Identifying a black hole X-ray transient in M31 with XMM-New

Stochastic variability in two out of four XMM-Newton observations of XMMU J004303+4115 along with its power spectra and X-ray luminosities suggest a low-mass X-ray binary (LMXB) with a black hole primary. However, Chandra observations resolve the object in

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002 yaM 6 1v6015040/hp-ortsa:viXraAstronomy&Astrophysicsmanuscriptno.aa20030661February2,2008

(DOI:willbeinsertedbyhandlater)

IdentifyingablackholeX-raytransientinM31with

XMM-NewtonandChandra

R.Barnard1,U.Kolb1,andJ.P.Osborne2

1

TheDepartmentofPhysicsandAstronomy,TheOpenUniversity,WaltonHall,MiltonKeynes,MK76BT,U.K.

2

TheDepartmentofPhysicsandAstronomy,TheUniversityofLeicester,Leicester,LE17RH,U.K.

Abstract.StochasticvariabilityintwooutoffourXMM-NewtonobservationsofXMMUJ004303+4115alongwithitspowerspectraandX-rayluminositiessuggestalow-massX-raybinary(LMXB)withablackholeprimary.However,Chandraobservationsresolvetheobjectintotwopointsources.Weusedatafrom35Chandraobservationstoanalysethecontributionsofeachsource,andattributethevariabilitytoCXOM31J004303.2+411528(knownasr2-3),whichvariesinintensitybyafactorof～100betweenobservations.WeassumethatthepowerdensityspectraofLMXBsaregovernedbytheluminosity,andthatthetransitionbetweentypesofpowerdensityspectraoccursatsomecriticalluminosityinEddingtonunits,lc,thatappliestoallLMXBs.WeuseresultsfromtheseXMM-Newtonobservationsandpastresultsfromtheavailableliteraturetoestimatethistransitionluminosity,and ndthatallresultsareconsistentwithlc～0.1inthe0.3–10keVband.CXOM31J004303.2+411528exhibitsalowaccretionratepowerdensityspectrumata0.3–10keVluminosityof5.3±0.6×1037ergs 1.Knownstellarmassblackholeshavemassesof4–15M⊙;henceourobservationsofCXOM31J004303.2+411528areconsistentwithlc～0.1ifithasablackholeprimary.

Keywords.X-rays:general–Galaxies:inpidual:M31–X-rays:binaries

1.Introduction

TheAndromedaGalaxy(M31)isanattractiveandimportanttargetforX-rayastronomy,sinceitisthenearestspiralgalaxy(760kpc,vandenBergh2000),anditsX-rayemissionisdominatedbypointsources.ThesepointsourcesarethoughttobemostlyX-raybinaries,alongwithafewforegroundobjects,back-groundactivegalacticnuclei(AGN),andsupernovaremnants(SNR).ThetwomostrecentX-rayobserva-tories,ChandraandXMM-Newton,have nallyal-lowedstudiesofvariabilityinextra-galacticX-raysourcesovertime-scalesofafewhundredseconds,aswellasbetweensuccessiveobservations.Suchshort-termtimevariabilityisoftencharacteristicofwell-studiedphenomenaandsometimesallowsclassi ca-tionoftheobjectsfromX-rayobservationsalone,inconjunctionwiththeirX-rayspectralproperties;forexample,thermonuclearX-raybursts(Lewinetal.1995)identifyanX-raysourceasanX-raybinarywithaneutronstarprimary.Todate,analysisoftheXMM-NewtonobservationsofthecoreofM31hasre-sultedinthediscoveryofapulsatingsupersoftsourcewithaperiodof865seconds(Osborneetal.2001),the

Stochastic variability in two out of four XMM-Newton observations of XMMU J004303+4115 along with its power spectra and X-ray luminosities suggest a low-mass X-ray binary (LMXB) with a black hole primary. However, Chandra observations resolve the object in

2R.Barnardetal.:BlackholetransientinM31

Table1.JournalofXMM-NewtonobservationsoftheM31core

x1x2x3x425/06/0027/12/0029/06/0106/01/02(rev0100)(rev0193)(rev0285)(rev0381)5172051906520895228034135661ksksksksMediumMediumMediumThin

L,

(1)

Edd

whereLcistheluminosityoftransitionbetweenTypeAandTypeBPDS,andLEddistheEddingtonlumi-nosity.

InSect.3weanalyselongtermlightcurvesofr2-3andr2-4from35Chandraobservations,payingpar-ticularattentiontothosethatweremadewithin30daysofoneoftheXMM-Newtonobservations.IfwecanassociateTypeAPDSwitheithersourcewhenitsluminosity,L,exceedsLcforaneutronstar,wecanestablishthattheprimaryisablackhole.Themostlikelycandidateforsuchblackholebehaviourisr2-

3,sincemostblackholebinariesaretransients(e.g.McClintock&Remillard2003),andglobularclusterX-raysourcesmostlycontain1.4M⊙neutronstars(seeHeinkeetal.2003,andreferenceswithin).WepresentinSect.3evidencethatr2-3containsablackholeprimary.

InSect.4we rstestablishthatr2-3islocatedinM31.Wethenobtainanempiricalvalueforlc,usingresultsfromtheseXMM-NewtonobservationsofglobularclustersourcesinM31andpublishedresultsfromanalysisofaGalacticneutronstarLMXBandaGalacticblackholeLMXB.WethenuseourvalueoflctocalculateLcforaneutronstarwithamassof3.1M⊙,thetheoreticalmaximum(Krishan&Kumar1978).Weassertthatr2-3exhibitsaTypeAPDSat

aluminositythatexceedsthislimitandconcludethattheprimaryinr2-3isablackhole,consistentwithitstransientbehaviour.

2.Observations

FourXMM-Newton(Jansenetal.2001)observationsweremadeofthecoreofM31;detailsoftheobserva-tionsaregiveninTable1.Foreachobservation,0.3–10keVlightcurveswereextractedfromsourceandbackgroundregionsforeachofthethreeEPICin-struments:onePNcamera(Str¨uderetal.2001)andtwoMOScameras(Turneretal.2001).Thesourceregionwascircularwitharadiusof20′′andcen-tredonXMMUJ004303+4115whilethebackgroundregionwasanequivalentsource-freeregiononthesameCCDandatasimilaro setfromtheopticalaxis.Thelightcurveswereaccumulatedwith2.6sec-ondbinning.Background-subtracted,summedEPIClightcurves,werethenproducedfollowingtheproce-Fig.1.AdetailoftheACIS-Iimagefromthe2000July29ChandraobservationofthecoreofM31;northisup,eastisleft.Theimageislogscaledand2′across.Thelargeblackcircleiscentredbetweenr2-3(thenorthernsource)andr2-4(thesouthernsource)andhasa20′′radius,indicatingtheextractionregionusedintheXMM-Newtonobservationsx1–x4.Thesmallercircleatthebottomlefthasaradiusof3′′;extrac-tionregionsofthissizewereusedforr2-3andr2-4inobservationsc1–c35.

Stochastic variability in two out of four XMM-Newton observations of XMMU J004303+4115 along with its power spectra and X-ray luminosities suggest a low-mass X-ray binary (LMXB) with a black hole primary. However, Chandra observations resolve the object in

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