An XMM-Newton search for X-ray sources in the Fornax dwarf galaxy

1.2 MB·English

by  A. A. Nucita

Astronomy&Astrophysicsmanuscriptno.ms˙20152 (cid:13)c ESO2013
December11,2013
An XMM-Newton search for X-ray sources in the Fornax dwarf
galaxy
A.A.Nucita1,2,L.Manni1,2,F.DePaolis1,2,D.Vetrugno1,2,andG.Ingrosso1,2
1 DipartimentodiMatematicaeFisica“EnnioDeGiorgi”,Universita`delSalento,CP193,I-73100Lecce,Italy
2 INFN,Sez.diLecce,viaPerArnesano,CP193,I-73100,Lecce,Italy
Received,;accepted,
3
1 ABSTRACT
0
2 Wereport the resultsof adeep archive XMM-Newton observation of theFornax spheroidal galaxy that weanalyzed withtheaim
of fully characterizing the X-ray source population (in most of the cases likely to be background active galactic nuclei) detected
n
towardsthetarget.Acrosscorrelationwiththeavailabledatabasesallowedustofindasourcethatmaybeassociatedwithavariable
a
starbelongingtothegalaxy.WealsosearchedforX-raysourcesinthevicinityoftheFornaxglobularclustersGC3andGC4and
J
foundtwosourcesprobablyassociatedwiththerespectiveclusters.ThedeepX-rayobservationwasalsosuitableforthesearchof

2 theintermediate-massblackhole(ofmass≃ 104 M )expectedtobehostedinthecenterofthegalaxy.InthecaseofFornax,this
⊙
searchisextremelydifficultsincethegalaxycentroidofgravityispoorlyconstrainedbecauseofthelargeasymmetryobservedinthe

] opticalsurfacebrightness.SincewecannotfirmlyestablishtheexistenceofanX-raycounterpartoftheputativeblackhole,weput
E
constraintsonlyontheaccretionparameters.Inparticular,wefoundthatthecorrespondingupperlimitontheaccretionefficiency,
H withrespecttotheEddingtonluminosity,isaslowasafew10−5.
.
h Keywords.X–rays:individual:FornaxdSph–blackholephysics
p
-
o
r 1. Introduction valuesobtainedbyextrapolationoftherelevantquantitiesfrom
t spiralgalaxies.
s
Diffusedwarfgalaxies(DDGs)arelow-luminositygalaxiesthat
a ThedSphgroupisalsointerestingsinceitprovidesanopti-
[ seem to be characterized by structural parameters (luminosity,
stellarscalelength)fundamentallydifferentfromthosefoundin mallaboratorytostudytheevolutionofaparticularstellarpopu-
1 lation(ofknownmetallicityandage)withoutsufferingextreme
spiral and elliptical galaxies (Kormendy 1985) with the dwarf
v crowdingconditionsasoftenhappensinglobularclusters.Inthis
spheroidalgalaxies(dSphs)attheextremeendofthissequence.
0 respect,thehigh-energyviewofthesegalaxies,suchasthatof-
9 Inparticular,dSphsarethoughttobesatellitegalaxiesinthe fered by deep XMM-Newton observations, allows the study of
1 Local Group (see e.g., Mashchenkoetal. 2006), have approx-
the faint end of the X-ray luminosity function of an old stellar
0 imately spheroidal shapes (sometimes typical of irregular and
population.Furthermore,bystudyingthelowmassX-raybinary
. late-typespiralgalaxies),and are usuallyatleast two ordersof
1 (LMXB)populationcharacteristicsindSphsandglobularclus-
magnitudelessluminousthanthefaintestknownspiralgalaxies.
0 ters,itispossibletogetinformationabouttheformationhistory
3 Thesesystemshavestellarcontentsin therange3×103 M⊙ to (still challenging,see e.g. Maccarone2005b) of such systems.
1 2×107 M⊙ (Martinetal. 2008) on length scales of a few kpc SinceanypersistentlybrightLMXBwouldentirelyconsumethe
: or less. Additionally, they show evidence of being dark-matter
v mass of the companionvia accretion in a few hundredmillion
dominated at all radii (for a review see Mateo 1997) as shown
i years(seee.g.thediscussionontheLMXBformationhistoryin
X by the measurementsof the centralvelocitydispersionthat are
Maccarone2005b),thepresenceofbrightX-raybinariesinold
r muchlargerthantypicalvalues(seeMateo1998a,Kleynaetal. stellarsystemsrepresentsaproblemthatisyettobesolved.An
a 2001,Kleynaetal.2002andreferencestherein).Thecentralve-
exampleofthesechallengingtargetsistheSculptordSphgalaxy.
locitydispersionallowsthemass-to-lightratiostobeestimated.
When studying a deep Chandra survey of this dwarf galaxy,
Thedifferencesbetweenthe normalgalaxiesandthedSphs Maccarone (2005b) found at least five X-ray sources with op-
familiesprobablycomefromadifferentformationhistorywith tical counterparts hence pushing towards alternative formation
the most favored theory being that dSphs have low mass den- theoriesofthelocalLMXBpopulation.Apushinthisdirection
sity since past supernova winds removed large amounts of gas wouldbetheobservationoftargetswithnoglobularclustercon-
(Silketal. 1987). Despite being verydifferentin their physical tamination(liketheSculptorgalaxy)and,possibly,withashort
propertiesfrom spirals and ellipticals, dSphsshow kinematical epochofstarformation.
properties that can be modeled using dark matter (DM) halos
Basedontheextrapolationtoglobularclustersofthefunda-
with the same mass profiles as those which reproduce the ro-
mental M − M relation derived from the study of super
tation curves of spirals (Saluccietal. 2012). Thus, the derived BH Bulge
massiveblackholesingalacticnuclei(seee.g.Magorrianetal.
centraldensitiesandcoreradiifordSphsareconsistentwiththe
1998),oneexpectstofindintermediatemassblackholes(here-
afterIMBHs)inglobularclusters,i.e.,sphericalsystemsofstars
Sendoffprintrequeststo:A.A.Nucita([email protected]) which survived the interactions with the surroundings objects
1
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
andnoworbitthecenterofthehostinggalaxy.Sinceitiscom- with known catalogs, and the identification of a few objects as
monly accepted that the galaxies and associated globular clus- genuine X-ray sources in Fornax dSph. We further discuss the
tersformedatthesametime(seee.g.Ashman&Zepf1998and IMBHhypothesisandaddressourconclusionsinSect.4.
Westetal.2004),itisnaturaltoexpectthatatleastsomeofthese
sphericalsystemsmayhostanIMBH.
ApartfromtheDMandstellarpopulationissues,dSphsare 2. X-rayobservationsanddataprocessing
intriguingplacestosearchforIMBHs,i.e.,collapsedobjectsin
The Fornaxdwarf galaxy(at J2000coordinatesRA = 02h 39m
themassrange102-105M whichareconsideredtobethemiss-
⊙ 59s.3 and Dec = −34◦ 26′ 57.1′′) was observed on August 8,
inglinkbetweentheobservedstellarmassblackholes(ofafew
2005for≃ 100ks(ObservationID0302500101)withthethree
tensofsolarmasses)andthesupermassiveones(106−108M )
⊙ EuropeanPhotonImagingCameras(EPICMOS1,MOS2,and
residingatthecenterofmostgalaxies.Oneofthereasonswhy
pn)(Stru¨deretal.2001,Turneretal.2001)onboardtheXMM-
theexistenceofsuchobjectsisexpectedistheymightplayacru-
Newtonsatellite.Thetargetwasobservedinimagingmodewith
cialroleintheformationofthesupermassiveobjectswhichare
thefull-framewindowandmediumfilter.
thoughtto growfromapopulationofseed objectswithmasses
in the IMBH range (see e.g. Ebisuzakietal. 2001). Obviously,
those seeds that did not accrete a substantial amount of matter 2.1.Datareductionandscreening
and/ordidnotmergetoformacentralsupermassiveblackhole
remainasIMBHs. The observation data files (ODFs) were processed using the
XMM-ScienceAnalysisSystem(SASversion11.0.01) withthe
WeexpecttofindIMBHsindSphsaswell(Maccaroneetal.
latest available calibration constituent files. The event lists for
2005).Forexample,Reinesetal.(2011)reportedthatthenearby
thethreecameraswereobtainedbyprocessingtherawdatavia
dwarf starburst galaxy Henize2-10 harbors a compact radio
thestandardemchainandepchaintools.
source at its dynamical center spatially coincident with a hard
X-ray source (possibly an ≃ 106 M accreting black hole). We followed standard procedures in screening the data. In
⊙
particular, for the spectral analysis we rejected time intervals
Farrelletal.(2009)foundthebrightestknownultra-luminousX-
affected by high levels of background activity. These time in-
raysourceHLX-1(seee.g.vanderMarel2004forareview)in
tervals (particularly evident in the energy range 10–12 keV)
thehalooftheedge-onS0agalaxyESO243-49,possiblyasso-
wereflagged,strictlyfollowingtheinstructionsdescribedinthe
ciated with an IMBH whose mass was initially evaluatedto be
>9×103 M (Servillatetal.2011)andthenbetterconstrained XRPSUser’sManual2byselectingathresholdof0.4countss−1
∼ ⊙ and0.35countss−1 forthe pnandMOScameras,respectively.
to the range 9×103-9×104 M (Webbetal. 2012). A further
⊙ This allowed us to compile lists of good time intervals (GTIs)
step towards the IMBH hypothesis was given by Farrelletal.
which were used to discard high background activity periods.
(2012) who detected evidence for a young (< 200 Myr) stellar
Theresultingexposuretimesforthetwo MOSandpncameras
cluster of total mass ∼ 106 M⊙ around the putative black hole were ≃ 82 ks and ≃ 62 ks, respectively. We screened the data
andconcludedthatHLX-1islikelytobethestrippedremnantof
by using the filter expressions #XMMEA EM (for MOS) and
anucleateddwarfgalaxy. #XMMEA EP (for pn). We also added the FLAG==0 selec-
InthecaseoftheFornaxdSph,vanWassenhoveetal.(2010) tion expression in order to reject events close to CCD gaps or
assumed that an IMBH of mass MBH ≃ 105 M⊙ is hosted in badpixels,takingintoaccountallthevalidpatterns(PATTERN
the galactic core and suggested that measuring the dispersion in[0:12])forthetwoMOScamerasandonlysingleanddouble
velocity of the stars within 30 pc from the center would allow events(PATTERNin[0:4])forpn.
that hypothesisto be tested. Jardel&Gebhardt(2012) recently
constructedaxisymmetricSchwarzschildmodelsinordertoesti-
matethemassprofileoftheFornaxdSphand,oncethesemodels 2.2.Sourcedetection
weretested versusthe availablekinematicdata,it waspossible
For each camera, the list of events was divided into 5 energy
toputa1-σupperlimitof M = 3.2×104 M ontheIMBH
BH ⊙ bandschosenaccordingto those used in the 2XMM catalogof
mass.
serendipitousX-raysources(Watsonetal.,2009),i.e.,B :0.2−
1
In this work, we concentrate on the Fornax dSph re- 0.5keV, B : 0.5−1.0keV, B : 1.0−2.0keV, B : 2.0−4.5
2 3 4
analyzing a set of XMM-Newton data previously studied by keV, and B : 4.5−12.0 keV. For the three EPIC cameras, we
5
Orioetal.(2010)whosearchedfortheX-raypopulationinthe producedone image for each energyband and a mosaic image
LeoI and FornaxdSphs.Inourwork,we used the mostrecent inthe0.3−10keVenergybandforinspectionpurposesonly.
analysissoftwareandcalibrationfiles.Apartfromthecharacteri-
We then performed the source detection using the SAS
zationofthehigh-energypopulationdetectedtowardsthegalaxy
task edetect chain. For each camera and input image, the tool
and the cross correlation with the available databases, we dis-
first evaluates the corresponding exposure map (via the task
cuss the possible identificationof a few genuineX-raysources
eexpmap)takingintoaccountthecalibrationinformationonthe
belonging to the Fornax dSph. We also considered the possi- spatialquantumefficiency,filtertransmission,andvignetting.In
ble existence of an IMBH in the galaxy core as suggested by
the next step, we producedimage masks that delimited the re-
vanWassenhoveetal.(2010)andJardel&Gebhardt(2012)and
gionswherethesourcesearchingwasperformed.
showthatoneofthedetectedX-raysourcescoincideswithone
Thesourcesidentifiedwith the localandmapsearchingal-
of the possible Fornax dSph centers of gravity. We then con- gorithms3werethenusedbythetaskemldetectwhichperforms
strained the black hole accretionparametersand notedthatad-
ditional importantinformationmay be obtained by moderately
1 http://xmm.esa.int/sas/
deepradioobservationsandhigh-angularresolutionX-raydata.
2 http://xmm.esac.esa.int/external/xmm user support/
The paper is structured as follows: in Sect. 2 we describe /documentation/rpsman/index.html
theX-raydataanalysisandinSect.3wereportourfindingson 3 Formoredetails,thereaderisaddressedtotheon-linethread:
theX-raypopulationobservedtowardsthegalaxy,itscorrelation http://xmm.esac.esa.int/sas/current/documentation
2
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
a point spread function (PSF) fitting in each of the EPIC cam- smoothedwith a 3-pixelGaussiankernel)of theMOS1,MOS
erasfor the five energybandssimultaneously.In particular,for 2, and pn exposuresin the 0.3-10 keV energyband is given in
anyofthedetectedsources,thelocationandextentwerefixedto Figure 1. Here, each of the identified sources is indicated by a
thesamevaluesinallbandswhileleavingthecountratesfreeto 35′′ radiuscircle(containing(cid:27)90%ofenergyat1.5keVinthe
varyamongthedifferentenergybands. pncamera,seee.g.theXRPSUser’sManual2008),labeledwith
After the task completion, three lists of sources remained asequentialnumberandwithacolordependingonwhichcam-
(onepercamera),eachcontainingtherefinedcoordinates,count era(orsetofcameras)thesourcewasdetectedby:yellow(pn),
rates,hardnessratios,andmaximumdetectionlikelihoodofthe magenta(pnandMOS1),red(pnandMOS2),black(pn,MOS
sourcecandidates. 1,andMOS2),blue(MOS1andMOS2),cyan(MOS1),and
Foreachof thedetectedsources,we derivedthe X-rayflux green (MOS 2). The red-dashedellipse indicates the extension
(inunitsofergs−1 cm−2)inagivenbandas of the galaxy which is characterized by semi-major and semi-
minoraxesof≃ 17′and≃ 13′(seetheNASA/IPACextragalac-
B
F = i , (1) ticdatabase7 -NED-)andamajor-axispositionangleof≃ 480
i
ECF
i (Mateo1998b).
where B is the countrate in the i bandand ECF is an energy In Table 1 we summarize our source analysis, ordering the
i i
conversionfactorwhich hasbeencalculatedusingthe mostre- list by increasing X-ray flux in the 0.2-12 keV energy band.
centcalibrationmatricesfortheMOS1,MOS2,andpn.ECFs Here,wegiveasequentialnumber(Scr)foreachsource,alabel
for each camera, energy band, and filter are in units of 1011 (#)indicatingwhichEPICcamera(orsetofcameras)identified
counts cm2 erg−1. In particular, we used the ECFs4 obtained thesource,i.e.,A(pnonly),B(pnandMOS1),C(pnandMOS
assuming a power-law model with photon index Γ = 1.7 and 2),D(pn,MOS1,andMOS2),E(MOS1andMOS2),F(MOS
a Galactic foreground absorption of N ≃ 3.0 × 1020 cm−2 1),andG(MOS2).WealsogivetheJ2000coordinateswiththe
H
(Watsonetal., 2009). Note also thatthe adoptedhydrogencol- associatederrors(Column3-5),thehigh-energyhardnessratios
umn density is of the same order of magnitude as the average HR andHR (seenextsection)(Columns6-7),the0.2−12keV
1 2
valueestimatedtowardsthetargetbyusingthe“N ”onlinecal- absorbedflux(Column8)andthecrosscorrelations(Column9-
H
culator5,i.e.,2.7×1020cm−2(Kalberlaetal.,2005). 10).
We refined the absolute astrometry by matching the candi- We used the Two Micron All-Sky Survey (2MASS),
date source lists to the USNO-B1 catalog (Monetetal., 2006). the Two Micron All-Sky Survey Extended objects (2MASX)
Fromthiscatalog,weextractedasub-sampleofstarswithcoor- (Skrutskieetal.,2006),theUnitedStatesNavalObservatoryall-
dinateswithin20′fromtheFornaxcenterandusedtheSAStask sky survey (USNO-B1) (Monetetal., 2006), and the variable
eposcorr(withparameterofmaximumdistanceof1′′)toobtain starcatalogintheFornaxgalaxy(Bersier&Wood,2002)tocor-
thecoordinatecorrectionwhichwas(onaverage)−0.56′′±0.50′′ relatetheX-raysourcecatalogwithopticalcounterparts.Indo-
in RA and −1.77′′ ± 0.34′′ in Dec. We purged the candidate ingthis,weassociatedwiththecoordinatesofeachoftheiden-
source lists (one per EPIC camera) by accepting only sources tifiedX-raysourceanerrorasresultingfromthequadraturesum
withamaximumlikelihooddetection(asprovidedbythesource oftheXMM-Newtonpositionalaccuracy(≃2′′at2σconfidence
detectionalgorithm)largerthan10 (equivalentto 4σ) andthen level, see Kirsch 2004, and Guainazzi 2010) and the statistical
cross correlated the source lists via the IDL routine srcor.pro6 errorasdeterminedbytheedetect chaintool8.Similarly,theer-
by requiring a critical radius of 1′′ outside which the correla- rorassociatedwiththeopticalcounterpartwasderivedfromthe
tionswererejectedandobtainedthesourceparameters(i.e.,co- relevantcatalog.
ordinates,countrates,fluxes,andhardnessratios)byweighting
WhenanX-raysourceisfoundtobewithin1σfromanopti-
(withtheerrors)thevaluesofinterestassociatedwiththesources
calcounterpartinagivencatalog,wereportinTable1thecorre-
identifiedinthethreecameras. spondingdistanceinarcsecondsintherelevantcolumn.We re-
Afterremovingafewspurioussources(i.e.,thosepositioned mindthereaderthattheFornaxdSphgalaxywasalreadythetar-
at the borders of the cameras and those not appearing as such getofaROSATX-rayobservation(seeGizisetal.1993forde-
inthe0.2-12kevband),ourcatalogresultedin107sourcesde- tails)withthepurposeofcharacterizingtheX-raypopulation(if
tected towards the Fornax dSph galaxy with, in particular, 32 any)inthegalaxyandofconstrainingtheextendedgascompo-
sources identified contemporarily in MOS 1, MOS 2, and pn, nent.TheanalysisoftheROSATdataresultedinthecompilation
of a catalog (hereinafter GMD093) listing 19 discrete sources
4 sources only in MOS 1 and MOS 2, 7 sources only in MOS
congruous with the expected number of sources in the extra-
1 and pn, 18 sources only in MOS 2 and pn, 30 sources only
galacticbackground.InTable2wegivetheGMD093labelsand
in pn, 2 sources only in MOS 1, and 14 sources only in MOS
coordinates (columns 1-3) of the sources observed by ROSAT
2.Notethatthenumberofdetectedsourcesagreesveryclosely
andthelabelsofthecorrespondingsourcesinourcatalog(col-
withthe104sourcesfoundbyOrioetal.(2010)whenanalyzing umn4).Thedistancebetweenthesourcesinthe GMD093cat-
thesamedataset,thediscrepancyprobablyarisingfromslightly alogandthe correspondingsourcesfoundin theXMM-Newton
differentchoicesinthedatascreeningprocedureandthedetec- list is given in the fifth column with the error evaluated as the
tionthresholdused.Amosaicimage(logarithmicallyscaledand sum in quadraturebetween the XMM-Newton and ROSAT po-
sitionalaccuracies9.Finally,apossibleclassification(basedon
4 For details on the adopted ECFs, see Saxton (2003). Please note
that,since2008,improvementsinthecalibrationofEPICcameraslead
tochangesintheECFs.Thus,thequotedECFsmustbemultipliedby 7 http://ned.ipac.caltech.edu/
acorrectionfactortogetabetteragreementamongthefluxesevaluated 8 Sincetheresultingpositionaluncertaintyisofafewarcseconds,we
ineachcamera(Stuhlingeretal.,2006).TheECFsandassociatedcor- donotover-plotthesourceerrorcirclesinanyofthefiguresappearing
rectionfactorsarereportedintheUser’sGuideofthe2XMMcatalog inthepaper.
ofserendipitoussourcesavailableat 9 Becauseofthelackofanypositionaluncertaintyonthesourcesre-
http://xmmssc-www.star.le.ac.uk/Catalogue/2XMMi-DR3. portedinGizisetal.(1993),weassociatedeachoftheROSATdetected
5 Availableinthetoolsectionofhttp://heasarc.nasa.gov sourceswithanerrorof≃15′′onboththecelestialcoordinates:weare
6 http://idlastro.gsfc.nasa.gov awarethatthiserrormaybeunderestimatedsincethepositionalaccu-
3
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
0 120.0
0.
0
5:
0
60.2
0
0.
0
10: 10094
71
10396 30.2
0
0.
15:0 45 18 44 7352 41 2371970219555 15.0
Dec -34:20:00.025:00.030:00.0 7786679270164262163556844273657134908114653564123272798973493561662912755118407348586936074115284507015181360222024754173599338833439018518 89896268844099 137...875
35:00.0 173685 107 38 102 2960 0.8
106
0.0 29
0
40: 0.4
0
0.
0
5: 0.1
4
30.0 41:00.0 30.0 2:40:00.0 30.0 39:00.0 38:30.0
RA
0.0
Fig.1.AmosaicimageoftheMOS1,MOS2,andpnexposuresinthe0.3−10keVenergyband(seetextfordetails).
the SIMBAD10 and NED web sites) is attempted in the sixth following the convention used in Ramsay&Wu (2006) in studying
column(QSO-quasar-,EmG-emissionlinegalaxy-,H2G-HII the Sagittarius and Carina dwarf galaxies and first introduced by
galaxy-,SyG-Seyfertgalaxy-):whenasourceisnotcrosscor- Prestwichetal.(2003)andSoria&Wu(2003)S,M,andHcorrespond
relatedwithSIMBADand/orNED,thisissimplylabeledasXrs tothecountratesin0.3-1keV,1-4keV,and4-10keVenergybands.
(X-raysource).Notethat,amongthe19sourcesbelongingtothe
GMD093catalog,6 are outof the XMM-Newtonfield of view.
Furthermore,fromtheL -L relation(L beingtheFornaxdSph Since we expect that the detected X-ray sources are mainly ac-
X B B
Bbandluminosity),Gizisetal.(1993)estimatedthatatleastone creting compact objects such as background AGN (Active Galactic
accretingbinaryshouldbepresentinthefieldofview.Forcom- Nuclei), X-raybinaries, and thebrighter end of cataclysmic variables
pleteness, we mention that as far as the extended component (CVs),wecomparedthesourcemeasuredhardnessratios(redsquares)
is concerned, the ROSAT data did not show any diffuse emis- with two spectral models (power-law, for simulating AGN and X-
sion different from the normalX-ray backgroundand no trend ray binaries, and bremsstrahlung, for CVs) which may be used to
withradiuswasevident.Here,weconcentrateonthepoint-like haveanoverall,although simplified,descriptionof thesource spectra
sourcesidentifiedintheXMM-NewtonobservationoftheFornax (see e.g. Ramsay&Wu 2006 for a similar analysis made on the Sgr
dSph. and Car dSphs). The model tracks, appearing in the color-color dia-
gram, were obtained by simulating synthetic spectra within XSPEC
(An X-Ray Spectral Fitting Package, Arnaud 1992), version 12.0.0.
In particular, we give the expected set of color-color contours for
bremsstrahlung (grey region) and power-law (black region) compo-
nents.Inbothcases,theequivalenthydrogencolumndensityN varies
3. Thehigh-energyviewoftheFornaxdSph H
intherange1019cm−2to1022cm−2:eachofthealmosthorizontallines
3.1.X-raycolorsandX-ray-to-NIRfluxratios correspondstomodelswithequal NH whichincreasesfrombottomto
top. The temperature kT of the bremsstrahlung models (taken in the
Using the results presented in the previous section and with the pur- range0.1-3keV)andthepower-lawindexΓ(intherange0.1-3)is
poseofatentativeclassificationofallthesourcesidentifiedtowardsthe associatedwithprimarilyverticallines:thevaluesofkTandΓincrease
FornaxdSph,weconstructedthecolor-colordiagraminFigure2.Here, fromlefttorightandfromrighttoleft,respectively. Arepresentative
weconsideredthecolors error bar, obtained by averaging all the data point error bars, is also
given.Mostofthedetectedsourceshavecolorsconsistentwiththoseof
HR = H−M and HR = M−S , (2) theabsorbedpower-laworabsorbedbremsstrahlungmodels,although
1 S +M+H 2 S +M+H afewofthemmayrequirecombinedspectra.Mostofthesourcesap-
peartohavespectraconsistentwiththatofatypicalAGN(whichinthe
racyofROSATPSPCdecreaseswithincreasingoffsetfromthedetector hardnessdiagramofFigure2wouldcorrespondtoadotlyingcloseto
axis. theharder color-color tracks,seee.g.Ramsay&Wu2006).However,
10 http://simbad.u−strasbg.fr/simbad/ becausethelargeerrorbarsaffectthehardnessratiodata,aclassifica-
4
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
GMD093 RA Dec Src Distance Type Ref.
ID (J2000) (J2000) (arcsec)
1 24015.7 -341323.1 100 27±15 Xrs –
2 23914.7 -341504.8 – – Xrs –
3 23912.2 -341818.2 101 3±15 Xrs –
4 23908.5 -341838.1 95 19±15 Xrs –
5 23950.0 -342010.9 104 18±15 QSO/EmG 1,2
6 23934.8 -342201.9 82 15±15 Xrs –
7 23933.5 -342539.0 105 16±15 QSO 1,2
8 24034.2 -342711.0 84 8±15 Xrs –
9 23820.8 -343017.6 – – Xrs –
10 23926.2 -343246.7 93 23±16 Xrs –
11 24008.1 -343425.3 97 6±15 QSO 2,3
12 23915.9 -343532.3 102 8±15 Xrs –
13 23903.7 -343646.4 90 8±15 Xrs –
14 23848.8 -343659.9 – – Xrs –
15 24019.0 -343727.1 107 7±15 QSO/H2G 1,2,4
16 24038.9 -343906.2 106 8±15 QSO/SyG 2,3
17 24208.1 -344006.8 – – Xrs –
18 23855.5 -344052.4 – – QSO 1,2
19 24039.1 -344810.7 – – Xrs –
Table 2. List of the detectedX-raysourcescross-correlatedwith the catalog GMD093(Gizisetal. 1993). TheGMD093sources
labeledas2,9,14,17,18,and19areoutoftheXMM-NewtonFOV.
References. (1)Tinney(1999);(2)Veron-Cetty&Veron(2006);(3)Tinneyetal.(1997);(4)Jonesetal.(2006).
tedblacklineinFigure3)and F /F < 3×10−2,respectively11.For
X J
the sources of our sample which correlate with2MASS counterparts,
wegiveinTable3the0.2-2.4keVbandflux,theNIRJbandfluxand
theJandKmagnitudes.Notethatthe0.2-2.4keVbandfluxhasbeen
obtained from that in the 0.2-12 keV band (given in Table 1) assum-
ing in webPIMMS a power-law model witha spectral index Γ = 1.7
and an absorption column density N = 3 × 1020cm−2. In Figure3,
H
wegivethecolor-colordiagram(X-raytoJbandfluxratioagainstJ-K)
forthesourceslistedinTable3.Thesourceslyingabovethehorizon-
taldashedlineareconsistentwithbackground AGN,whiletheothers
seem to have characteristics similar to X-ray active stars and binary
sources:threeofthem(19,41,and82)havemeasuredpropermotions
in thePPMX catalog (seealso Table 1), confirming the stellar nature
of theseobjects. Note that the X-raysource labeled as 107 correlates
(within1.0′′)withasourceinthe2MASXand,accordingtoourcolor-
colordiagram,ispossiblyassociatedwithabackgroundAGN(seealso
Mendezetal.2011whereitisusedasareferencebackgroundAGNfor
propermotionestimates),whileitwasprobablyreportedinthePPMX
catalogbymistake(sourcelabeledas024019.0-343719):thisconfirms
thepredictivepowerofthiscolor-colordiagram.
Fig.2. The color-color diagram of the sources detected by the
EPICcamerastowardstheFornaxdSphgalaxy.Thesolid lines
represent the theoretical tracks expected for different emitting 3.2.BackgroundsourcesfromthelogN-logSplot
models(seetextfordetails).Arepresentativeerrorbar(obtained
Weestimatedthenumberofbackgroundsourcesexpectedtowardsthe
averagingalldatapointerrorbars)isalsoshown.
FornaxdwarfgalaxythroughthelogN-logSdiagram(Hasingeretal.,
2005) and by using the minimum absorbed flux in the 0.2-12 keV
band among the sources of our analysis, i.e., FAbs = 2.24 ×
0.2−12
10−15ergs−1cm−2. Using webPIMMS v3.912 and assuming a power-
tionbasedonlyonthehardnessratiocannotconstrainthenatureofthe
law model with a spectral index Γ = 1.7 and an absorption column
objectsinoursample.
density N = 3 × 1020cm−2, we obtained an unabsorbed flux in
A better diagnostic tool was found by Haakonsen&Rutledge H
(2009) when studying the cross associations of the ROSAT All-Sky
SurveyBrightSourceCatalog(RASS/BSC)withtheinfraredcounter- 11 NotethatabetterdiagnosticwouldusetheunabsorbedX-rayflux
partsfoundin2MASS.Inparticular,theseauthorsfoundthatacolor- inthe0.2-2.4keVbandandtheextinction-correctedNIRmagnitudes:
colordiagram(basedontheratiobetweenthe0.2−2.4keVflux(F ) intheproposedcolor-colordiagramofHaakonsen&Rutledge(2009),
X
andtheNIRfluxinJband(F )versustheJ-Kcolor)isusefulinstudy- thegalaxiesarepositionedintheregiondescribedby(J−K) > (J−
J
ingthecharacteristicsofthesources.Inthisplane,thegalaxies(Quasars K)0 > 0.6 and (FX/FJ)0 > FX/FJ > 3×10−2. On the contrary, the
andSeyfert1objects)andthecoronallyactivestars(includingpre-main coronally active starssatisfy the relations (J−K)0 < (J−K) < 1.1
sequenceandmain-sequencestars,highproper-motionobjects,andbi- and(FX/FJ)<(FX/FJ)0<3×10−2,withthesubscript0indicatingthe
nary stars) occupy distinct regions in such a way that galaxies have resultofthede-reddeningprocedure(seeHaakonsen&Rutledge2009
(J−K) > 0.6and F /F > 3×10−2 (dashed redlinesinFigure3), fordetails).
X J
whilealmostalltheobjectsinthesecondclasshave(J−K)<1.1(dot- 12 http://heasarc.gsfc.nasa.gov/Tools/w3pimms.html
5
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
Src F F J K Annulus R R #Exp #Obs
X J in ex
(×10−14cgs) (×10−13cgs) (arcmin) (arcmin)
6 0.14±0.12 1.82±0.20 16.12±0.11 15.18±0.20 1 0.00 0.76 0.4±0.1 0
10 0.18±0.16 10.90±0.32 14.17±0.03 13.35±0.04 2 0.76 3.60 10.2±1.7 6
19 0.34±0.16 17.11±0.43 13.68±0.03 13.14±0.04 3 3.60 6.50 24.2±4.0 21
40 0.57±0.28 1.77±0.16 16.15±0.10 15.32±0.19 4 6.50 9.00 32.1±5.0 29
41 0.59±0.45 102.71±2.30 11.74±0.02 11.30±0.02 5 9.00 16.00 144.5±20.0 51
46 0.69±0.28 8.86±0.43 14.40±0.05 13.42±0.10
Table4.ListofsourcesexpectedthroughthelogN-logSdia-
48 0.74±0.28 1.53±0.20 16.30±0.15 14.10±0.15
gramandobservedinannuliaroundFornaxcenter.Here,R and
75 1.42±0.47 1.50±0.21 16.33±0.15 15.14±0.18 in
R representtheinteriorandexteriorannulusradii,respectively.
82 1.75±0.26 76.42±1.70 12.06±0.02 11.47±0.02 ex
84 1.87±0.25 54.25±1.10 12.43±0.02 11.58±0.02
86 2.03±0.38 1.32±0.22 16.46±0.17 14.95±0.20
102 4.25±0.93 2.10±0.19 15.97±0.10 15.32±0.20
3.3.HintofalocalvariablesourceassociatedwithanX-ray
107 9.84±1.22 6.97±0.48 14.66±0.10 13.43±0.10
source
Table 3. A few sources of the X-ray sample detected towards
the FornaxdSph correlate with the 2MASS catalog. Following WhensearchingforopticalcounterpartstothedetectedX-raysources
Haakonsen&Rutledge (2009), we can try to constrain the na- (seeTable1)intheavailablecatalogs,wefoundthatsourcenumber61
ispossiblyassociated,within2”,withonesource(J023941.4-343340)
tureofthesourcesbyusingtheX-ray(inthe0.2-2.4keVband)
belongingtoacatalogofvariablestars(Bersier&Wood,2002),making
and NIR (J and K bands) fluxes (see text and Figure 3 for de-
it a good candidate for a genuine X-ray source in the Fornax galaxy.
tails).
TheangularsensitivityofXMM-Newton,however,makesitdifficultto
separate this source from the one labeled 80 (see Figure 1) which is
only≃22”apart.Onesolutionistoreducethecontaminationfromthe
nearbysourcebyreducingthesourceextractionradiusandbyselecting
suitable background regions. In the particular case of source 61, we
usedasourceextractioncirclewitharadiusof≃11”(correspondingto
≃50%ofthesourceencircledenergy,seeXRPSUser’sManual2008)
and extracted the background from acircular region having the same
radius as the source extraction area and localized at a distance of ≃
22” from source 80. The latter choice allowed us to properly remove
thebackground noiseaswellasanypatternhiddeninthedatacaused
by the proximity of source 80. Wethen extracted the pn spectrum of
source61(whichconsistedofonly≃ 200counts)andfitteditwithan
absorbedpower-lawmodelwithhydrogencolumndensityfixedtothe
value2.7×1020cm−2(seeSection2.2).Thebestfit,whichconvergedto
apower-lawindexΓ≃2.0,correspondedtoafluxinthe0.2−12keV
energybandof≃2.0×10−14ergs−1cm−2consistentwiththatobtained
fromtheanalysisdiscussedinSection2(seealsoTable1).
Using the extraction regions previously described, we generated
source and background pn light curves (with a bin-size of 500 sec-
onds) from the cleaned event files and used the SAS task epiclccorr
toaccountforabsoluteandrelativecorrectionsandforthebackground
subtraction.TheresultinglightcurvewasthenanalyzedwiththeLomb-
Scarglemethod(Scargle1982)tosearchforperiodicitiesintherange
Fig.3.Thecolor-colordiagramfortheX-raysourcesofoursam- 1000−10000secondsandthesignificanceofanypossiblefeaturesap-
ple with a counterpart in the 2MASS catalog (see text for de- pearingintheLomb-Scargleperiodogramwasevaluatedbycomparing
tails). thepeakheightwiththepowerthresholdcorrespondingtoagivenfalse
alarmprobabilityinwhitenoisesimulations(Scargle,1982).Whenap-
pliedtothepresentlyavailabledata,thismethoddidnotdetectanyclear
periodicity.
3.4.TheFornaxglobularclusters
the same band FUnabs = 2.47 × 10−15ergs−1cm−2, hence FUnabs =
6.95 × 10−16ergs0−.12−c1m2−2inthe0.5-2keVband. 0.5−2 AmongalltheotherdSphswhicharesatellitesoftheMilkyWay,the
FornaxdSphhasthepeculiarcharacteristicofhostingfiveglobularclus-
We used this value as an input parameter of the Hasingeretal. ters(GCs) whose (J2000) coordinates and main structural parameters
(2005) method, obtaining a theoretical number of AGN asa function (mass M, core radius r , projected distance R from the galaxy cen-
c p
of the angular distance fromthe galactic center. Then wedivided the ter, tidal radius r, and maximum radius r ) are reported in Table 5.
t M
Fornaxfieldofview(FOV)intofiveringsandcomparedthenumberof Apartfromthepuzzlingquestionofwhydynamicalfrictionhasnotyet
sourcesobservedineachannulustotheexpectednumberusingthelog draggedanyoftheFornaxGCstowardsthecenterofthegalaxy(seee.g.
N-logSrelation(Hasingeretal.,2005),seeTable4.Thenumber of Coleetal.(2012)foranintroductiontothisintriguingproblemandto
sourcesdetectedineachannulusisconsistentwiththeexpectednumber Goerdtetal.2006forapossiblesolution13),asearchforX-raysources
exceptintheexternalannulus,astheexpectedvaluedoesnotaccount associated withtheGCsisalsointeresting.Weperformed thissearch
forbordereffectsalthoughallthedetectedsourcesintheFornaxFOV towardsthefiveFornaxdSphGSsandfoundthatafewX-raysources
maybebackgroundobjects,becauseoftheintrinsicstatisticalmeaning
ofthelogN-logSdiagram,itcannotberuledoutthatsomeofthem 13 Goerdtetal.(2006)haveshownthatdynamicalfrictioncannotdrag
actuallybelong tothegalaxy itself.Thisconclusion issomehow sup- globularclusterstothegalaxycenter inthecaseofcoreddark-matter
ported by Figure3 where theX-ray sources in thebottom-left region halodistributions.Inthiscasethedragisstoppedatthepointwherethe
aremostlikelyoflocalorigin. dark-matterdensityremainsconstant,i.e.,≃200pc.
6
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
GC RA Dec M r R r r
c p T M
(J2000) (J2000) (pc) (kpc) (pc) (pc)
12 2233780424 --334414180310 01..48 150.8.0 11..70 3479..10 4590..74 9:59.9 37 2
3 23948 -341530 3.6 1.6 0.4 51.0 50.4 2 5
4 2407.7 -343211 1.3 1.7 0.2 28.5 42.4
5 24221 -340606 1.8 1.4 1.4 49.0 50.4 28 28GC4
Table 5. The coordinates of the Fornax globular clusters are 7.9 26
etracak,cehanncdfluropstmreorjtewhceetegNdiAvdeSistAthae/nImPcAeaCsRspeMxftr(roiamngautlnhaicetstigcoafdl1aax0tay5bMcaes⊙ne)t,(ecNroEarseDrf)ao.duFinuodsr Dec -34:31:4 46 232011141519542211117386607121311829224 27
in Coleetal. (2012), while the valuesof the tidal radiusr and 29
t 25
maximumradiusr (expressedinparsecforadistanceof0.138
M
9
Mpc) were derivedfrom MacKey&Gilmore (2003) and refer- 5.
3
encestherein. 3:
3
20.0 15.0 10.0 05.0 2:40:00.0 39:55.0
RA
arelocatedclosetoGC3andGC4(seethezoomedviewsoftheXMM-
Fig.4.AzoomedviewoftheregionaroundtheFornaxglobular
NewtonfieldaroundthetwoclustersinFigures4and5.
clusterGC4(seetextfordetails).
ForGC4,thethreegreendashedconcentriccirclesarecenteredon
theGCcoordinatesandhaveradiiof2.6′′,43′′,and64′′,corresponding
tothe cluster core radius, tidal radius, and maximum radius (seealso
Table5)asgiveninColeetal.(2012)andMacKey&Gilmore(2003). 6_ V49021
InthecaseofGC3,forclarity,wehaveplottedonlythecircleshavinga 71
radiusequaltothecoreradius(2.4′′)andtidalradius(77′′).Alltheother 7.9 103 96
cthirecXle-sra(wyistohuarcsseoscdiaetteedcteraddiiniooufr3a5n′′a)lyaspipse(asreiengSeinct.th2e.2fi).guresindicate 13:4 NGC 1049 (GC3) 6_V44543
InthecaseofGC4,weidentifiedoneX-raysource(labeledas28
inthesourcelistappearinginTable1)whichisatadistanceof≃ 37′′ 5.9
fromtheglobularclustercenter14.Consequently,consideringtheGC4 Dec 15:3 6_V40765
structuralparametersreportedinTable5,oneobservesthatsource28is 6_V369_1V5318 8 75
67_3V38403
waseslolcwiaittehdinwtihtehtiitd,aaslraalrdeiaudsy(≃no4t3ed′′)boyfOthrieogeltoablu.l(a2r0c1l0u)stwerhaenndanpaolsysziibnlgy 7:23.9 44 6_ V37637 6_V3547197
thesameXMM-Newtondataset. 34:1 52 6_V33971
In the case of GC 3, two variable Sx Phe sources15 (6 V40765 -
7_V21373
and6 V38403)wereidentifiedbyPorettietal.(2008)asbelongingto
9
theglobular cluster.Bothsourcesappear tobeatadistanceof≃ 30′′ 1.
1
and ≃ 78′′ from the globular cluster center and, therefore, are possi- 19: 05.02:40:00.0 55.0 50.0 45.0 40.0 35.0 39:30.0 25.0
bly associated with GC 3. As one can see from a close inspection of
RA
Figure5,thereisnoclearassociationofanyofthevariablestars(blue
crosses)observedinGC3withthedetectedX-raysources.Moreover,
one X-ray source (labeled as 103 at ≃ 72′′ from the globular cluster
center) and identified intheXMM-Newton fieldof view iswithinthe
Fig.5.AsinFigure4,butthecirclesarecenteredonthecoordi-
globularclustertidalradius(≃ 77′′)and,therefore,maybeassociated
natesoftheglobularclusterGC3.
with it (as already claimed by Orioetal. 2010). Finally, we note that
for an estimated distance of 0.138 Mpc to the Fornax dSph, the ob-
served 0.2-12 keV band luminosities associated with the sources 28
and 103 are ≃ 2.3×1034 erg s−1 and ≃ 2.6×1035 erg s−1, respec- 3.5.Anintermediate-massblackholeintheFornaxgalaxy:
tively. While the source possibly associated with GC 4 has a lumi- high-energyconstraints
nosity comparable to that of a typical CV, the source possibly resid-
When the galactic globular clusters were identified in X-rays,
ing in GC3has a luminosity well within the typical ranges (although
Bahcall&Ostriker(1975)andSilk&Arons(1976)suggestedthatthe
towards the lower limit) for LMXBs and high-mass X-ray binaries
observedemissionwasduetothepresenceofIMBHsinthemassrange
(HMXBs)(seee.g.Fabbiano&White2006,Kuulkersetal.2006,and
∼102M –105M accretingmaterialfromtheintraclustermedium.
Tauris&vandenHeuvel2006). ⊙ ⊙
The discovery of ultra-luminous compact X-ray sources (ULXs,
withluminositygreaterthan∼ 1039 ergs−1)initiallypushedthecom-
munitytointerpretsuchobjectsasIMBHs.Notehoweverthatthecur-
14 We checked that none of the X-ray sources of our sample has a
rent accepted sceario is that all the ULXs (with the exception of the
counterpartinthecatalogofvariablestars(indicatedasbluecrossesin
highestluminosityobjectsstillcontainanIMBH)arestellar-sizedblack
Figure4)asreportedbyGrecoetal.(2007).
holeswhichaccreteatsuper-Eddingtonrates(Miller&Colbert2003).
15 ASxPheobjectisavariablepulsatingstarwhosemagnitudecan Moreevidencecomesfromthestudyofthecentralvelocitydispersion
varyfromafew0.001magtoseveral0.1magwithtypicalperiodsof of starsinspecificglobular clusters(asG1intheAndromeda galaxy,
P<∼0.10daysandisoftenusedasadistanceestimator.Foradescription seee.g.Gebhardtetal.2002,butalsoPooleyetal.2006,orM15and
onthemainpropertiesofthisclassofvariablestarswereferthereader ωCentauri intheMilkyWay,seee.g. Gerssenet al.2002, 2003,and
toPychetal.(2001)andreferencestherein. Miocchi 2010, respectively ) which may contain central IMBHs. As
7
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
far as M15 is concerned, milli-second pulsar timing studies have al-
lowed us to put an upper limit of ≃ 3×103 M on the IMBH mass
⊙
(DePaolisetal.1996).Finally,compactobjectsofIMBHsizearealso
predictedbyN-bodysimulations(seee.g.PortegiesZwartetal.2004)
asaconsequenceofmergingofmassivestars.Asnotedbyseveralau-
thors(seee.g.Baumgardtetal.2005,andMiocchi2007),photometric
studiesmayprovideafurtherhintfortheexistenceofIMBHsinglob-
ular clusters. It is expected that the mass density profile of a stellar
systemwithacentralIMBHfollowsacuspyρ ∝ r−7/4 law,sothatthe
projecteddensityprofile,aswellasthesurfacebrightness,shouldalso
haveacuspprofilewithslope−3/4.
AsshownbyMiocchi(2007),theglobularclustersthatmostlikely
hostacentralIMBHarethosecharacterizedbyaprojectedphotometry
wellfittedbyaKingprofile,exceptinthecentralpartwhereapower-
law deviation (α ≃ −0.2) from a flat behavior is expected. However,
theerrorswithwhichtheslopesofcentraldensitiescanbedetermined
areapproximately0.1−0.2(Noyola&Gebhardt2006),sothatoptical
surface densityprofilesdonot give clearevidence of theexistenceof
IMBHinglobularclusters.
It isthen natural to expect that observations in different bands of
the electromagnetic spectrum, such as radio and X-ray bands, would
Fig.6.A zoomedviewaroundthe FornaxdSphcenter. Thedi-
permitfurtherconstraintsontheIMBHparameters.Thisissuewascon-
amondsindicatethe globalcentroidsof the galaxyobtainedby
sideredbyseveralauthors.Forexample,Grindlayetal.(2001)provided
thecensusofthecompactobjectsandbinarypopulationsintheglobu- several authors. In particular, the green diamonds (with labels
larcluster47TucandobtainedanupperlimittothecentralIMBHof SHS98 500 pxl and SHS98 2000 pxl) correspond to the galaxy
afewhundredsolarmasses;Nucitaetal.(2008)showedthatthecore centers obtained by Stetsonetal. (1998), the red and black di-
of the globular cluster NGC 6388 harbors several X-ray sources; and amonds indicate the centroids as derived by Hodge&Smith
Csehetal.(2010)refinedtheanalysisputtinganupperlimittothecen- (1974)andDemers&Irwin(1987),respectively(seetextforde-
tralIMBHmassofafewthousandsolarmasses(seealsoBozzoetal. tails).
2011,andNucitaetal.2012).
One expects to find IMBHs in dSph as well (Maccaroneetal.
2005). Inthe specificcase of theFornax dSph, vanWassenhoveetal. sources(labeled2)detectedinouranalysis,thusallowingustoestimate
(2010) assumed that an IMBH of mass MBH ≃ 105 M⊙ exists in the theX-rayluminosityfromaputativeIMBH.Inalltheothercases,the
galactic core and suggested that measuring the dispersion velocity of centroidsfallinaregionapparentlyfreeofsources,sothatonlyanup-
thestarswithin30 pcfrom thecenter would allow ustotest that hy- perlimitontheIMBHfluxcanbeobtained.Asareferenceforthiscase
pothesis. Furthermore, Jardel&Gebhardt (2012) recently constructed weuseSHS98500pxlasthepositionofthecentroid.
axisymmetricSchwarzschildmodelsinordertoestimatethemasspro- Inthefollowingparagraphs wespeculate about thepossibleexis-
file of the Fornax dSph. These models were tested versus the avail- tenceofanIMBHintheFornaxdSphandwhetheritisintheposition
able kinematic data allowing the authors to put a 1-σ upper limit of labeledSHS982000pxlorcoincidentwiththecentroidSHS98500pxl.
MBH =3.2×104M⊙ontheIMBHmass.Wewillusethelattervaluein As in the case of the G1 (Pooley&Rappaport, 2006) and M15
thefollowingdiscussions. (Hoetal.,2003)globularclusters,theX-rayemissionfromtheputative
Since any Brownian motion of IMBH at the center of the galaxy IMBHatthecenteroftheFornaxdSphmaybeduetoBondiaccretion
isnegligible16,wesearchedforX-raysourcesclosetotheFornaxcen- (Bondi&Hoyle1994)ontotheblackhole,eitherfromtheclustergas
ter.Thissearchwasmadedifficultowingtotheuncertaintywithwhich orfromstellarwinds.Thus,assumingthatlow-angularmomentumgas
the Fornax dSph center position is known. This is caused mainly by closetothecompactobjectaccretesspherically,forablackholeofmass
thelargeasymmetryinthegalaxysurfacebrightnessasfirstobserved M movingwithvelocityvthroughagaseousmediumwithhydrogen
BH
byHodge(1961)(seealsoHodge&Smith1974).Theasymmetrywas numberdensityn,theaccretionrateis
alsoconfirmedbyStetsonetal.(1998)whoevaluatedtheglobalgalaxy
M˙ ≃4π(GM )2(v2+c2)−3/2m n, (3)
centroidusingacirclemovingontheskyplaneuntilthemedian(x,y) BH s p
positionofallthestarscontainedwithintheaperturecoincidedwiththe
wherem andc aretheprotonmassandsoundspeedinthemedium,
centerofthecircle.Dependingonthecircleradius(500or2000pixels), p s
respectively.TheexpectedX-rayluminosityisthen
Stetsonetal.(1998)obtainedtwocenterpositions(separatedby≃ 3′)
indicatedbythegreendiamonds(withlabelsSHS98500pxlandSHS98 L ≃ǫηM˙c2, (4)
X
2000pxl)inFigure6.Inthesamefigure,wealsogivethecoordinatesof
thecentroidasobtainedbyHodge&Smith(1974)(reddiamond)and whichcanbeparametrizedas
Demers&Irwin(1987) (black diamond). For comparison, the yellow
diamond (on aCCDgapof thepncamera) representsthecentroid as L ≃ǫη8.8×1036 MBH 2 V −3 n ergs−1cm−2,(5)
recentlydeterminedbyBattagliaetal.(2006)andthebluediamondis X 103M⊙! (cid:18)15kms−1(cid:19) (cid:18)0.1cm−3(cid:19)
centeredontheNEDcoordinatesofFornaxdSph.
whereV =(v2+c2)1/2,ǫistheefficiencyinconvertingmasstoradiant
Note that the centroid labeled SHS98 2000 pxl, as estimated by s
energyandηisthefractionoftheBondi-Hoyleaccretionrateontothe
Stetsonetal.(1998)whenconsideringthelargestructureofthegalaxy,
is at a distance of ≃ 28′′ from the centroid position according to blackhole.
Thehydrogen number density of themassfeeding theblack hole
Battagliaetal.(2006)andisapparentlyveryclosetooneoftheX-ray
canbeestimatedbyusingthestructuralparametersoftheFornaxdSph
as given in McConnachie (2002). In particular, it was found that the
16 Fromsimulations,oneexpectsanIMBHwithinaglobularcluster
galaxyhostsatleast M ≃ 0.17×106 M ofgaswithintheobserved
(or a spheroidal galaxy) tomove randomly when interacting withthe HI ⊙
half-lightradiusofr ≃ 710pc.Thus,alowerlimittothegasdensity
surrounding stars. In the assumption that all the stars have the same h
canbeevaluatedas
mass m, the IMBH moves with an amplitude ∼ r (m/M ) (see e.g.
c BH
Bahcall&Wolf1976,Gurzadyan1982,andMerrittetal.2006)where n≃ 3MHI ≃5×10−3cm−3. (6)
rcisthecoreradiusandMBH theblackholemass. 4πrh3mp
8
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
Assumingthatv≃c ≃10kms−1,onehasV ≃14−15kms−1.Thus, where M˙ istheaccretionrateandr istheradiusoftheinneredgeof
s I
usingtheIMBHupperlimitofM =3.2×104M quotedabove,Eq. the disk. For a disk that extends inward to the last stable orbit (r =
BH ⊙ I
5finallygivestheexpectedX-rayluminosity,i.e., 6GM /c2 for a non-rotating black hole), it is easy to verify that the
BH
efficiencyinconvertingtheaccretingmasstoradiantenergyis≃8.3%.
L ≃ǫη4.50×1038ergs−1. (7) When comparing the observed X-ray luminosity in the 0.2−12 keV
X
bandofsource2withthevalueexpectedinthethindiskscenario(Eq.
For an estimated distance of 0.138 Mpc to the Fornax dSph, the ex- 8), we can estimate a mass accretion rate of M˙ ≃ 1.87×1014 g s−1.
pectedIMBHluminosityLXcorrespondstoanobservablefluxofFX ≃ Assumingagaintheradiationefficiencytobe8.3%,ablackholeofmass
ǫη1.98×10−10ergs−1cm−2. 3.2×104M hasanEddingtonmassaccretionrateofM˙ ≃5.5×1022
⊙ Edd
AsonecanseefromacloseinspectionofFigure6,inthecasewhen g s−1. Thus, the ratio M˙/M˙ ≃ 4×10−9 implies that the IMBH in
Edd
theputativeIMBHpositioncoincideswiththecentroidSHS98500pxl FornaxdSph(ifany)accretesveryinefficientlyeveninthecontextofa
17,therewasnocleardetectionofX-raycounterpartsinthe0.2−12keV Keplerianthindiskmodel.
band.Tobeconservative, theexpectedfluxcanbecomparedwiththe Theaboveconsiderations,theobservedlowluminosity,andthees-
minimum(unabsorbed)flux(≃2.5×10−15ergs−1cm−2,seeSection2.4 timated power-law index Γ = 1.3+1.0 (marginally consistent with an
−0.8
forfurtherdetails)detectableintheXMM-Newtonobservation.Hence, indexintherange 1.4-2.1, seee.g.Remillard&McClintock2006 for
oneeasilyconstrainstheaccretionefficiencyoftheIMBHtobeǫη ≤ aclassificationofblackholebinaries)allowustodepictascenarioin
1.3×10−5. whichtheFornaxdSphIMBHmaybeinaquiescentstate.
Onthecontrary,adetectionappears(sourcelabeled2)onthecen- Recently, it has also been proposed that a relationship between
troidSHS98 2000 pxl position. Inthis case, using the fluxvalues ob- blackholemass,X-rayluminosity,andradioluminositydoesexist(see
tained by the automatic procedure described in Section 2, we found e.g.Merlonietal.2003andKoerdingetal.2006a).Thisfundamental
thatsource2hasanunabsorbedflux(obtainedviawebPIMMS)inthe plane can be used to test the IMBH hypothesis in globular clusters
0.2−12keVof≃ 3.0×10−15 ergs−1 cm−2which,attheFornaxdSph and dwarf spheroidal galaxies (Maccaroneetal. 2005). In particular,
distance,correspondstoanintrinsicluminosityofL =7.0×1033 erg Maccarone(2004)scaledthefundamental-planerelationtovaluesap-
X
s−1.Thus,intheIMBHhypothesis,theaccretionefficiencyturnsoutto propriateforanIMBHhostinaGalacticglobularcluster;byrescaling
beǫη≃1.6×10−5. theirequationtoestimatetheexpectedIMBHradiofluxat5GHz,i.e.,
With a more detailed analysis, we further extracted the MOS 1,
L 0.6 M 0.78 10kpc 2
MOS 2, and pn spectra of source 2 using a circular region centered F =10 X BH µJy, (9)
onthetargetcoordinatesandwithradiusof35′′.Wethenaccumulated 5GHz 3×1031cgs! 100M⊙! d !
thebackgroundonaregionlocatedonthesamechipandinaposition which,fortheaboveX-rayfluxestimates,correspondsto
apparently free of sources. This resulted in MOS 1, MOS 2, and pn
spectrawith≃230,≃250,and≃480counts,respectively. F ≃0.1 ǫ 0.6 mJy. (10)
Aftergroupingthedataby25counts/bin,weimportedtheresulting 5GHz (cid:18)10−5(cid:19)
background-reducedspectrumwithinXSPEC.Amodelconsistingofan For an accretion efficiency as low as a few 10−5, the expected radio
absorbedpower-law,withhydrogencolumndensityfixedtotheaverage fluxiswellwithinthedetectionpossibilitiesoftheAustraliaTelescope
value observed towards the target (2.7×1020 cm−2, see Section 2 for CompactArray(ATCA)which,foranintegrationtimeof≃12hr,may
furtherdetails),resultedinthebestfitparametersΓ = 1.3−+01..80 andN = reachanRMSsensitivityof≃10µJy/beamat5GHz.
(5.0±4.0)×10−7 kev−1 cm−2 s−1 with χ2 = 1.4 (for 34 d.o.f.). The
ν
0.2-12keVbandabsorbedfluxofsource2isthen(0.6+1.7)×10−14erg
−0.4 4. Conclusions
s−1 cm−2 (90%confidence level)whichcorresponds toanunabsorbed
flux of ≃ 6.2×10−15 erg s−1 cm−2 and an unabsorbed luminosity of Inthispaperwere-analyzedadeeparchiveXMM-Newtondataofthe
LX ≃1.4×1034ergs−1.Assumingasphericalaccretion,onefinallyhas FornaxdSphgalaxywiththeaimofcharacterizingtheX-raypoint-like
anaccretionefficiencyofǫη≃3×10−5. sourcepopulation.Byusingarestrictiveanalysis,wedetected107X-
Notealsothatfor η = 1, theradiativeefficiencyǫ of theputative raysources.Mostofthemarelikelytobebackgroundobjectssincethe
FornaxdSphIMBHissimilartothevaluesfoundforothersystemsin numberofdetectedsourcesisstatisticallyconsistentwiththatexpected
theradiativelyinefficientregime(10−5−1,seeBaganoffetal.2003,but from the logN-logS relation. However, we cannot exclude that a few
also Fenderetal. 2003, and Koerdingetal. 2006a,b). The same con- ofthedetectedobjectsbelongtotheFornaxdSph.Thecolor-colordi-
clusioncanbereachedwhencomparingtheobservedX-rayluminosity agram(basedontheratiobetweenthe0.2−2.4keVflux(F )andthe
inthe 0.2−12 keV band of source 2 (LX ≃ 1.4×1034 erg s−1) with NIRfluxinJband(FJ)versustheJ-Kcolor)fortheX-raysouXrceswith
theexpectedEddingtonluminosity(i.e., LEdd ≃ 1.3×1038(MBH/M⊙) acounterpartinthe2MASScatalogshowsthepresenceofafewobjects
ergs−1) intheIMBHhypothesis. Inthiscase, LX/LEdd ≃ 3×10−9 is (seebottom-leftpartofFigure3)possiblyofstellarnatureandlocalori-
obtained, thusimplying that theIMBH must beextremely radiatively gin.Furthermore,sourcenumber61appearstobespatiallycoincident
inefficient (for a comparison see the M15 case discussed in Hoetal. (within≃2′′)withalong-periodvariablestarfoundinthecatalogcom-
2003). Accretion onto a black hole may be different from a simple piledbyBersier&Wood(2002).
spherical model, since the accreting gas has angular momentum. In Asdiscussedpreviously,amongalltheotherdwarfspheroidalsatel-
such a case a Keplerian disk forms and the accretion occurs due to litesoftheMilkyWay,apeculiarcharacteristicoftheFornaxdSphis
the presence of viscous torques, which transport the angular momen- thatofhostingfiveglobularclusters.WenotedthattwoX-raysources,
tum outward from the inner to the outer regions of the disk (see e.g. detectedtowardsthegalaxywithouranalysis,arecoincidentwiththe
Shapiro&Teukolsky1983).Inthesimplifiedcaseofathindiskstruc- twoFornaxglobularclustersGC3andGC4.Inparticular,forGC4we
ture (see Shakura&Sunyaev 1973), the total luminosity of the disk identifiedoneX-raysource(labeled28inthesourcelistappearingin
(mainlyoriginatinginitsinnermostparts)is Table1)whichisatadistanceof≃37′′ fromtheglobularclustercen-
ter.Consequently,consideringtheGC4structuralparametersreported
L≃ 1GMBHM˙ , (8) inTable5,oneobserves that source28 iswellwithinthetidal radius
2 r (≃ 43′′)oftheglobularclusterand,hence,possiblyassociatedwithit
I
(asalreadyclaimedbyOrioetal.2010).InthecaseofGC3,weidenti-
17 InthecaseinwhichtheFornaxcentroidpositioncoincideswithone fiedoneX-raysource(labeled103atadistanceof≃72′′ fromtheGC
ofthoseevaluatedbyHodge&Smith(1974),Demers&Irwin(1987), center)thatmightbeattheoutskirtsoftheglobularcluster.
orBattagliaetal.(2006)(red,black,andyellowdiamondsinFigure6, Finally,wediscussedtheIMBHhypothesisandfoundthatoneof
respectively),wecansetonlyanupperlimittotheIMBHunabsorbed theX-raysources(labeled2)mightbeassociatedwithoneofthepos-
fluxof≃2.5×10−15 ergs−1cm−2which,atthedistanceoftheFornax siblegalaxycentroidsidentifiedbyStetsonetal.(1998).Inthisframe-
dSph,correspondstoaluminositylimitof≃5.7×1033ergs−1. work, we estimated the IMBH accretion parameter to be ǫη ≃ 10−5.
9
A.A.Nucitaetal.:TheXMM-NewtonviewoftheFornaxdSph
However, sincethere isalargeuncertaintyintheidentificationof the Mateo,M.,1998b,ARA&A,36,435
galaxy’scenterofgravity,thelattervaluecanbeconsideredasanupper McConnachie,A.W.,2012,AJ,144,1
limittotheIMBHaccretionparameters. Mendez,R.,etal.,2011,AJ,143,93
Merloni,A.,Heinz,S.,DiMatteo,T.,2003,MNRAS,345,1057
Acknowledgements. We are grateful to F. Strafella, V. Orofino, and B.M.T. Merritt,D.,Berczik,P.,Laun,F.,Astron.J.133,533
Maiolofortheinterestingdiscussionswhilepreparingthemanuscript.Wealso Miller,M.C.,&Colbert,E.,2003,IJMPD,13,1
acknowledgetheanonymousRefereeforhissuggestions. Miocchi,P.,2007,MNRAS,381,10
Miocchi,P.,2010,A&A,514,52
Monet,D.,G.,etal.,2003,AJ,125,984
References Noyola,E.,&Gebhardt,K.,2006,AJ,132,447
Nucita,A.A.,etal.,2008,A&A,478,763
Arnaud,K.A.1996,XSPEC:TheFirstTenYears,inAstronomicalDataAnalysis Nucita,A.A.,DePaolis,F.,Saxton,S.,Read,A.M.,2012,NewAstronomy,17,
SoftwareandSystemsV,ed.G.Jacoby,J.Barnes,ASPConf.Ser.,101,17 589
(SanFrancisco:ASP) Orio,M.,etal.,2010,AIPConferenceProceedings,1314,337
Ashman,K.M.,Zepf,S.E.,1998,GlobularClusterSystems,CambridgeUniv. Pooley,D.,&Rappaport,S.,2006,ApJ,644,L45
Press,Cambridge Poretti,E.,etal.,2008,ApJ,685,947
Baganoff,F.K.,etal.,2003,ApJ,591,891 PortegiesZwart,S.F.,etal.,2004,Nature,428,724
Bahcall,J.N.,&Ostriker,J.P.,1975,Nature,256,23 Prestwich,A.,etal.,2003,ApJ,595,719
Bahcall,J.N.,&Wolf,R.A.,1976,ApJ,209,214 Pych,W.,etal.2001,A&A,367
Battaglia,G.,etal.,2006,A&A,459,423 Ramsay,G.,&Wu,K.,2006,A&A,2006,459,777
Baumgardt,H.,etal.,2005,ApJ,589,25 Reines,A.E.,Sivakoff,G.R.,Johnson,K.E.,Brogan,C.L.,2011,Nature,470,66
Bersier,D.,&Wood,P.R.,2002,AJ,123,840 Remillard,R.A.,&McClintock,J.E.,2006,ARA&A,44,49
Bondi,H.,Hoyle,F.,1944,MNRAS,104,273 Roeser,S.,etal.,2008,A&A,488,401
Bozzo,E.,etal.,2011,A&A,535,1 Salucci,P.,etal.,2012,MNRAS420,2034S
Cole,D.R.,Dehnen,W.,Read,J.I.,Wilkinson,M.I.,2012,arXiv:1205.6327 Saxton,R.,2003,XMM-SOC-CAL-TN-0023,version2.0
Cseh,D.,etal.,2010,MNRAS,406,1049 Scargle,J.,D.,1982,ApJ,263,835
Demers,S.,Irwin,M.J.,1987,MNRAS,226,943 Servillat,M.,etal.,2011,ApJ,743,6
DePaolis,F.,Gurzadyan,V.G.,Ingrosso,G.,1996,A&A315,396 Shakura,N.I.,&Sunyaev,R.A.,1973,A&A,24,337.
Ebisuzaki,T.,etal.,2001,ApJ,562,L19 Shapiro,S.L.,&Teukolsky,S.A.,1983,BlackHoles,WhiteDwarfsandNeutron
Fabbiano, G., & White, N.E., 2006, in Compact Stellar X-ray Sources, Eds. Stars:thephysicsofcompactobjects,Eds.JohnWiley&Sons,NewYork
W.H.G. Lewin & M. van der Klis, pag. 475, Cambridge Univ. Press, Silk,J.,&Arons,J.,1975,ApJ,200,L131
Cambridge Silk,J.,Wyse,R.,Shields,G.A.,1987,ApJ322,59
Farrell,S.A.,etal.,2012,ApJ,747,L13 Skrutskie,M.F.,etal.,2006,AJ,131,1163
Farrell,S.A.,Webb,N.A.,Barret,D.,Godet,O.,Rodrigues,J.M.,2009,Nature, Soria,R.,&Wu,K.,2003,A&A,2003,410,56
460,73 Stetson,P.B.,Hesser,J.E.,Smecker-Hane,T.A.,1998,PASP,110,533
Fender,R.P.,Gallo,E.,Jonker,P.G.,2003,MNRAS,343,99 Stru¨der,L.,Briel,U.,Dennerl,K.,etal.2001,A&A,365,L18
Gebhardt,K.,Rich,R.M.,Ho,L.C.,2002,ApJ,578,L41 Stuhlinger,M.,etal.,2006,XMM-SOC-CAL-TN-0052,version3.0
Gerssen,J.,etal.,2002,AJ,124,3720 Tauris,T.M.,&vandenHeuvel,E.P.J.,2006,inCompactStellarX-raySources,
Gerssen,J.,etal.,2003,AJ,125,376 Eds.W.H.G.Lewin &M.van der Klis, pag. 623, Cambridge Univ. Press,
Gizis,J.E.,Mould,J.R.,Djorgovski,S.,1993,PASP,105,871 Cambridge
Goerdt,T.,etal.,2006,MNRAS368,1073 Tinney,C.G.,1999,MNRAS,303,565
Greco,C.,etal.,2007,ApJ,670,332 Tinney,C.G.,DaCosta,G.S.,&Zinnecker,H.,1997,MNRAS,285,111
Grindlay,J.E.,etal.,2001,Science,292,2290 Turner,M.J.L.,Abbey,A.,Arnaud,M.,etal.2001,A&A,365,L27
Guainazzi, M., 2010, XMM-SOC-CAL-TN-0018, van der Marel R. P., 2004, Coevolution of Black Holes and Galaxies, from
XMM-Newton Science Operations Centres, theCarnegie Observatories Centennial Symposia.PublishedbyCambridge
http://xmm2.esac.esa.int/docs/documents/CAL-TN-0018.ps.gz UniversityPress,aspartoftheCarnegieObservatoriesAstrophysicsSeries.
Gurzadyan,V.G.,1982,A&A,114,71 EditedbyL.C.Ho,37
Haakonsen,C.B.,&Rutledge,R.E.,2009,ApJSS,184,138 vanWassenhove, S.,Volonteri, M.,Walker, M.G.,Gair, J.R.,2010,MNRAS
Hasinger,G.,Miyaji,T.,Schmidt,M.,2005,A&A,441,417 408,1139
Ho,L.C.,Terashima,Y.,&Okajima,T.,2003,ApJ,587,35 Veron-Cetty,M.P.,&Veron,P.,2006,A&A,455,773
Hodge,P.W.,1961,AJ,66,249 Watson,M.,etal.,2009,A&A,493,339
Hodge,P.W.,&Smith,D.W.,1974,ApJ,188,19 Webb,N.,etal.,2012,Science,337,554
Jardel,J.R.,&Gebhardt,K.,2012,ApJ,746,89 West,M.J.,Coˆte´,P.,Marzke,R.O.,&Jordan,A.,2004,Nature,427,31
Jones,D.H.,Peterson,B.A.,Colless,M.,&Saunders,W.,2006,MNRAS369, XRPSUser’sManual,2008,Issue2.6,(Eds.EhleM.etal.)
25
Kalberla,P.M.W.,Burton,W.B.,Hartmann,D.,etal.2005,A&A,440,775
Kirsch,M.G.F.,etal.2004,inProc.SPIE,5488,103
Kleyna,J.T.,Wilkinson,M.I.,EvansN.W.,GilmoreG.,2001,ApJ,563,L115
Kleyna,J.T.,Wilkinson,M.I.,Evans,N.W.,Gilmore,G.,2002,MNRAS,330,
792
Koerding,E.,Falcke,H.,Corbel,S.,2006,A&A,456,439
Koerding,E.G.,Fender,R.P.,Migliari,S.,2006,A&A,456,439
Kormendy,J.,1985,ApJ,295,73
Kuulkers,E.,Norton,A.,Schwope,A.,&Warner,B.,2006,inCompactStellar
X-raySources,Eds.W.H.G.Lewin&M.vanderKlis,pag.421,Cambridge
Univ.Press,Cambridge
Maccarone,T.J.,2004,MNRAS,351,1049
Maccarone,T.J.,2005,MNRAS,364,L61
Maccarone,T.J.,etal.,2005,AstrophysicsandSpaceScience,300,239
Mackey,A.D.&Gilmore,G.F.,2003,MNRAS,340,175
Magorrian,J.,etal.,1998,AJ,115,2285
Martin,N.F,deJong,J.T.A.,Rix,H.-W,2008,ApJ,684,1075
Mashchenko,S.,Sills,A.,Couchman,H.M.,2006,ApJ,252,269
Mateo,M.,1997,inASPConf.Ser.Vol.116,TheNatureofEllipticalGalaxies,
Astron.Soc.Pac.,SanFrancisco,(Eds.,ArnaboldiM.etal.)
Mateo, M., 1998a, in Proc. Bonn/Bochum-Graduiertenkolleg Workshop, The
Magellanic CloudsandOtherDwarfGalaxies.ShakerVerlag,Aachen,53,
(Eds.Richtler,T.&Braun,J.M.)
10