XMM-Newton and Chandra Observations of Abell 2626

20 Pages·2008·1.51 MB·English

TheAstrophysicalJournal,682:155Y174,2008July20 A
#2008.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A.
XMM-NEWTON AND CHANDRA OBSERVATIONS OFABELL 2626: INTERACTING RADIO JETS
AND COOLING CORE WITH JET PRECESSION?
Ka-Wah Wong,1 Craig L. Sarazin,1 Elizabeth L. Blanton,2 and Thomas H. Reiprich3
Received2007June30;accepted2008March11
ABSTRACT
WepresentadetailedanalysisoftheXMM-NewtonandChandraobservationsofAbell2626focusedontheX-ray
andradiointeractions.Withintheregionof theradiominihalo((cid:1)70kpc),therearesubstructuresthatareprobably
producedbythecentralradiosourceandthecoolingcore.Wefindthatthereisnoobviouscorrelationbetweenthe
radiobarsandtheX-rayimage.ThemorphologyofAbell2626ismorecomplexthanthatofthestandardX-rayradio
bubblesseeninothercoolcoreclusters.Thus,Abell2626providesachallengetomodelsforthecoolingflowYradio
sourceinteraction.WeidentifiedtwosoftX-ray(0.3Y2keV)peakswiththetwocentralcDnuclei;oneofthemhasan
associatedhardX-ray(2Y10keV)pointsource.Wesuggestthatthetwosymmetricradiobarscanbeexplainedby
twoprecessingjetsejected fromanAGN.Beyondthecentralregions,we findtwoextendedX-ray sourcestothe
southwestandnortheastoftheclustercenterthatareapparentlyassociatedwithmergingsubclusters.ThemainAbell
2626clusterandthesetwosubclustersareextendedalongthedirectionof thePerseus-Pegasussupercluster,andwe
suggestthatAbell2626ispreferentiallyaccretingsubclustersandgroupsfromthislarge-scalestructurefilament.We
alsofindanextendedX-raysourceassociatedwiththeclusterS0galaxyIC5337;themorphologyofthissourcesug-
geststhatitisinfallingfromthewestandisnotassociatedwiththesouthwestsubcluster,ashadbeenpreviously
suggested.
Subject headingsg: cooling flows — galaxies: clusters: general — galaxies: clusters: individual (Abell 2626) —
intergalactic medium — radio continuum: galaxies — X-rays: galaxies: clusters
Online material: color figures
1. INTRODUCTION 2005).Theyarebelievedtobepreviouseruptionsof theradio
sources, which are episodic. In some cases, buoyantly rising
Thecentralregionsofclustersofgalaxiesareamongthemost
bubbles may entrain cooler X-ray gas from the centers of the
interestingandphysicallyactiveareasintheuniverse.Inmany
coolingcores(Churazovetal.2001;Fujitaetal.2002,2004).
clustersofgalaxies,thereisacentralpeakinX-raysurfacebright-
Someradiosourcespreviouslyclassifiedasclustermergerradio
ness,withacentralcoolingtimethatislessthantheageof the
relicsmayactuallybedisplacedradiobubblesfromthecentral
cluster(e.g.,Chenetal.2007).Intheclassicalcoolingflowpic-
radiosources(Fujitaetal.2002,2004).Insomerecentstudies,
ture,therapidcoolingofgasinthisregionleadstothelossof
thereisevenevidenceofradiojetsrestartedatadifferentposition
centralpressure(Fabian1994).If thereisnoadditionalheating,
anglefromapreviousoutburst,whichmaybeduetoaprecessing
thecooledgaswouldflowintothecenterinacoolingflow.The
supermassiveblackhole(Gittietal.2006).Withhigh-resolution
gasshouldcontinuetocooltoverylowtemperatures.However,re-
three-dimensionalsimulations,Vernaleo&Reynolds(2006)have
centhigh-resolutionX-rayspectrafromXMM-NewtonandChandra
shownthatmultipleburstsfromtheactivegalacticnucleus(AGN)
showthatthereisnotenoughcoolerX-raygastobeconsistent
alongthesamedirectionfailtoproducealong-termbalanceof
withaclassicalcoolingflow(Petersonetal.2001,2003;Peterson
heatingandcooling.Thereasonisthatalow-densitychannelis
&Fabian2006).
evacuatedbyapreviousjet,whilethesubsequentjetscanpass
The lack of sufficient cooler materials probably means that
throughthechannelfreelywithoutheatingthecoolingcorere-
thereisanadditionalheatingsourcewithinthecentralcooling
gion.Ithasbeensuggestedthatjetprecessionisapossiblesolu-
core region. Very often, the clusters host central dominant gal-
tiontothisproblem.Alternatively,turbulence,rotation,orbulk
axiesthatarealsostrongradiosources.Insomeoftheclusters,
motionsintheintraclustermedium(ICM)mightalsomoveor
thereisstrongevidencethatthecentralradiosourcesareinteract-
ingwiththesurroundingX-rayYemittingplasma.Theradiosources disruptthechannelmadebytheradiosourceandkeeptheenergy
blow ‘‘bubbles’’ in the X-rayYemitting gas, displacing the gas fromjustgoingdownthesamechannel(e.g.,Heinzetal.2006).
Abell2626(z¼0:0573;Abelletal.1989)hostsamoderate
andcompressingitintoshellsaroundtheradiolobes.Atthesame
time,theradiosourcesareconfinedbytheX-rayYemittinggas coolingflowof(cid:1)50M(cid:2)yr(cid:3)1basedonEinsteindata(Whiteetal.
1997).AdoublenucleicDgalaxyissittingatthecenterofthe
(Fabianetal.2000;Blantonetal.2001).Insomeclusters,‘‘ghost
cluster.Fromhigh-resolutionVLAimages,Gittietal.(2004)rec-
bubbles’’atlargerradiiareseenthatareweakinradioemission
ognizedthatthereisanunresolvedradiocorecoincidentwiththe
exceptatlowfrequencies(McNamaraetal.2001;Clarkeetal.
centralcDgalaxy.Thereisasmalljetlikefeatureextendedtoward
thesouthwestdirectionfromtheunresolvedradiocore.Inaddi-
1 Department of Astronomy, University of Virginia, Charlottesville, VA tion,therearetwounusualamorphous,symmetricradio‘‘bars’’
22904-4325;[email protected],[email protected]. runningparallelatoppositesidesofthecentralcDgalaxy.These
2 InstituteforAstrophysicalResearch,BostonUniversity,Boston,MA02215;
compactfeaturesaredistinctfromandembeddedindiffuseex-
[email protected].
3 Argelander-Institut fu¨r Astronomie, Universita¨t Bonn, D-53121 Bonn, tendedradioemission(aminihalo).Gittietal.(2004)believed
Germany;[email protected]. thattheradiobarsmayrepresentanearlierevolutionarystage
155
156 WONG ETAL. Vol. 682
of jetsinjectedbythecentralsource.Thus,Abell2626isagood observationstoderiveaccuratelythepropertiesoftheICM(e.g.,
candidateforstudyingtheinteractionbetweentheX-rayYemitting coolingradius,massaccretionrate,aswellasmorphologyofthe
ICMandtheradio-emittingplasma(Rizzaetal.2000;Gittietal. X-rayemission).Ontheotherhand,Pfrommer&Ensslin(2004)
2004;Markovic´ etal.2004).PreviousROSATX-rayandVLA suggested that the reacceleration is of hadronic origin, which
radioobservationssuggestedthatthereisanX-rayexcessthatis shouldbebesttestedby(cid:1)-rayobservations.
spatiallycorrelatedwiththeradiosource,buttheyfailedtofind InthispaperwepresentadetailedstudyofAbell2626from
strongX-raydeficits(holes)inAbell2626(Rizzaetal.2000). XMM-NewtonandChandraobservationsfocusedontheX-ray
Rizzaetal.(2000)carriedoutnumericalsimulationsandshowed andradiointeraction.Thepaperisorganizedasfollows.Inx2
thatevenaweakjet(internalMachnumberof5)shouldproduce wedescribeobservationsanddatareduction.TheX-rayimages
a significantholealong thelineofsighttotheradiojet.They are analyzed in x 3, with an emphasis on the X-ray morphol-
suggestedthatthelackofX-rayholesmaybeduetomixingof ogyoftheoverallregion,thecentralregion,andtheS0galaxy
thermalandradioplasmaintheregion,althoughtheirnumerical IC5337inx3.1.Inparticular,twoextendedX-rayemissionre-
simulationsdidnotdistinguishbetweenthesetwocomponents. gionspossiblyassociatedwithsubclustersarediscussedinx3.1.1.
Theyalsoransimulationsinwhichthejetturnedoff,whichre- ThepointsourcesdetectedintheX-raydataarediscussedinx3.2.
sembled the observations of weak X-ray holes. Another possi- TheX-raysurfacebrightnessprofileisstudiedinx3.3.Various
bilitytheynotedistheimprecisemodeltheyusedtosubtractout substructuresidentifiedonresidualmapsarepresentedinx3.4.
fromthesymmetricX-rayemissiontoobtainaresidualimage. Spectralanalysisispresentedinx4.Inparticular,clusterprofiles,
TheysearchedforX-rayandradiocorrelationsfromtheresidual massdepositionrate,coolingandthermalconductiontimescales,
imagebutfailedtofindstrongX-raydeficits(holes)displacedby hardnessratiomaps,andpossibleAGNsassociatedwiththecD
theradioplasma.Withtheexcellentspatialresolutionprovided galaxyIC5338andtheS0galaxyIC5337arediscussedinthis
byChandraanddeeperobservationsprovidedbybothChandra section.Throughoutthedataanalysissections(xx3and4),some
andXMM-Newton,weareabletostudythecomplicatedcentral discussions related to the interpretation of the results are pre-
regionofAbell2626inmoredetail. sentedtohelpreadersunderstandthedata.Wethendiscussthe
WhilemanyradiosourcesareassociatedwithAGNsincluster kinematics of the S0 galaxy and the cooling flow interaction
galaxies,thereexistothertypesofextendedradioemissionas- withthecentralradiosourceinxx5and6indetail,respectively.
sociatedwiththediffuseICMwithoutcleargalaxyhosts.These We summarize our work in x 7. We assume H ¼71 km s(cid:3)1
0
extended radio sources can be divided into cluster-wide halos, Mpc(cid:3)1, (cid:1) ¼0:27, and (cid:1) ¼0:73 throughout the paper. At
M (cid:2)
relics,andminihalos(Feretti&Giovannini1996,2007).Abell aredshiftz¼0:0573,theluminositydistanceofAbell2626is
2626containsacandidateminihalo(Gittietal.2004).Cluster- 252.8Mpc,and100 ¼1:10kpc.Thevirialradiusisapproximately
wide halos and relics are large in scale ((cid:1)1 Mpc), with halos 1.53Mpc(Markevitchetal.1998)fortheaverageclustertem-
locatedinthecentersof clusters,whiletherelicsaregenerally peratureofT ¼3:0keV(Whiteetal.1997).
X
locatedattheperiphery.Veryoften,thesetwotypesoflarge-scale
2. OBSERVATIONS AND DATA REDUCTION
diffuseradioemissionarefoundinclusterswithoutcoolingcores.
Thisindicatesapossibleorigininclustermergers,whicharebe- 2.1.XMM-Newton Data
lievedtodisruptcoolingcoresandtoreaccelerateradio-emitting
Abell2626wasobserved(ObsID0148310101)on2002De-
particles(Brunetti2004;Blasietal.2007).However,somecool-
cember28byXMM-Newtonfor(cid:1)38and(cid:1)42kswiththeEPIC
ingcoreclusterswithdominantradiogalaxieshostminihalosat
pn and MOS cameras, respectively. Extended full frame mode
theclustercenterwithextendedradioemissionupto0.5Mpc.
(pn) and full frame mode (MOS) were used with thin optical
The morphology of the minihalos is different fromthat ofthe
filters. XMM-Newton data reduction was done with SAS ver-
radiobubblesatthecentersofmanycoolcoreclusters,which
sion5.4.1.Allthedatawerereprocessedusingthetasksepchain
show a strong X-ray/radio interaction in which the radio lobes
andemchain.WeincludedonlyeventswithFLAG¼0inour
displaceX-raygas.Thishintsatadifferenttransportmechan-
analysis.WechoseonlyeventswithPATTERN¼0Y4(0Y12)
ism(notjets)toexplaintheextendedpropertyoftheminihalos.
forpn(MOS)data.
Thetypicaltimescalesforradiativelossesbyrelativisticelectrons
((cid:1)107Y108yr)incoolingcoreswithstrongmagneticfieldsare Backgroundflareswererejectedbyperforminga2(cid:2)clipping
ofthe100sbinnedlightcurvesinthe12Y14keVand10Y12keV
muchshorterthanthediffusiontimes((cid:1)109yr)fortheelectrons
bandsforthepnandMOScameras,respectively,whichyielded
tobetransportedfromthecenterofthecoolingcoreouttoafrac-
theout-of-timecorrectedexposuretimesof35,563,40,300,and
tion of a megaparsec. Hence, the diffuse radio emission from
40,600sof cleaneddataforthepn,MOS1,andMOS2cameras,
minihalossuggeststhataninsitureaccelerationmechanismis
respectively.Blank-skybackgroundfilescollectedbyD.Lumb4
needed for the electrons. The origin of the minihalos has re-
wereusedduringtheanalysis.Theobservedbackgroundlevels,
mainedunclear,butithasbeensuggestedthattheinteractionof
estimatedfromthetotalcountrateinthewholefieldofview(FOV)
thecentralradiogalaxyandtheICM(re)acceleratesparticlesto
inthehardband(10Y12keVand12Y14keVforMOSandpn,
relativisticenergies.Recently,Gittietal.(2004)modeledAbell
respectively),are1.07,0.99,and1.26timesthoseintheblank-
2626withanelectronreaccelerationmodeldrivenbyMHDtur-
skybackgrounds forMOS1, MOS2, andpn,respectively. We
bulenceamplifiedbythecompressionofthemagneticfieldinthe
haverenormalizedtheblank-skybackgroundsaccordingly.The
coolingcoreregion.Theyhaveshownthatthemodelagreeswith
uncertaintyof thepnbackgroundislargerthanthatfortheMOS
variousobservationalconstraintssuchastheobservedradiobright-
detectors.Theeffectsofthelargerpnbackgrounduncertainty
nessprofile,theintegratedsynchrotronspectrum,andtheradial
andsomeotherbackgrounduncertaintiesonthespectralanaly-
steepeningof theradiospectrum.Theyconcludethatonlyatiny
sisareaddressedinx4.1.1.Theimageandhardnessratiomap
fraction((cid:1)0.7%)ofthemaximumpowerthatcouldbeextracted
analysesweredoneusingtheMOSdataonly(xx3and4.6),and
fromthecoolingcore(basedonthestandardcoolingflowmodel)
wecheckedthattheresultsof theanalysiswouldnotbeaffected
isneededforreacceleration.Sincethereaccelerationmodelfor
theoriginoftheminihaloassumesaphysicalconnectionbetween
thermalandrelativisticplasma,itisessentialtoperformnewX-ray 4 Seeftp://xmm.vilspa.esa.es/pub/ccf/constituents/extras/background/.
No. 1, 2008 X-RAY OBSERVATIONS OFABELL 2626 157
byachangeinthebackgroundnormalizationof(cid:4)10%.Thevi-
gnetting effect for the image and hardness ratio map analysis
hasbeencorrectedusingtheexposuremaps.
2.2. Chandra Data
AChandraobservation(ObsID3192)of(cid:1)25kswasmadeon
2003January22inveryfaintmode.Thecenteroftheclusterwas
positioned30fromtheedgeoftheS3chip(midwaybetweenthe
nodeboundaries).Thefocalplanetemperaturewas(cid:3)120(cid:5)C.
TheChandradatareductionwasdonewithCIAOversion3.1,
using calibration products from CALDB 2.28. The latest gain
file(CALDBGAIN2.28)andgeometryfile(CALDB2.9)atthe
timeofreductionwereused.Thereductionwasdoneinveryfaint
modetoremovemoreparticlebackground.Backgroundflares
were rejected by performing the standard clipping using the
lc_clean routine written by M. Markevitch.5 This yielded
22,959sofcleaneddata.SincethewholeFOVisdominatedby
theclusterX-rayemission,internalbackgroundcannotbeused.
Theblank-skybackgroundfilescollectedbyM.Markevitchwere
usedduringtheanalysis.Thedifferencebetweentheobserved
andtheblank-skyhigh-energy(9Y12keV)backgroundcount
rateswaslessthan1%,andthissmalldifferencewasremovedby
renormalizingthebackground.Theinfluenceof thebackground
Fig.1.—Background-subtracted,exposure-corrected,adaptivelysmoothed
onthespectralfittedmeasurements(temperatureprofile)hasbeen mosaicoftheXMM-NewtonEPICMOS1andMOS2imagesinthe0.3Y10keV
checked by varying the background normalizations by (cid:4)10%, band.Theimagewassmoothedtoasignal-to-noiseratioof3.Theunitsforthe
and the results are not significantly changed. To check if there colorscalearecountsperbinnedpixel,whereeachbinnedpixelhasasizeof
4:100;4:100.ThearrowsontheleftandrightindicatethecDgalaxyIC5338and
mightbeasignificantdifferencebetweentheGalacticbackground
theS0galaxyIC5337,respectively.Twoouterextendedemissionregionsare
inthesourceandblank-skyobservations,anotherthermalcom- indicatedwithwhitecircles.Itshouldbenotedthatthereisachipgapatabout5.50
ponentwithafixedlowtemperatureof 0.2keVhasbeenadded fromthecenter,whichmakesthenortheasternextendedX-raysourceappeartobe
(Markevitch etal.2003;Lumbetal.2002). Normalizationof moredistinctthanitactuallyis.
theadditionalcomponenthasbeenallowedtobenegativeforthe
case of more Galactic emission in the blank-sky observations
axiescoversaregionwheretheexcessX-rayemissionisseen.
comparedtotheclusterobservation.Alltheresultsarebasically
Mohretal.(1996)showthatthesubclusterisboundtoandfall-
unchanged.Fixingthetemperatureoftheadditionalcomponent
ingintothemaincluster.OurXMM-Newtonimageshows,for
tobetheoutermosttemperaturethatcanbedeterminedfromthe
thefirsttime,extendedX-rayemissionpossiblyassociatedwith
Chandra FOValso gives essentially the same results. We con-
thesubclusterinthesouthwestdirection.TheextendedX-ray
cludethattheChandraresultsareinsensitivetouncertaintiesin
emissioncanbeviewedonabackground-subtracted,exposure-
thebackground.
corrected, adaptively smoothed image. The location of the ex-
tendedemissionpossiblyassociatedwiththesubclusteriscircled
3. X-RAY IMAGE ANALYSIS atthesouthwestcornerinFigure1.Theextendedemissionatthe
3.1. X-Ray Morphology southwestcornerisnotobvious,buttheslightdeviationfrom
sphericalsymmetryinthatregioncanbenoted.Theshapeofthe
3.1.1.GlobalMorphology extendedemissionregionisdifficulttocharacterizeduetothelow
The background-subtracted, exposure-corrected, adaptively signal-to-noiserationeartheedgeof thecluster.Byinspecting
smoothedmosaicoftheXMM-NewtonEPICMOS1andMOS2 variousimages,wechoseacircularregioncenteredonthehighest
imagesisshowninFigure1.Thepnimagewasnotincludedbe- excessX-rayemission,witharadiuschosentobesmallenough
causeofitsdifferentspectralresponseandbecausethechipgaps sothattheregiondidnotoverlapwiththenearlycircularcluster
andbadcolumnsproducedcosmeticartifactsinthesmoothedim- emissionofAbell2626.Ourchoiceforthisregionisjustifiedby
age.Theimagewassmoothedtoasignal-to-noiseratioof3per thehardnessratiomapbelow(Fig.14),whichshowsthattheex-
smoothingbeam. tendedregionchosenappearstobeharderthanitssurrounding.
TheglobalX-rayimageofAbell2626appearstoberoughly ComparedtotheX-rayemissionatthesameradiusfromthemain
azimuthallysymmetric.Thebrightestregionatthecentercorre- clustercenter(thecDgalaxyIC5338)withallpointsourcesand
spondstothepositionofthecDgalaxyIC5338.Thereisasource otherextendedemissionexcluded,theexcessemissionissignifi-
associatedwiththeclusterS0galaxyIC5337thatis1.30 west cantatthe4.6(cid:2)level.Ourresidualimages(x3.4)alsoindicate
ofthecenterof thecluster.ExcessX-rayemissionisseentothe thepossiblesignaturesofamerger.
southwestofthecluster,extendingfrom(cid:1)10outto(cid:1)70(seex3.4). AnotherextendedX-raysourcecanbeseenabout70northeast
Mohretal.(1996)identifiedasubclustertothesouthwestofthis ofthecenterofAbell2626(Fig.1).Thecriteriausedtodefine
regionbasedonopticalobservationsofthegalaxypopulation. thenortheastregionarethesameasthesouthwestone.Thisfea-
The subcluster identified is centered roughly at the southwest tureissignificantatthe11.6(cid:2)level.Itshouldbenotedthatthere
edgeoftheXMM-NewtonFOV,butthedistributionofthegal- isachipgapatabout5.50fromthecenterthatmakestheextended
X-raysourceappeartobemoredistinctthanitactuallyis.No
clusterorgroupstructurehasbeenidentifiedpreviouslyinthis
5 Seehttp://cxc.harvard.edu/contrib/maxim/acisbg/. region.However,Abell2626isknowntobeassociatedwiththe
158 WONG ETAL. Vol. 682
Fig.2.—Left:Background-subtracted,exposure-corrected,adaptivelysmoothedChandraimageofthecentral3:50;2:40regionoftheclusterinthe0.3Y10keV
band.Theimagewassmoothedtoasignal-to-noiseratioof3.TheChandraimageshowsthedetailsof theinnerregionofthecoolingcore.Thewhitesolidlinesare
1.5GHzCconfigurationradiocontoursshowingtheminihalo.Thegreensolidlinesare1.5GHzBconfigurationradiocontoursshowingtheradiobarstructures.Both
radiocontoursweretakenfromGittietal.(2004).ThearrowsontheleftandrightindicatethecDgalaxyIC5338andtheS0galaxyIC5337,respectively.Right:
Background-subtracted,exposure-corrected,adaptivelysmoothedChandraimagecenteredontheS0galaxyIC5337.Theimagewassmoothedtoasignal-to-noise
ratioof4,withanintensityscalechosentoshowthebowshockYlikeshape.Thesouthernsource(redellipse)wasremovedandreplacedbytheaveragesurrounding
X-rayintensitybeforeadaptivelysmoothing.ThegreencontoursarefromtheVLA1.5GHzBconfigurationradioimagetakenfromGittietal.(2004),withthelowest
contourlevelbeingasignal-to-noiseratioof3.
Perseus-Pegasussupercluster,afilamentof clustersofgalaxies correspond to the curved radio jet feature to the south of the
extendingforasmuchas(cid:1)300Mpc(Batuski&Burns1985a, core.
1985b;Zuccaetal.1993;Einastoetal.2001).Thesouthwest-to- However,themostunusualfeaturesoftheradioimagearethe
northeast extension to the structure in our X-ray image around twoelongatedradiobarstothenorthandsouthoftheradiocore.
Abell2626iselongatedalongthePerseus-Pegasusfilament.This Wefindthatthereisnoobviouscorrelationbetweenthetwora-
mayindicatethattheclusterispreferentiallyaccretingsubclusters diobarsandtheChandraX-rayimageinFigure2(leftpanel).
andgroupsfromthissupercluster. NoobviousX-raydeficit(holes)isfoundontheChandraX-ray
image,whichisconsistentwiththefindingof Rizzaetal.(2000).
3.1.2.Central Region
Wehavegeneratedresidualmapstofurtherinvestigateourfind-
The background-subtracted, exposure-corrected, adaptively ing,andtheresultsarepresentedinx3.4.Ourinterpretationis
smoothedChandraimageofthecentral3:50;2:40 isshownin thattheradiobarsarethintubes(seex6).
Figure2(leftpanel).Theimagewassmoothedtoasignal-to- OneinterestingfeatureisthatthereisanX-rayexcess‘‘tongue’’
noiseratioof3.ThebrightestregionoftheX-rayemissioninthe thatextendsfromthecentralcDgalaxytothesouthernradiobar,
cluster is centered on the southwest nucleus of the central cD andthereisasimilarbutweakertongueinthenortherndirection
galaxy.Thissourceispossiblyextendedinbothsoft-band(0.3Y aswell(Fig.3,toprightpanel).Thetonguesarebestseeninthe
2keV)andfull-band(0.3Y10keV)imageswhendetectedby residualmapinFigure8ofx3.4.
wavdetectinCIAO,butnothingwasfoundbywavdetectin Figure2 (leftpanel)alsoshowsradiocontours(white)from
the hard band (2Y10 keV). However, examination of the raw theVLA1.5GHzCconfiguration(Gittietal.2004)overlaidon
soft-band(0.3Y2keV)imageshowstwopeakssuperposedon theChandraimageshowingtheradiominihaloinAbell2626.The
thetwoopticalnucleiof thecentralcDgalaxy(Fig.3,bottom minihaloisroughlyconfinedwithin6000 withadiamondshape.
left panel). There is also a hard point source seen in the raw Thedistancefromthecentertothenortheasternorsouthwestern
hard-band(2Y10keV)image(Fig.3,bottomrightpanel).Thepo- cornersofthediamondisabout5000,whiletothenorthwesternor
sition of this hard point source agrees with the southwest cD southeasterncornersitisabout7000.Adiscussiononthemini-
nucleus. The hard point source is separated from the peak of haloispresentedinx6.
theradiocorebylessthan100.Thissourcecanalsobeseeninthe
3.1.3. TheS0RadioGalaxy IC5337
hardnessratiomap(seeFig.16below).Thehardpointsourceand
itslikelyassociationwiththeAGNatthecenterofthesouthwestcD About1.30westofthecentralcDgalaxy,thereisanX-raysource
nucleusandradiocorearediscussedfurtherinx4.7.Thissource associatedwiththeS0galaxyIC5337(Fig.2,leftpanel).This
is located along an arc of X-ray emission extending from the source is clearly extended in the soft-band image, but a hard-
northeasttothewesttothesouthoftheclustercenter. bandimageshowsthatthereisapointsourcecoincidentwith
PreviousROSATX-rayandVLAradioobservationssuggest thenucleusofIC5337.ThepossibilitythattheS0nucleusisan
thatthereisenhancedX-rayemissionspatiallycoincidentwith AGNwillbediscussedbelow(x4.7).Tothesouthof IC5337,
theradiosource(Rizzaetal.2000).InFigure2(leftpanel),the thereisanotherX-raypointsourcethatisnotassociatedwithany
contoursfromtheVLA1.5GHzBconfigurationradioimageare knowngalaxy.AsindicatedinFigure2,thereisanearbyradio
showningreen(Gittietal.2004).Thepositionof theradiocore sourcethatmaybeassociatedwiththisX-raysource.
agreeswiththebrightestregionofX-rayemission.Inparticular, Therightpanelof Figure2showsthebowshockYlikeshape
theradiocoreiscenteredatthesouthwestcDnucleusratherthan oftheextendedX-rayemissionassociatedwiththeS0galaxy
the northeast one. There is an arc of X-ray emission that may IC5337.Theimageiscreatedbyremovingandreplacingthe
No. 1, 2008 X-RAY OBSERVATIONS OFABELL 2626 159
Fig.3.—ImagesofthetwonucleiofthecDgalaxyIC5338inAbell2626.AllimagesshowthesameFOVof3000;3000.Topleft:OpticalimagefromHSTarchive.
TheimagewastakenwithWFPC2usingtheF555Wfilter.Thetwogreencircles,includedintheotherpanels,arecenteredatthetwoopticalnucleiobservedbyHST.
Topright:Background-subtracted,exposure-corrected,adaptivelysmoothedChandraimageinthe0.3Y10keVband.Theimagewassmoothedtoasignal-to-noise
ratioof 3.ThecolorrepresentstheX-rayintensityfromhigh(white-yellow)tolow(darkblue).Bottomleft:RawChandraimageinthesoft(0.3Y2keV)band.Two
intensitypeakscanbeidentified.Bottomright:RawChandraimageinthehard(2Y10keV)band.Onlythesouthwesternnucleuscorrespondstothepeakinthehard
band.
southernpointsourcebytheaveragesurroundingX-rayintensity Infact,allopticalcounterpartsidentifiedarewithin1100.Weadopt
andthenadaptivelysmoothingtoasignal-to-noiseratioof4. theNEDsourcewithanaccuratepositionthatisclosesttothe
TheS0galaxyIC5337alsoisaradiosource(Fig.2).Radio X-rayposition.Thepossibleassociationsof X-raypointsources
contours(Gittietal.2004)areoverlaidinFigure2(rightpanel). withAGNsinthecentralcDgalaxyIC5338andtheS0galaxy
ThereisacomponentcenteredontheS0galaxyandacompo- IC5337werenotedabove(xx3.1.2and3.1.3)andarediscussed
nentlocatedatapositionof about190(cid:5)(measuredfromnorthto inmoredetailbelow(x4.7).NineX-raysourcesareincludedin
east)oftheS0galaxyonthe1.5GHzBconfigurationmap.The thelist(Table1).Figure4isanopticalimagefromtheDigitalSky
1.5GHzCconfigurationmapshowsthreeradiotailsinthesouth, SurveywiththenineX-raysourcesmarked.Twoofthesearethe
southwest,andwestdirections.Thekinematicsof theS0galaxy centralcDgalaxyIC5338andtheS0galaxyIC5337discussed
isdiscussedinx5. above.Interestingly,threeoftheX-raysourcescorrespondtogal-
axiesthatarelocatedintheregionofextendedX-rayemission
3.2. Source Detection
about70northeastofthecenterofAbell2626(Fig.1).However,
WeusedtheXMM-NewtonSAStaskeboxdetecttodetecta oneofthesepossibleopticalIDs(2MASXJ23365722+2114032)
total of 90 distinct point sources on the XMM-Newton images; has a redshift of 0.038 and thus is not directly associated with
thedetectionsweredoneseparatelyoneachofthethreedetec- Abell2626.
tors,andthenthesourcelistswerecombined.Thesourceswere On the Chandra image, 14 point sources were detected by
confirmedbyinspection,andsomelow-leveldetectionsattheedge wavdetectinCIAOusingthedefaultsettings.Ofthese,only
ofthe FOVorbadpixelgapswereremoved.Herewe discuss thecentralcDIC5338galaxyandtheS0galaxyIC5337have
onlythesourceswithpossibleopticalcounterpartswithin2000. possibleidentificationsinNEDwithpositionsthatagreewiththe
160 WONG ETAL. Vol. 682
TABLE1
XMM-NewtonX-RayPointSourceswithNEDIdentifications
Separationb
X-RayR.A. X-RayDecl. PossibleNEDObject Typea NEDR.A. NEDDecl. Redshift (arcsec)
233604.8 +210611.2 2MASXJ23360455+2106127 G/IrS 233604.6 +210613 0.066130 3
233609.6 +212025.1 MG3J233615+2120 RadioS 233609.7 +212025 ... 1
233615.8 +210611.5 Abell2626:[SPS89]08 VisS/RadioS 233616.2 +210621 ... 11
233624.5 +210903.6 IC5337 G/IrS/RadioS 233625.1 +210902 0.054988 9
233630.5 +210847.0 IC5338 G/IrS/RadioS 233630.6 +210850 0.054900 3
233653.8 +211332.9 2MASXJ23365372+2113322 G/IrS 233653.7 +211332 0.056100 2
233654.2 +211531.7 2MASXJ23365415+2115302 G/IrS 233654.2 +211529 0.054200 3
233657.1 +211405.6 2MASXJ23365722+2114032 G/IrS 233657.2 +211403 0.038040 3
233732.4 +210907.6 2MASXJ23373222+2109020 G/IrS 233732.2 +210902 0.184400 6
Note.—Unitsofrightascensionarehours,minutes,andseconds,andunitsofdeclinationaredegrees,arcminutes,andarcseconds.
a ThesourcetypeasclassifiedbyNED:G=Galaxies;IrS=InfraredSource;RadioS=RadioSource;VisS=VisualSources.
b SeparationbetweentheX-raypointsourceandsourceidentifiedbyNED.
X-raytowithin1000.Table2liststhepositionsofthe14Chandra whichhasbeenshowntoprovideagoodfittorelaxedclusters
X-raypointsources.Figure5isanopticalimagefromtheDigital withcoolingcores(Xue&Wu2000).
SkySurveywiththelocationsofthe14X-raysourcesmarked. The background-subtracted and exposure-corrected surface
Unlessspecified,allthesourcesdetectedareexcludedinthe brightnessprofilefromtheXMM-NewtonMOS1andMOS2data
analysisofextendedemissionsinthesubsequentsections. isshowninFigure6.Thepndatawerenotincludedsincethe
spectral response is different, which complicates the deprojec-
3.3. Surface Brightness Profile
tionoftheelectrondensityprofilebelow(x4.3).Also,thefitted
The azimuthally averaged X-ray surface brightness profile surface brightness model was used to create residual images
wasextractedseparatelyfortheChandraandXMM-Newtondata. (x 3.4), and including the pn image would produce cosmetic
ExceptforthetwoextendedX-rayclumpstothenortheastand artifactsduetothelargerchipgapsandgreaternumberof bad
southwestandthestructureintheverycenter,Abell2626appears columns in the pn camera. All of the X-ray point sources ex-
to be a nearly relaxed cluster. The profiles were fitted using a cept the central cD galaxy were excluded from the data. The
double(cid:3) modelprofileoftheform extended X-ray emission around the S0 galaxy was also ex-
cluded,asitproducedafeatureintheclustersurfacebrightness
X " (cid:1) r (cid:2)2#(cid:3)3(cid:3)iþ1=2 profile.WealsoexcludedtheextendedX-rayemissionregions
SðrÞ¼ Sið0Þ 1þ ; ð1Þ tothenortheastandsouthwestoftheclusterthatweredescribed
r
i¼1;2 ci in x 3.1.1, and the criteria of defining the regions were also
explained in that section. The annuli were chosen such that
all the widths are larger than the FWHM of the point-spread
function(PSF)atthatradius.Withthischoice,allthebinscon-
tain at least 1000 counts after background subtraction for the
XMM-Newton MOS1 and MOS2 data. With these very good
statistics,theerrorbarsonthesurfacebrightnessaretoosmall
tobeeasilyseeninthetoppanelof Figure6;instead,wepre-
sent the residuals in the bottom panel. The parameters of the
TABLE2
ChandraX-RayPointSources
PointSource R.A. Decl. NEDObject
1............................ 233615.82 +210855.4 ...
2............................ 233621.31 +211329.7 ...
3............................ 233621.41 +211135.4 ...
4............................ 233624.61 +210847.7 ...
5............................ 233624.81 +211016.7 ...
6............................ 233625.04 +210903.0 IC5337
7............................ 233625.60 +211307.7 ...
8............................ 233630.52 +210847.0 IC5338
9............................ 233633.30 +210836.7 ...
Fig.4.—OpticalimagefromtheDigitalSkySurveyshowingthenineXMM- 10.......................... 233636.98 +211051.8 ...
11.......................... 233637.46 +210621.0 ...
NewtonX-raysourceswithidentificationsinNED(Table1).Thedarkgraycir-
clesarecenteredontheX-raypositionsdeterminedwithXMM-Newton.Thereare 12.......................... 233638.43 +210826.1 ...
lightgraycirclescenteredontheopticalpositionsfromNED,buttheyoverlapso 13.......................... 233645.60 +210749.3 ...
completelywiththedarkgraycirclesthattheyarebarelyvisible.NEDobjectsare 14.......................... 233646.84 +210807.8 ...
consideredpossibleidentificationsiftheyarewithin2000ofanX-raysource.In
fact,allopticalcounterpartsidentifiedarewithin1100.[Seetheelectronicedition Note.—Unitsofrightascensionarehours,minutes,andseconds,andunits
oftheJournalforacolorversionofthisfigure.] ofdeclinationaredegrees,arcminutes,andarcseconds.
No. 1, 2008 X-RAY OBSERVATIONS OFABELL 2626 161
Fig.5.—OpticalimagefromtheDigitalSkySurveyshowingthepositions
(darkgraycircles)ofthe14X-raypointsourcesdetectedwithChandra(Table2).
[SeetheelectroniceditionoftheJournalforacolorversionofthisfigure.]
best-fitdouble(cid:3)modelareS (0) ¼ 0:40þ0:02countss(cid:3)1arcmin(cid:3)2
1 (cid:3)0:02
andS (0)¼0:10þ0:01countss(cid:3)1arcmin(cid:3)2,r ¼34:8þ7:1arcsec
andr2¼128:5þ8:(cid:3)10a:0rc1sec,and(cid:3) ¼0:91þ0:261and(cid:3) ¼(cid:3)05::673þ0:02. Fig.6.—Top:AzimuthallyaveragedX-raysurfacebrightnessprofilefrom
Visua2lly,thedo(cid:3)u7:b2le(cid:3)modelfit1seemsa(cid:3)de0:q18uate.H2owever,(cid:3)w0:i0t1h theXMM-NewtonMOS1andMOS2observationsofAbell2626.Thesquares
aretheX-raydata,whilethesolidlineisthefittoadouble(cid:3)model.Thetinyerror
theverysmallerrorsonthemeasuredsurfacebrightness,thefit
barswithinthesquaresonthesurfacebrightnessaretoosmalltobeeasilyseen.
isactuallynotverygood,withareduced(cid:4)2of 2.3.Inaddition Bottom:Residualstothebest-fitdouble(cid:3)model,inunitsof(cid:2).Thehorizontal
totheexcessatthecenter,associatedwiththecentralcDgalaxy errorbarsshowthewidthsofthebinsusedtoaccumulatethecounts.Thevertical
andradiosource,within20000thereareseveralotherregionsof errorbarsareatthe1(cid:2)level.[SeetheelectroniceditionoftheJournalforacolor
versionofthisfigure.]
excessordeficient emissioncomparedtothe model.These fea-
turesarereproduced(albeitwithpoorerstatistics)intheChandra
surfacebrightnessdataandinthedatafromtheXMM-Newtonpn far southwestexcesslabeledwithagreenpolygonintheresidual
detector.Theseresidualsmayindicatethatthecentralregionis map.
disturbedbyeffectsofthecentralradiosourcebeyondthecurrent Mostofthestructuresintheinnerregionoccuronascalesim-
extentofthesource,orthattheclusterisnotcompletelyrelaxed ilartothatof theradiosourcesinthecenter.Inordertostudythe
duetoamerger. radio/X-rayinteractioninmoredetail,Figure8showsradiocon-
toursfromtheVLA1.5GHzCconfiguration(whitecontours)
3.4.Residual Images
andthe1.5GHzBconfigurationradioimages(greencontours;
Tobetterunderstandtheoriginoftheresidualsinthesurface Gittietal.2004),overlaidonthecentralpartof theChandrare-
brightness fit and the dynamics of the intracluster gas, we cre- sidualmap.Ingeneral,thediamond-shapedoutlineoftheextended
atedresidualmapsforbothXMM-NewtonandChandraimages radioemission(theradio‘‘minihalo’’)correspondstoaregionof
(Figs.7and8).SimulatedazimuthallysymmetricX-rayimages reduced X-ray emission, and there are excess X-ray regions
wereproducedthatexactlyfollowedthedouble(cid:3)modelfitsto north-northeastandsouth-southwestoftheedgeof theminihalo
thesurfacebrightness.Thesesimulatedimagesweresubtracted wheretheradiosurfacebrightnessisdroppingrapidly.Themini-
fromtheadaptivelysmoothedimages.Figure7showstheXMM- haloisroughlyconfinedwithin6000.Thedistancefromthecen-
NewtonandChandraresidualimageswiththesameFOV.Both tertothenortheasternorsouthwesterncornersofthediamondis
residualmapsshowsimilarstructures;theXMM-Newtonimage about5000,whiletothenorthwesternorsoutheasterncornersitis
hasgreatcontrastduetothebetterstatistics,whiletheChandra about7000.NotethatintheinterpretationofGittietal.(2004)the
imageshowsmoredetailedstructures.Substructurescanbeiden- tworadiobarsaredistinctfromthediffuseradioemission,andin
tifiedwithmanyoftheresidualsseeninthemaps.Alargeregion factaftertheirsubtractionthemorphologyoftheminihalobe-
ofexcessemissioncanbeseenabout11000 southwestofthecD comesroughlycircular(seeFig.6ofGittietal.2004).However,
galaxy,whichcorrespondstothepeakofthesurfacebrightness intheresidualimageinGittietal.(2004)wecanstillseesome
residualsaroundthatradius.Otherregionsofexcessanddeficit excessradioemissioninthenorthwesternandsoutheasterncor-
within10000canalsobeidentifiedfromtheresidualmapofthe ners.Asnotedpreviously,thereisasignificantexcessassociated
Chandraimage(Fig.7,rightpanel).Inthesurfacebrightness withtheradiocoreandthesouthernradiojet.Thesouthernradio
residuals,thecentralexcesscorrespondstothecentralbrightpoint barisaregionofX-rayexcess,butthenorthernbarshowsanexcess
inFigure7(rightpanel).Thetrougharound1500correspondsto totheeastandadeficittothewestinX-rays.Theinterpretation
thedarkringaroundthecentralexcessintheresidualmap,which oftheinteractionbetweenthermalandradioplasmaisdiscussed
canbeseenmoreclearlyinFigure8.Thepeakataround3000 is inx6indetail.
probablyduetothetwobrightregionsduenorthandsouthinthe The one major X-ray residual in the inner cluster that is not
residualmap(labeledastwogreencircles).Thetrougharound onthescaleofthecentralradioemissionistheextendedexcess
5000correspondstothedarkregionindicatedbyawhitepolygon tothesouthwest.Itispossiblethatthishasbeenproducedbythe
intheresidualmap.Thepeakaround11000 correspondstothe actionofthecentralradiosource.However,thesouthwestexcess
162 WONG ETAL. Vol. 682
Fig.7.—Left:ResidualmapfromtheXMM-NewtonMOS1andMOS2observationsofAbell2626,generatedfromabackground-subtractedimage.Yellowrepre-
sentsthelargestexcess,whileblackisthegreatestdeficit.Right:ResidualmapfromtheChandraobservationofAbell2626,generatedfromabackground-subtracted
image.Yellowrepresentsthelargestexcess,whileblackisthegreatestdeficit.ThereisanexcesscoincidentwiththecenterofthecentralcDgalaxy,whichispartly
surroundedbyatroughofreducedX-raysurfacebrightnessataradiusofabout1500.Therearetwolocalizedregionsofexcesssouthandnorthofthecentralpeak,which
areindicatedwithgreencircles.Thereisanextendeddeficitabout5000westofthecenter,whichisindicatedwithawhitepolygon.Anextendedregionofexcess
emissionisseenabout11000southwestofthecenter,whichisindicatedwithagreenpolygon.
is between the cD galaxy and the subcluster southwest of the 4. SPECTRAL ANALYSIS
mainclustercenter.Itispossiblethatthisexcessistheresultof
4.1. Temperature Profile
thismergerorapreviousmergerfromthisdirection.Thereisno
obvious sharp surface brightness discontinuity associated with The azimuthally averaged, projected temperature profile for
thisexcess,whichmeanseitherthatitisnotashockorcoldfront Abell2626wasextractedusingbothXMM-NewtonandChandra
orthatthemergerisnotoccurringprimarilyintheplaneofthe data(Fig.9,toppanel).Thespectrawerefittedusinganabsorbed
sky.WenotethatMohretal.(1996)showthattheline-of-sight thermalmodel(WABS(cid:6)MEKAL)withXSPECversion12.3.1.
velocitydifferencebetweenthemainclusterandthesubclusteris FortheXMM-Newtondata,allthespectraofthethreecameraswere
(cid:1)2600 km s(cid:3)1, which is greater than the typical sound speed fittedsimultaneously.Theenergybandsusedare0.6Y10.0keVand
withinclusters.Iftheexcessistheresultofamerger,thissug- 0.4Y7.0keVforXMM-NewtonandChandradata,respectively.
geststhatthemergerismainlyalongthelineofsight. Using0.6Y7.0keVand0.5Y10.0keVbandsforXMM-Newton
andChandradatagivesessentiallythesameresults.Thespectra
weregroupedtohaveaminimumof 25countsbin(cid:3)1.Wefixed
theabsorptiontobe4:2;1020cm(cid:3)2(Dickey&Lockman1990)
andtheredshifttobe0.0573(Abelletal.1989).Freeingthese
valuesgivesessentiallythesameresults.Excepttheoutermost
datapointwherethereduced(cid:4)2valueis(cid:1)1.6,alltheotherre-
duced(cid:4)2valuesareP1,whichindicatesthatthefitsaregood.
Toensurethatthespectraarenotaffectedbyenergy-dependent
PSFeffects,theannulioftheXMM-Newtondatawerechosen
suchthatallofthewidthsareatleasttwicethePSFhalf-energy
widthofthepncamera.
The generaltrend isthattheprojectedtemperature increases
fromacentralvalueof (cid:1)2.5keVwithin900((cid:1)10kpc)toamax-
imumof(cid:1)3.5keVat10000Y20000 (110Y220kpc).Sucharadial
increaseisnormallyseeninaclustercoolingcore.Theprojected
temperaturethendecreasesto(cid:1)1.4keVat(cid:1)54000 ((cid:1)600kpc).
Interestingly,thereisasharpdropintheprojectedtemperature
Fig.8.—ResidualmapfromChandraobservationofthecentral3:50;2:40 from(cid:1)2.9keVat21000(230kpc)to(cid:1)2.0keVat27000(296kpc)in
regionofAbell2626,generatedfromabackground-subtractedimage.Thecolor theXMM-Newtondata.Outsideofthissteepdrop,theprojected
scalefromyellowtoredtobluetoblackrepresentstheresiduals,runningfrom temperaturegradientflattens.Theradiusofthedropcorresponds
excesstodeficit.Thewhitesolidlinesare1.5GHzCconfigurationradiocon- tothedistancefromthecDgalaxytotheouteredgeofthelarge
toursshowingtheminihalo.Thegreensolidlinesare1.5GHzBconfiguration
excessemissioninthesouthwestdirection.Ifthelargeexcessemis-
radiocontoursshowingtheradiobarstructures.Bothradiocontoursweretaken
sionindicatesamergersignature,thetemperaturejumpmightin-
fromGittietal.(2004).ThearrowsontheleftandrightindicatethecDgalaxy
IC5338andtheS0galaxyIC5337,respectively. dicatethatgasisheatedwithinthatregion.Unfortunately,the
No. 1, 2008 X-RAY OBSERVATIONS OFABELL 2626 163
accumulationofthecontaminantlayerontheChandraACISop-
ticalblockingfilterwasunderestimated.
Tostudythetruetemperatureofthecluster,wehavealsode-
projectedthetemperatureprofilebyusingtheXSPECPROJCT
model.Foreachannulus,themodelsumsuptheappropriatefrac-
tionsofemissionscontributedfromtheouterannuli.Thespectra
ofalltheannuliwerefittedsimultaneouslytogetthedeprojected
temperature profile. The middle panel of Figure 9 shows the
azimuthallyaveraged,deprojectedtemperatureprofileforAbell
2626usingbothXMM-NewtonandChandradata.FortheXMM-
Newton data, all thespectra fromthe three cameras were fitted
simultaneously.TheXMM-NewtonandChandradataareconsis-
tentwithinthe90%confidenceuncertainties,buttheuncertain-
tiesoftheChandradataaremuchlarger.SincetheXMM-Newton
dataextendfurtheroutandhavesmallererrorbars,wedescribe
results obtained from XMM-Newton in the following. We also
adopttheXMM-Newtonspectralfits(Fig.9,bottompanel)for
thedeprojectionandthemassprofileanalysisbelow(xx4.3,4.4,
and4.5).
Thedeprojectedcentraltemperatureisslightlylowerthanthe
projected one because the latter is contaminated by the hotter
emissionalongthelineofsight.Thedifferenceissmallsincethe
hottestregionhasatemperatureonly1keVhigherthanthecen-
tral one. A more noticeable difference is that the temperature
Fig.9.—Top:Profileoftheazimuthallyaveraged,projectedtemperatureTof
jumpataround24000ismoreobviousinthedeprojectedprofile.
Abell2626.TheXMM-Newton(Chandra)dataareshownasfilled(open)circles.
Middle:Profileoftheazimuthallyaveraged,deprojectedtemperatureTofAbell Itdropsfromatemperatureof(cid:1)3.5keVto(cid:1)2.2keV.Thepeak
2626.TheXMM-Newton(Chandra)dataareshownasfilled(open)circles.The of thedeprojectedtemperatureprofileislocatedjustwithinthe
blackcrossesaretheprojectedtemperaturesfromtheXMM-Newtondatainthe temperaturejump((cid:1)18000Y24000).
toppanelforcomparison(withouterrorbarsforclarity).Inboththetopand
middlepanels,theerrorbarsareatthe90%confidencelevel.Bottom:Crossesare 4.1.1. Effects ofXMM BackgroundUncertainties
thetemperaturescalculatedfrominterpolationoftheoriginalXMM-Newtonde-
projectedtemperatureprofile;thisinterpolatedprofilewasusedforthepressure/ SincetheclusteremissioncoversthewholeFOVandthecor-
entropy/timescaledeprojectionandthemassprofileanalysisinxx4.3,4.4,and rectbackgroundcannotbeestimatedeasily,theso-calleddouble
4.5.TheinterpolatedtemperatureprofilefortheChandradataanalysisissimilar backgroundsubtractionmethod,commoninXMM-Newtondata
butnotshownforclarity.Theerrorsareassignedas5%(20%)atradiiofsmaller
analysis(Arnaudetal.2002),cannotbeapplied.Thenaturalback-
(larger)than25000toincludepossiblesystematicerrors.Thefilledcirclesarethe
originalXMM-Newtonmeasureddeprojectedtemperaturesasshowninthemid- groundsubtractionmethodistoperformarenormalizedblank-
dlepanel,withouterrorbarsforclarity. skybackgroundsubtractionwithoutthesecondstepofextracting
ablank-fieldYsubtractedspectruminanemission-freeregionin
jumpcannotbeconfirmedwithChandraastheChandradatado thedoublebackgroundsubtractionmethod.Weusethis‘‘single
notextendfarenoughinalldirectionstoreachthedistanceofthe backgroundsubtraction’’method.Notethatthismethodhasalso
jumpseenintheazimuthallyaveragedtemperatureprofile.Itonly beenappliedtothestudyofaclusterwithasimilarredshiftto
extendsfarenoughtowardthenorthwestdirection,wherethere Abell2626(Arnaudetal.2001).AsmentionedinArnaudetal.
arenotenoughcountstoconstrainthetemperature. (2001),ourtreatmentdoesproperlyaccountforthecosmic-ray
In general, the XMM-Newton data give lower values for the (CR)inducedbackground.However,thecontributionsfromvari-
projectedtemperatureprofile.Thereareatleastthreepossibleex- ationsinthelocalsoftGalacticbackground(whichdependson
planationsforthisdifference.First,itmightbeduetothelarger thepositiononthesky)andtheextragalacticX-raybackground
PSFofXMM-Newton.Thesofterphotonsfromthebrighter,cooler component(whichdependsontheabsorbinghydrogencolumn
inner regions may be scattered to outer regions, hence making densityalongthelineofsightandcosmicvariance)werenotin-
theouterregionsappearcooler.Thiseffectmaybeseenfromthe cluded.Inourcase,theemissionfromAbell2626usuallydom-
secondbinoutwardandshouldbemoreseriousinregionswhere inatesthelocalsoftGalacticbackground,andtheCR-induced
thegradientofthesurfacebrightnessisgreatest(i.e.,nearthecen- backgroundusuallydominatesthehardX-rayextragalacticback-
ter;Markevitch2002).Thismightexplainthediscrepancyinthe groundcomponent.UsingtheROSATAllSkySurvey(Snowden
secondandthirdbinsofourXMM-Newtondata.Thesecondpos- etal.1997),wehavecomparedthesoftX-rayemission(0.47Y
sibilityisthattheremighthavebeenasteadyandmildenhance- 1.21keV)aroundAbell2626andtheexposure-weightedemission
mentinthehardX-raybackgroundduringtheChandraobservation intheregionsusedtoextracttheblank-skybackground,andthe
thatwasnotremovedbythestandardcleaningprocedure;thiscould differenceislessthan4%.Thehydrogencolumndensitytoward
maketheChandraemissionappearhotter,particularlyinouter, Abell2626isaboutafactorof 2higherthanthatformostof the
lowsurfacebrightnessregions.Suchaneffecthasalsobeennoted blank-skyfields(Dickey&Lockman1990)butisnothighenough
byMarkevitch(2002).Wehavetriedtoassesstheimportanceof toaffectthecosmicX-raybackgroundstrongly.Also,asmentioned
suchabackgroundenhancementbyincludingacomponentfor previously, the CR-induced background usually dominates the
thehardbackground,modeledasapowerlawwithanexponen- hard X-ray extragalactic background component. Hence, the
tialcutoff(Markevitch2002;Markevitchetal.2003)inthefits blank-skybackgroundsshouldrepresentthesoftGalacticback-
oftheChandraspectra.However,thiscomponentdidnotsignifi- groundandthehardX-raybackgroundreasonably.
cantlyimprovethefits,andthebest-fitamplitudeofthecompo- We were concerned that, while the background renormal-
nent was negligible. A third possibility is that the continuous izationsforMOS1andMOS2aremodest(P7%correction),the
164 WONG ETAL. Vol. 682
usingdifferentnormalizationswasatmost(cid:1)1keVintheouter
region.
ThedeprojectedtemperatureprofilesobtainedfromtheMOS1+
MOS2+pnspectra(blackdiamonds),theMOS1+MOS2spectra
(magentatriangles),andthepnspectraalone(cyantriangles)all
agree reasonably well. Thus, the pn data appear to give fairly
reliablespectralinformation,eventhoughtheuncertaintyinthe
pnbackgroundnormalizationmaybelarger.
Finally,varyingtheamountofsoftGalacticbackground(orange
crosses)gavedeprojectedtemperaturesingoodagreementwith
thoseusingtherenormalizedblank-skybackgrounds(blackdia-
monds)exceptfortheoutermostannulus.Thelargedeviationof
theoutermostdatapointmayindicatethatthereislessofsoftGa-
lacticemissioninthedirectionofAbell2626thanintheblank-sky
fieldsbecausethefittednormalizationof theextrasoftGalactic
backgroundcomponentinthemodelisnegative.ThesoftGalac-
ticcomponentshouldbeuniformonthescaleofacluster.How-
ever,areductioninthesoftGalacticbackgroundsimilartothat
suggestedbythelastannuluswasfoundtobecompletelyincon-
sistentwiththefitsintheinnerannuli.Hence,weconcludedthat
thereducedsoftcomponentintheoutermostannuluswasnotdue
toareductioninthesoftGalacticbackgroundtowardthecluster.
Thereducedsoftcomponentintheoutermostannulusmaybethe
resultofadditionalthermalstructureinthisregion.
Fig.10.—Profileoftheazimuthallyaveraged,deprojectedtemperatureTof
Tosummarize,allourresultsareindependentof background
Abell2626withdifferentXMM-Newtonbackgrounds.Theblackdiamondsgive
ouradoptedXMM-Newtontemperatureprofile(theblackfilledcirclesinthe usedwithin(cid:1)18000.Fortheouterregions,thetemperatureprofile
middlepanelofFig.9).Theothertemperatureprofilesaregeneratedbyincreas- isonlyslightlyaffectedbythechoiceofthebackground,butthe
ingtheblank-skybackgroundnormalizationsby10%(20%)forMOS(pn)(blue differences are mostly well within the error bars. All the fea-
diamonds),decreasingtheblank-skybackgroundnormalizationsby10%(20%)
turesdescribedinthepaperrelatedtotemperaturemeasurements
forMOS(pn)(greendiamonds),usingtheblank-skybackgroundswithoutrenor-
(the temperature jump, pressure, entropy profiles, etc.) are not
malization(noerrorbarsforclarity;redcrosses),usingtheMOS1+MOS2spectra
only(datashiftedtotherightforclarity;magentatriangles),usingthepnspec- affected.
traonly(datashiftedtotheleftforclarity;cyantriangles),andaddinganextra
0.2keVMEKALcomponenttorepresentvariationsinthesoftGalacticcompo- 4.2. Central X-Ray Spectrum and Cooling Rate
nent(normalizationfreeandallowedtobenegativeineachannulus,datashifted
WedeterminedtheradiativecoolingratebyfittingtheX-ray
totheleftforclarity;orangecrosses).Theerrorbarsareatthe90%confidence
level. spectrum of thecentral regions of thecluster from both XMM-
NewtonandChandradata.Weextractedthespectrumfromacir-
cularregionwitharadiusof6500,whichisroughlythecooling
renormalizationofthepnbackgroundislarger((cid:1)26%correction). radius determined in x 4.4. In XSPEC, we used the WABS(cid:6)
Whiletheuncertaintiesinthebackgroundwillaffectmoststrongly (MKCFLOW+MEKAL)model,whichcombinesacoolingflow
thederivedtemperaturesof theouterregionsofthecluster,we component(MKCFLOW)withanisothermalmodel(MEKAL)
emphasizeherethatmostofourfocusinthispaperisontheX-ray toaccountforthegasintheouterregions.Theabsorptionwas
and radio interaction in the innermost region where the results fixedattheGalacticcolumnof4:2;1020 cm(cid:3)2asbefore.The
arenearlyindependentofbackgroundassumed.Weperformeda upper temperature (kT ) and abundance of the MKCFLOW
high
numberofteststoaddresstheeffectsoftheXMM-Newtonback- componentweresettobethesameastheMEKALcomponent,
grounduncertaintiesonthedeprojectedtemperatureprofiles.We whichisexpectedifthegascooledfromtheambientICM.Initially,
didthisby(1)varyingtheblank-skybackgroundnormalizations the lower temperature of the MKCFLOW component is set to
by(cid:4)10%((cid:4)20%)fortheMOS(pn),(2)usingtheblank-skyback- bethelowerlimitforthemodel(kT ¼0:08keV),asexpected
low
groundswithoutrenormalization,(3)usingtheMOS1+MOS2 fortheclassicalcoolingflowmodelinwhichthegascoolsradia-
spectraonly,(4)usingthepnspectraonly,and(5)addinganextra tivelytoverylowtemperatures.Withthisassumption,thebest-
0.2 keV MEKAL component (with normalization free and al- fit cooling mass deposition rate is 2(cid:4)1 (3(cid:4)3) M yr(cid:3)1 for
(cid:2)
lowedtobenegativeineachannulus)asamodelforanyvariation XMM-Newton (Chandra) data in the 0.6Y10.0 (0.5Y7.0) keV
inthesoftGalacticcomponent.Thetestresultsaresummarized energybandwitha(cid:4)2/dof ¼1:18(0.90).Thefitteduppertemper-
inFigure10.Ouradopteddeprojectedtemperatureprofileinthis atureandabundancewerekT ¼2:8(cid:4)0:05(3:3(cid:4)0:2)keV
high
paperisshownwithblackdiamonds. and0:52(cid:4)0:03(0:9(cid:4)0:1)solar,respectively.AllowingkT
low
Withinr <18000,allthedeprojectedtemperaturesareinvery tovaryincreasedthemassdepositionratetobe74(cid:4)6M yr(cid:3)1
(cid:2)
goodagreement.Allthesetestsshowasignificantjumpintem- fortheXMM-Newtondata,withkT ¼1:5(cid:4)0:1keV,kT ¼
low high
perature near 24000 and a flattening of temperature beyond this 4:2þ0:2keV,andanabundanceof0:45þ0:03solar,witha(cid:4)2/dof ¼
(cid:3)0:5 (cid:3)0:02
jump.Asexpected,increasingthebackgroundnormalization(blue 1:13.TheChandradatagaveverypoorconstraintsonthemass
diamonds) gave lower deprojected temperatures for the outer depositionrateorlowertemperaturewitha(cid:4)2/dof ¼0:87ifkT
low
regions(r>18000),whiledecreasingthebackgroundnormal- wasallowedtovary.FromtheF-test,theprobabilitythatthefitto
ization (green diamonds) gave higher temperature.Using the themodelwithkT fixedisbetterthanthefitwithfreeingkT
low low
blank-skybackgroundswithoutrenormalization(redcrosses) isonly2;10(cid:3)11(9;10(cid:3)3)fortheXMM-Newton(Chandra)data.
alsogavehighertemperatures.Thevariationinthetemperatures Forcomparison,themassdepositionratedeterminedfromEinstein