi´sicadeAndaluci´a(CSIC),Apdo3004,18080Granada,Spain
2InstituteofAstronomy,MadingleyRoad,CambridgeCB30HA
6
0 5February2008
0
2

n ABSTRACT
a We report the results of a study of X-ray point sources coincident with the High Velocity
J System(HVS) projectedin frontofNGC1275.A verydeepX-rayimageofthe coreofthe

9 PerseusclustermadewiththeChandraObservatoryhasbeenused.We findapopulationof
Ultra-LuminousX-raysources(ULX;7sourceswithL (0.5−7.0keV)>7×1039ergs−1).
X
1 AswiththeULXpopulationsintheAntennaeandCartwheelgalaxies,thoseintheHVSare
v associatedwith a regionofveryactivestar formation.Severalsourceshavepossibleoptical
0 counterparts found on HST images, although the X-ray brightest one does not. Absorbed
8
power-lawmodelsfittheX-rayspectra,withmosthavingaphotonindexbetween2and3.
1
1 Keywords: galaxies:clusters:individual:Perseus-ULX-galaxies:individual:NGC1275
0
6
0
/
h
1 INTRODUCTION different fromthesampleof stellar-massBHCand areconsistent
p
withbeingIMBH.
- The study of Ultra-Luminous X-ray sources (ULX) has been
o Here we report on the discovery of a population of 8 point
greatlyexpandedbythehighspatialresolutionandspectralgrasp
r X-raysourcestotheNofthenucleusofNGC1275, whichisthe
t oftheChandraandXMM-Newtonobservatories,respectively.ULX
s centralgalaxyinthePerseuscluster.Allexceed1039ergs−1 inX-
a sources(Fabbiano&White2003;Miller&Colbert2004)have2–
: 10 keV X-ray luminosities exceeding 1039ergs−1 and are found rayluminosity,and7areformallyULX,iftheyareatthedistance
v ofthecluster.Thespatialregionwheretheyliecoincideswiththe
some distance from the centres of galaxies; they are not active
i HighVelocitySystemofNGC1275.Weassumethattheyarepart
X galactic nuclei. Their luminosity exceeds that for a 10M⊙ black
ofthatsystem.Weseenootherpoint sources(apartfromthenu-
r hole accreting at the Eddington limit which radiates isotropically
a andsohavecreatedmuchinterestinthepossibilitythattheycontain cleus)overthebodyofNGC1275(Fig.1).
even higher mass black holes, such as InterMediate Black Holes NGC1275 is embedded in a complex multiphase environ-
(IMBH) of ∼103M⊙ (Makishima et al 2000; Miller, Fabian & ment. Optical imaging and spectroscopy first established the ex-
Miller2004).Alternativelytheymayappearsoluminousbecause istence of two distinct emission-line system toward NGC1275: a
of beaming (Reynoldset al1999; Kinget al2001, Zezas&Fab- low-velocity component associated withthe galaxy itselfat 5200
biano2002)orduetosuperEddingtonaccretion(Begelman2002). kms−1 andahigh-velocitycomponent at8200kms−1 projected
ULXaremostcommoninstarburstgalaxiesandinveryactive nearbyonthesky(Minkowski1955,1957).Thislattercomponent
star-forming regions, such as in the Antennae and the Cartwheel is associated with a small gas-rich galaxy falling into the cluster
galaxy, where populations of tens of them are found (Zezas et al along our line of sight (Haschick, Crane & van der Hulst 1982).
2002; Gao et al 2003; Wolter & Trinchieri 2004). In some cases A merger scenario has been proposed (Minkowski 1955, 1957).
variability rules out the possibility that they are just clusters of However, interactionof thelow and/or high-velocitysystemwith
lower-luminosityobjects.TheoriginofIMBHisunclear.Theymay athirdgas-richgalaxyorsystemofgalaxies(Holtzmanetal.1992;
formasaresultofbinaryinteractionsindensestellarenvironments Conselice, Gallagher & Wyse 2001), or influences from the sur-
(PortegiesZwart&McMillan2002).AcomparisonofIMBHULX roundingdenseintraclustermedium(ICM)(Sarazin1988;Boroson
candidateswithanumber of wellknown stellar-massblackholes 1990;Cauletetal.1992)havebeendiscussed.
candidates(BHC;Milleretal2004)demonstratesthattheULXare DeepChandraobservationshaveclarifiedthepositionofthe
moreluminousbuthavecoolerthermaldiskcomponentsthanstan- HighVelocitySystem(HVS).ThedepthoftheobservedX-rayab-
dardstellar-massBHC.Therefore,ULXinthissampleareclearly sorption(e.g.Fig.1)isnorinfilledbyemissionfromhotgaspro-
jectedalongtheline-of-sightsotheHVSmustliewellinfrontof
NGC1275. Gillmon, Sanders & Fabian (2004) have estimated a
lowerlimitonthedistanceoftheHVSfromthenucleusof57kpc.
⋆ E-mail:[email protected] Thelow-andhigh-velocitysystemarethereforenotyetdirectlyin-
(cid:13)c 0000RAS
2 O. Gonza´lez-Marti´n,A.C. FabianandJ.S.Sanders
Figure1.NGC1275inthe0.3–7.0keVband.Pixelsare0.49arcsecinsizeandtheN-Sheightoftheimageis97arcsec.Northistothetopandeastistothe
leftinthisimage.ThepointsourceswhichweidentifyasULXarelocatedinthehigh-velocitysystemisseeninabsorptiontothenorthofthebrightnucleus.
Noneareseeninthesouthlobe
teracting.TheHVSishowever stronglyinteractingwiththeICM 29gain ctiN0003).Thedatasetsanalysedeachusedanaimpointon
of the Perseus cluster, which has triggered strong star formation. theACIS-S3CCD.Thedatasetswerefilteredusingthelightcurve
InthispaperwedescribethedetailedanalysisoftheX-rayspatial in the 2.5 to 7 keV band on ACIS-S1 CCD, which is a back-
andspectralpropertiesofthediscretesourcesinthehighvelocity illuminatedCCDliketheACIS-S3.TheCIAOLC CLEANtoolwas
system. used to remove periods 20 per cent away from the median count
Thepaperisorganizedasfollows:inSect.2and3wepresent rateforallthelightcurves.Thisprocedurewasnotusedfordatasets
reductionandresultsfromtheimaginganalysisandspectralanaly- 03209and04289whichdidnotincludetheS1CCD,howeverno
sis,respectively;inSect.4wediscusstheresults;andSect.5sum- flareswereseenintheseobservationsontheS3CCD.Eachofthe
marizesourfindings. observationswasreprojectedtomatchthecoordinatesystemofthe
Throughout this paper we use a redshift of 0.018 and H0= 04952observation.
70kms−1Mpc−1.Thisgivesaluminositydistancetothecluster The 900 ks X-ray image covering the energy range
of80Mpc;1arcseccorrespondstoaphysicaldistanceof370pc. 0.3−0.8keVisshowninFig2.ThebrightNGC1275nucleusis
clearlyseenatRA3h19m48sandDec.+41o30’42”(J2000)andthe
high-velocity systemisseen inabsorption tothenorth of thenu-
cleus.
2 IMAGINGANALYSIS
The CIAO CELLDETECT source detection routine was then
TheChandradatasetsincludedinthisanalysisarelistedinTable1. used on the reprocessed level 2 event data to produce a prelimi-
Thetotalexposuretime,afterremovingperiodscontainingflares, narylistofpointsources.Thecellsizerangesbetween4pixelsto
is890ks.Topreparethedataforanalysis,allofthedatasetswere 8pixels.Thisalgorithmstronglydependsonthelocalbackground
reprocessed tousethe latestappropriate gainfile(acisD2000-01- and the detection cell in not adjustable to the size of the source.
(cid:13)c 0000RAS,MNRAS000,000–000
X-raysourcesin theHVS ofNGC1275 3
Figure2.ThecentralregionofNGC1275inthe0.3–0.8keVband.Pixelsare0.49arcsecindimensionandtheentireimageis1.77×0.89arcmin2.Northis
tothetopandeastistotheleftinthisimage.Thehigh-velocitysystemisseeninabsorptiontothenorthofthebrightnucleuswhichisatRA3h19m48s,Dec.
+41o30’42”
Obs.ID Sequence Observationdate Exposure(ks) Nominalroll(deg) PointingRA PointingDec
3209 800209 2002-08-08 95.8 101.2 3:19:46.86 +41:31:51.3
4289 800209 2002-08-10 95.4 101.2 3:19:46.86 +41:31:51.3
6139 800397 2004-10-04 51.6 125.9 3:19:45.54 +41:31:33.9
4946 800397 2004-10-06 22.7 127.2 3:19:45.44 +41:31:33.2
4948 800398 2004-10-09 107.5 128.9 3:19:44.75 +41:31:40.1
4947 800397 2004-10-11 28.7 130.6 3:19:45.17 +41:31:31.3
4949 800398 2004-10-12 28.8 130.9 3:19:44.57 +41:31:38.7
4950 800399 2004-10-12 73.4 131.1 3:19:43.97 +41:31:46.1
4952 800400 2004-10-14 143.2 132.6 3:19:43.22 +41:31:52.2
4951 800399 2004-10-17 91.4 135.2 3:19:43.57 +41:31:42.6
4953 800400 2004-10-18 29.3 136.2 3:19:42.83 +41:31:48.5
6145 800397 2004-10-19 83.1 137.7 3:19:44.66 +41:31:26.7
6146 800398 2004-10-20 39.2 138.7 3:19:43.92 +41:31:32.7
Table1.Chandraobservations includedinthisanalysis.Theexposuregivenisthetimeremainingafterfilteringthelightcurveforflares.Allobservations
weretakenwiththeaimpointontheACIS-S3CCD.AllpositionsareinJ2000coordinates.
AstheX-raydiffuseemissionoftheNGC1275isverystrong,the Allthepoint-likesourcesarelistedinTable2,showingtheir
source list may well include false detections in high background positionsandcountrates.
levelregions.Thereforeproblematicsourcesembeddedinsuchre- We have used archival HST observations of NGC1275 in
gionshavebeenexcludedinouranalysis.Moreover,asmentioned order to search for optical counterparts. The galaxy was imaged
above,weonlyincludedsourcesassociatedwiththeHVS. withtheWFPC2cameraonHST usingtheF814W(∼I,on2001
We have detected 8 bright sources close to the nucleus of November6withanexposuretimeof1200s)andF702W(∼R,on
NGC1275,locatedinthenortherninnerradiolobeof3C84.Allof 1994March31withanexposuretimeof140s)broad-bandfilters.
thesesourceareembeddedinthesameregionastheHVS(seeFig. Several coincidences between X-raysources and optical knots of
1).Therearenosourcesassociatedwiththesouthernlobe(Fig.1), emission(F814W)canbeseeninFig.4(right),showingthesame
thusweassumethesesourcesareassociatedwiththeHVS. regionsasFig.4(left).
Fig.4(left)showsthesmoothed ACIS-S3imageinthe0.3– TheHST imageshowsmanyhighlyabsorbedfeatures.When
7.0keVband,includingnumberedlabelsofallthedetectedsources wecompareindetail,sourcesN7andN8arelocatedinstarforming
(top),centredonsourcelabelledN3(centre)andcentredonsource regions, while N2 and N6 have a point-like counterpart. Sources
N5(bottom). N1, N3, N4 and N5have no optical identification. Therefore, we
(cid:13)c 0000RAS,MNRAS000,000–000
4 O. Gonza´lez-Marti´n,A.C. FabianandJ.S.Sanders
N Position(J2000) CountRate N NH G c 2/d.o.f.
(countsks−1) (1021cm−2)
1.... 03:19:48.736+41:30:47.25 0.34±0.05 1.... 2.5(a) 3.20+0.23 112.90/101
−0.37
2.... 03:19:48.166+41:30:46.64 1.69±0.06 2.... 2.72+1.43 1.78+0.30 101.50/109
−0.87 −0.24
3.... 03:19:48.090+41:31:01.88 2.60±0.08 3.... 2.49+0.40 2.08+0.09 153.86/142
−0.40 −0.09
4.... 03:19:47.994+41:30:52.30 1.42±0.09 4.... 2.05+0.91 2.29+0.44 156.24/152
−0.96 −0.28
5.... 03:19:47.925+41:30:47.50 1.19±0.09 5.... 2.64+1.23 2.92+1.44 124.09/139
−0.93 −0.36
6.... 03:19:47.602+41:30:47.01 0.74±0.06 6.... 3.74+1.57 3.51+0.48 102.58/92
7.... 03:19:47.422+41:30:51.93 0.95±0.08 7.... 4.03+−11..7389 3.20+−10..3696 133.69/135
8.... 03:19:47.214+41:30:47.62 1.28±0.08 8.... 2.66−+11..4050 2.13−+00..4582 150.81/138
−0.91 −0.25
Table2.PositionsofsourcesdetectedneartheNGC1275centreanddis-
Table4.Spectralfits.(a)ThecolumndensityofsourceN1hasbeenfixed
played in Fig. 4(column 2) andcount rate in the energy range between
duetothelowcountrate.
0.5–7.0keV(column3).
N Fobs(0.5–7.0keV) Fcorr(0.5–7.0keV) logLX
N FX/FF814W FX/FF702W MF814W MF702W ergcm−2s−1 ergcm−2s−1 0.5–7.0keV
1.... 2.09×10−15 4.34×10−15 39.51
1.... >26.5 >16.2 >22.6 >22.1 2.... 7.59×10−15 9.97×10−15 39.86
2.... 25.4 23.6 20.4 20.3 3.... 1.64×10−14 2.28×10−14 40.22
3.... >18800 ... >26.8 ... 4.... 7.76×10−15 1.10×10−14 39.91
4.... >1081 >800 >24.5 >24.2 5.... 5.36×10−15 1.07×10−14 39.90
5.... >123 >60.6 >22.6 >21.9 6.... 3.02×10−15 9.23×10−15 39.84
6.... 26.2 28.4 21.7 21.7 7.... 4.28×10−15 1.18×10−14 39.95
7.... 76.7 51.4 22.3 21.9 8.... 7.67×10−15 1.16×10−14 39.93
8.... 134 90.1 22.2 21.7
Table 5. Fluxes (observed and k-corrected) and luminosities assuming a
Table 3. Optical analysis. X-ray to optical ratios (columns 2and3) and cosmologicalmodelwithH0=70kms−1Mpc−1andz=0.018.
magnitude determinations (columns 4 and 5) for the filters F814W and
F702W,respectively,withtheX-rayfluxbetween1.0–7.0keV.
eitherasource-freecircularannulusorseveralcirclessurrounding
eachsource,inordertotakeintoaccountthespatialvariationsof
havefoundapossiblecorrelationbetweencompactX-raysources thediffuseemissionandtominimizeeffectsrelatedtothespatial
and regions of vigorous star formation. The implications are dis- variationoftheCCDresponse.
cussedlater. For each source, we extracted spectra from each of the
InordertoinvestigatetheemissionmechanismoftheseULX, datasets.Thesespectraweresummedtoformatotalspectrumfor
the X-ray to optical flux ratios have been computed between the eachsource.Responseandancillaryresponsefileswerecreatedfor
F702WandF814WHSTbroad-bandsand1.0–7.0keVX-rayband. eachsourceineachoftheobservationsusingtheCIAOMKACIS-
Preliminaryprocessingoftherawimagesincludingcorrectionsfor RMFandMKWARFtools.Theresponsesforaparticularsourcewere
theflatfieldingwasdoneremotelyattheSpaceTelescopeScience summedtogether,weightingaccordingtothenumberofcountsin
Institutethroughthestandardpipeline.Foreachframe,cosmicrays eachobservation.
wereremovedbyimagecombination,usingtheIMCOMBINErou- ThespectrawerefittedusingXSPECv.11.3.2.Inordertouse
tine in IRAF. After cosmic ray removal, the frames were added the c 2 statistic,wegroupedthedatatoincludeatleast20counts
usingtaskWMOSAICinSTSDASpackage. Photometricmeasure- perspectralbin,beforebackground subtraction.Inspectralfitting
mentsweremadewithPHOTtask,withintheNOAOpackage.Fi- weexcludedanyeventswithenergiesabove7.0keVorbelow0.5
nally the fluxes and magnitudes have been determined using the keV.
photometriczero-pointinformationintheheaderofthecalibrated Table4summarizesourspectralresultsintermsoftheabsorb-
imagefiles. ingcolumndensityandphotonindex.
TheseresultsareshowninTable3,includingtheX-raytoopti- Thesourceshavebeenmodelledwithanabsorbedpowerlaw
calfluxratiosfromtheF814WandF702Wbroad-bandfilters,and slopewithphotonindexbetweenG =[1.78-5.56]andanequivalent
the magnitude determinations from the same filters. In the cases columndensityofNH=[2.05−4.03]×1021cm−2.Inallthecases
whereanopticalcounterparthasnotbeenfound(N1,N3,N4and thesinglecomponentpowerlawgivesatisfactoryfits.Thecolumn
N5),themagnitudesandfluxesarejustalowerlimit. densityofsourceN1hasbeenfixedduetothelowcountrate.The
fittedN values areconsistent withtheintrinsicabsorption mea-
H
sured e.g. in theoptical band; the value of A =0.54 corresponds
V
3 SPECTRALANALYSIS to NH∼1.1×1021cm−2, assuming AV=NH×5.3×10−22 for
RV=3.1(Bohlinetal.1978).Thisvalueshouldbealowerlimit
WeextractedspectraforallthedetectedsourcesclosetotheHVS, tothefittedN valuetobeconsistent,asisseeninTable4.
H
usingextractionregionsdefinedtoincludeasmanyofthesource As an example of our spectral fits, the brightest source, N3,
photonsaspossible,butatthesametimeminimizingcontamination hasbeenfittedwithapower-lawwithspectralindexof2.08±0.09
fromnearbysourcesandbackground.Thebackgroundregionwas andabsorptionofNH=2.5±0.4×1021 cm−2(seeFig.3).
(cid:13)c 0000RAS,MNRAS000,000–000
X-raysourcesin theHVS ofNGC1275 5
Figure4. Top-left:Broadband(0.5–7.0keV)X-raysmoothedimagewithsourceslabelled.Top-right:HST/WFPC2F814Wbroadbandimage.Centre-left:
Broadband(0.5–7.0keV)X-raysmoothedimagecentredinsource3.Centre-right:HST/WFPC2F814Wbroadbandimagecentredinsource3.Bottom-left:
Broadband(0.5–7.0keV)X-raysmoothedimagecentredinsource5.Bottom-left:HST/WFPC2F814Wbroadbandimagecentredinsource5.
InTable5welistthe0.5-7keVfluxand(absorptioncorrected) The brightest point source has a luminosity of
luminositiesoftheindividual sourcesbased onthebest-fitpower LX(0.5−7.0keV)=1.67×1040ergs−1, and is one of the
lawmodel. brightestindividualsourcesfoundinagalaxy.AULXsourcemore
luminousthantheentireX-rayluminosityofanormalgalaxyhas
The lower limit of the luminosity of point sources
been found intheCartwheel system withaluminosityof at least
in the image, if at the distance of NGC1275, is L ∼2−4×1040ergs−1 (Gao et al. 2003; Wolter & Trinchieri
LX(0.5−7.0keV)=3.2×1039ergs−1, which is already 20X04).Theyexplainthisluminositywithahigh-massX-raybinary
well above the Eddington limit for a neutron star binary
source (HMXB). The high X-ray luminosity suggests either a
(L ∼3×1038ergs−1) and is also above the limit of canonical
X singleextremelybrightsource,oraverydensecollectionofseveral
ULX,i.e.≥1039ergs−1.
(cid:13)c 0000RAS,MNRAS000,000–000
6 O. Gonza´lez-Marti´n,A.C. FabianandJ.S.Sanders
thePerseusgalaxyclusterhastriggeredstarclusterformationinthe
HVC,thentheclustersshouldhaveageslessthan∼108yr.
Ourinterpretationofthespatialcorrespondencewithstarclus-
tersisthattheregionsareespeciallyactive,indicatingareallinkbe-
tweenULXandstar-formingregions,andmeaningtheyareyoung
objects. However the optical limitson sources N3 and 4 rule out
anyassociation withmassive clustersinthose cases(thelimit on
theabsolutemagnitudeisabout−8).
In M31 and the Milky Way (Grimm, Gilfanov & Sunyaev
2003), XRB have luminosities consistent with the Eddington
limit of a ∼2M⊙ accreting object. They produce luminosities
∼3×1038ergs−1,aboutoneorderofmagnitudebelowthelimit-
ingluminosityinoursample(3.2×1039ergs−1).Itispossiblethat
ourULXconsistofatleast15(or130,inthecaseofthebrightest
sourcefound)‘normal’XRBclusteredtogether,perhapsinayoung
Figure3.ACIS-SspectrumofsourceN3.Thesolidlinecorrespondswith
starcluster.Howeverinotherobjectsweknowthatvariabilityre-
a power law model with a spectral index of G =2.08 and absorption of
NH=2.5×1021cm−2.Thefitresidualsarepresentedinthelowerpanel. quires the presence of intrinsically luminous X-ray sources (e.g.
M82;Griffithsetal.2000,Kaaretetal.2001).Alternativepossibil-
itiesarethatblackholesources,withmassesintherangeofgalactic
highL sources,whichwouldbeevenmorepeculiar.Evidenceof blackholebinaries,aremildlybeamed(Reynoldsetal.1999and
X
timevariabilitymightsuggestthatisasinglehighL source. Kingetal.2001).Spectralandtimingfeatureshoweverruleoutthis
X
Time variability analysis has been performed. The observa- possibilityinsomeULX(e.g.Strohmayer&Mushotzky2003).We
tionsspanabouttwoyears.Twodatafileswereobservedon2002 notethatcompactsupernovaremnantssometimeshaveULXlumi-
August8and10,andtheotherelevendatafileswereobservedfrom nosities(e.g.Fabian&Terlevich1998), butnorecentsupernovae
2004 October 4 to 2004 October 20, giving an almost daily cov- havebeen reported for NGC1275 (SN1968A wastotheS of the
erage. The exposure times are between 22 and 143 ks. The data HVS;Capetti2002).
characteristicsallowsusdetermineshortvariationin16days(sec- Finally,werecalltheIMBHmodelwhichhasspectralsupport
ondperiod)andlong-termvariabilityof2years.Becauseofthelow from some sources (Miller et al 2004; the level of absorption in
countratesofthesourcesinNGC1275(seeTable2),itisveryhard NGC1275 is too high for any soft excess to be observed). They
tosearchforshort-termvariability.Weextractedlight-curves,us- mayformindensestarclusters.
ingDMEXTRACTCIAOtaskforthetwobrightestsources(N3and OuropticalstudieshaveclearlyshownthattheULXhavevery
N4) (net count rate greater than 0.98 count s−1) binned with bin high X-Ray to optical flux ratios. X-ray selected AGN from the
sizesof500,1000,2500and5000s.Inbothcasesthepointswere Rosatallskysurveytendtohavelog(FX/Fopt)∼1.ThustheULX
consistentwiththerespectivemeanvaluesandvariabilityhasnot do not have the optical properties expected if their were simple
beenfound.Furthermore,themeanvaluesbetween2002and2004 extensionsofAGN(IMBH,aslowluminositylimit).However,low
are the same, including errors bars. Therefore, evidence of time massX-raybinariesintheMilkyWayhaveFX/Fopt∼100−10000
variabilityhasnotbeenfoundduringthewholesetofobservations. (Mushotzky2004).
Theresultsfoundinoursystemindicatethatwehaveamixed
groupofobjects(seeTable3).Atleast4outof8sources(N3,N4,
N5and N8) havehigh X-rayto optical fluxratios. Atleast 3 out
4 DISCUSSION
of8(N1,N2andN6)havelowerX-raytoopticalratios,possibly
Chandra has revealed significant populations of ULX in the in- becausetheylieinstarclusters.
teractingsystemsoftheAntennae(NGC4038/9;Zezas,Fabbiano Our data are consistent with no significant variability, simi-
&Murray2002) andtheCartwheelringgalaxy(Gaoetal.2003; lartotheresultobtainedonNGC3256byLiraetal.(2002).Time
Wolter&Trinchieri2004),wheredramaticeventshavestimulated variability is frequently observed in ULX (e.g. IC342, Sugiho et
massivestarformation.Wehavereportedhereonanotherexample al.2001 orM51X-1,Liuetal.2002), arguingthatmostof them
(Fig.4left)intheHVSofNGC1275whichisinteractingwiththe aresingle compact objects, rather than asum of numerous lower
ICMofthePerseuscluster. luminosityobjectsinthesameobject.WhilemostULXvary,many
The sources are spatially associated with the distribution of showlowamplitudevariabilityonlongtimescales(e.g.theAnten-
absorbing clouds seen in soft X-ray (Fig. 2) and optical (Fig. 3) naegalaxies,Zezasetal.2002),whichisverydifferenttogalactic
images. Two sources (N7 and N8) are directly linked with dust blackholes.PortegiesZwart,Dewi&Maccarone(2004)findthata
knotsandanothertwo(N2andN6)haveanopticalpoint-likecoun- persistentbrightULXrequiresadonerstarexceeding15M⊙.The
terpart (Fig. 4 bottom). Similar correspondence have been found searchforcharacteristicfrequenciesisoneofthemostproductive
in the Cartwheel galaxy withthe outer ring (Wolter & Trinchieri wayofdeterminingthenatureoftheULX.
2004) andintheAntennaegalaxieswith39X-raysourceswithin
theWFPC2field(Zezasetal.2002).Theopticalbrightnessofthe
counterparts in the HVC are too high to be individual stars and
5 CONCLUSIONS
somaybeassociatedwithyoung starclusters.Followingthedis-
cussionofyoungstarclustersinNCG1275givenbyRicheretal We have described the detailed analysis of the spatial and spec-
(1993),anobjectofmagnitude22correspondstoaclustermassof tral properties of thediscrete X-ray sources detected with adeep
about 106M⊙ if itsage isabout 107yr.The HVCsystem travels Chandra ACIS-S observation around NGC1275. Our results are
atleast30kpcin107yrsoifastronginteractionwiththecoreof summarizedbelow:
(cid:13)c 0000RAS,MNRAS000,000–000
X-raysourcesin theHVS ofNGC1275 7
(i) We have detected a total of 8 sources to the north of MinkowskiR.,1957,inVandeHulstH.C.,ed.,Proc.IAUSymp.4,Radio
NGC1275nucleus. Astron..CambridgeUniv.Press,Cambridge,p.107
(ii) ThesourcesarespatiallycoincidentwiththeHighVelocity MushotzkyR.,2004,Prog.ofTheo.Phys.Supplement,155,27
System and thus probably associated with it. They are therefore PortegiesZwartS.F.,McMillanS.L.W.,2002,ApJ,576,899
PortegiesZwartS.F.,DewiJ.,MaccaroneT.,2004,MNRAS,355,413
ULX.
ReynoldsC.S.,HenzS.,FabianA.C.,BegelmanM.C.,1999,ApJ,521,99
(iii) FourofthesourceshaveanopticalcounterpartintheIand
Richer,H.B.,Crabtree,D.R.,Fabian,A.C.,Lin,D.N.C.,1993,AJ,105,877
Rbands(fromHST images); twoofwhicharepoint-likesources
Sarazin C. L., 1988, X-Ray Emissions from Clusters of Galaxies, Cam-
andtheothertwoareassociatedwithstar-formingregions.
bridgeUniv.Press,Cambridge.
(iv) Inallthecasesasinglecomponent power lawgivessatis- SugihoM.,KotokuJ.,MakishimaK.,KubotaA.,MizunoT.,FukazawaY.,
factoryfits,withspectralindexofG =[1.78-3.51]andanequivalent TashiroM.,2001,ApJ,561,L73
columndensityofNH=[2.05−4.03]×1021cm−2. StrohmayerT.E.,MushotzkyR.F.,2003,ApJ,586,L61
(v) The minimum luminosity is WolterA.,TrinchieriG.,2004,A&A,426,787
LX(0.5−7.0keV)=3.2×1039ergs−1 (source N1), which is ZezasA.,FabbianoG.,,2002,ApJ,577,726
alreadyabovethelimitofcanonicalULX. ZezasA.,FabbianoG.,RotsA.H.,MurrayS.S.,2002,ApJSupplement,
142,239
(vi) No variability was detected in the two brightest sources
found.
Our results add to the growing evidence that some episodes
of rapid star formation lead to the production of ULX. Young,
massive, starclustersmaybeinvolvedinsome, but notall ofthe
sources.
ACKNOWLEDGEMENTS
OGMacknowledgesthefinancialsupportbytheMinisteriodeEd-
ucacionyCienciathroughtheprogramAYA2003-00128andgrant
FPIBES-2004-5044.ACFthankstheRoyalSocietyforsupport.
REFERENCES
BegelmanM.C.,2002,ApJ,568,L97
BohlinR.C.,SavageB.D.,DrakeJ.F.,1978,ApJ,224,132
BorosonT.A.,1990,ApJ,360,465
Capetti,A.,2002,ApJ,574,L25
CauletA,WoodgateBE.,BrownL.W.,GullT,R.,HintzenP,Lowenthal
J.D.,OliversenR.J.,ZieglerM.M.,1992,ApJ,388,301
ConseliceC.J.,GallagherJ.S.,WyseR.F.,2001,AJ,122,2281
FabbianoG.,WhiteN.E.,2003,astro-ph/0307077
FabbianoG.,ZezasA.,MurrayS.,2001,ApJ,554,1035
FabianA.C.,TerlevichR.,1996,MNRAS,280,L5
GaoY.,WangQ.D.,AppletonP.N.,LucasR.A.,2003,ApJ,596,L171
GebhardtK.,RichR.M.,HoL.C.,ApJ,inpress,astro-ph/0508251
GillmonK.,SandersJ.S.,FabianA.C.,2004,MNRAS,348,159
GriffithsR.E.,PtakA.,FeigelsonE.D.,GarmireG.,TownsleyL.,Brandt
W.N.,SambrunaR.,BregmanJ.N.,2000,Science,290,1325
GrimmH.,GilfanovM.,SunyaevR.,2003,MNRAS,339,793
HaschickA.D.,CraneP.C.,vanderHulstJ.,1982,ApJ,262,81
HoltzmanJ.A.,FaberS.M.,ShayaE.J.,LauerT.R.,GrothJ.,HunterD.
A.,BaumW.A.,EwaldS.P.,HesterJ.J.,LightR.M.,LyndsC.R.,
O’NeilE.J.,WestphalJ.A.,1992,AJ,103,691
KaaretP.,PrestwichA.H.,ZezasA.,MurrayS.S.,KimD.-W.,KilgardR.
E.,SchlegelE.M.,Ward,M.J.,2001,MNRAS,321,L29
KingA.R.,DaviesM.B.,WardM.J.,FabbianoG.,Elvis,M.,2001,ApJ,
552,L109
LiraP.,WardM.,ZezasA.,Alonso-HerreroA.,UenoS.,2002,MNRAS,
330,259
LiuJ.,BregmanJ.N.,IrwinJ.,SeitzerP.,2002,ApJ,581,L93
MakishimaK.,KubotaA.,MizunoT.,OhnishiT.,Tashiro,M.,ArugaY.,
AsaiK.,DotaniT.,MitsudaK.,UedaY.,UnoS.,YamaokaK.,Ebisawa
K.,KohmuraY.,OkadaK.,2000,ApJ,535,632
MillerJ.M.,FabianA.C.,MillerM.C.,2004,ApJ,614,L117
MillerJ.M.,ColbertE.,2004,IJMod.Phys.,13,1
MinkowskiR.,1955,CarnegieYearbook,54,25
(cid:13)c 0000RAS,MNRAS000,000–000