Mon.Not.R.Astron.Soc.000,000–000(0000) Printed23March2016 (MNLATEXstylefilev2.2) Another shock for the Bullet cluster, and the source of seed electrons for radio relics Timothy W. Shimwell1,2(cid:63), Maxim Markevitch3, Shea Brown4, Luigina Feretti5, 6 1 B. M. Gaensler6, M. Johnston-Hollitt7, Craig Lage8, Raghav Srinivasan7 0 2 1CSIROAstronomy&SpaceScience,AustraliaTelescopeNationalFacility,POBox76,Epping,NSW1710,Australia r 2LeidenObservatory,LeidenUniversity,POBox9513,NL-2300RALeiden,theNetherlands a M 3AstrophysicsScienceDivision,NASA/GoddardSpaceFlightCenter,Greenbelt,MD20771,USA 4DepartmentofPhysicsandAstronomy,UniversityofIowa,203VanAllenHall,IowaCity,IA52242,U.S.A 5INAF-IstitutodiRadioastronomia,viaGobetti101,40129Bologna,Italy 1 6SydneyInstituteforAstronomy,SchoolofPhysics,TheUniversityofSydney,NSW2006,Australia 2 7SchoolofChemical&PhysicalSciences,VictoriaUniversityofWellington,POBox600,Wellington6014,NewZealand 8CenterforCosmologyandParticlePhysics,DepartmentofPhysics,NewYorkUniversity,NY,NY10003,USA ] E H . h Accepted—;received—;inoriginalform23March2016 p - o ABSTRACT r t s With Australia Telescope Compact Array observations, we detect a highly elongated a [ Mpc-scale diffuse radio source on the eastern periphery of the Bullet cluster 1E0657-55.8, which we argue has the positional, spectral and polarimetric characteristics of a radio relic. 2 Thispowerfulrelic(2.3±0.1×1025 WHz−1)consistsofabrightnorthernbulbandafaint v lineartail.Thebulbemits94%oftheobservedradiofluxandhasthehighestsurfacebright- 4 nessofanyknownrelic.ExactlycoincidentwiththelineartailwefindasharpX-raysurface 6 0 brightness edge in the deep Chandra image of the cluster – a signature of a shock front in 1 thehotintraclustermedium(ICM),locatedontheoppositesideoftheclustertothefamous 0 bowshock.ThisnewexampleofanX-rayshockcoincidentwitharelicfurthersupportsthe . hypothesisthatshocksintheouterregionsofclusterscanformrelicsviadiffusiveshock(re- 2 )acceleration. Intriguingly, our new relic suggests that seed electrons for reacceleration are 0 comingfromalocalremnantofaradiogalaxy,whichweareluckytocatchbeforeitscom- 5 1 pletedisruption.Ifthisscenario,inwhicharelicformswhenashockcrossesawell-defined : regionoftheICMpollutedwithagedrelativisticplasma–asopposedtotheusualassumption v thatseedsareuniformlymixedintheICM–isalsothecaseforotherrelics,thismayexplain i X anumberofpeculiarpropertiesofperipheralrelics. r Keywords: radiationmechanisms:non-thermal–accelerationofparticles–shockwaves– a galaxies:clusters:individual:1E0657-55.8–galaxies:clusters:intraclustermedium–radio continuumgeneral 1 INTRODUCTION and temperature of the ICM (e.g Markevitch et al. 2002, Marke- vitchetal.2005,Russelletal.2010andOwersetal.2011).Un- Inthehierarchicalschemeofstructureformation,massivegalaxy fortunately,intheclusteroutskirts,wheremorestrongshocksare clustersareassembledbythecomingtogetheroflessmassivecom- expected (e.g. Hong et al. 2014), the X-ray flux is low and the ponents(e.g.Press&Schechter1974).Shocksplayacrucialrole X-ray identification of shocks is difficult (see e.g. Ogrean et al. inthisprocess,asduringclusterassemblytheydissipateasignif- 2013a). However, it is thought that a powerful shock can accel- icantquantityofgravitationalenergyintotheintraclustermedium erate electrons in the ICM (via diffusive shock acceleration, see (ICM).Shocksingalaxyclustersaredifficulttodetect;theywere Blandford & Eichler 1987) with sufficient efficiency to produce initiallydetectedinsomeofthemostmassivegalaxyclustersusing detectable synchrotron emission in the cluster outskirts, forming X-rayobservationstoidentifysharpedgesinthesurfacebrightness a radio relic (Ensslin et al. 1998). Indeed, regions of highly ex- tended, diffuse synchrotron emission on cluster peripheries have been frequently observed (see Ferrari et al. 2008, Bru¨ggen et al. (cid:63) E-mail:[email protected] (cid:13)c 0000RAS 2 Shimwelletal. 2012,Ferettietal.2012andBrunetti&Jones2014forrecentre- Table1.AsummaryofourATCAobservationstowards1E0657-55.8.The views),andinseveralcasesdeepX-rayobservationshaverevealed quoted synthesised beam FWHM and sensitivity correspond to a natural thatthesynchrotronemissionisco-locatedwithX-rayshocksig- weightingofthevisibilities,howeverthedatawereimagedatavarietyof natures (see e.g Giacintucci et al. 2008, Finoguenov et al. 2010, resolutionsbetween2.7(cid:48)(cid:48)and23.3(cid:48)(cid:48). Macarioetal.2011,Bourdinetal.2013,Akamatsu&Kawahara 2013andOgrean&Bru¨ggen2013c).However,theX-rayshocks Coordinates(J2000) 06:58:32.7-55:57:19.0 andsynchrotronemissionarenotalwayscolocated(seeOgreanet Amplitudecalibrator PKSB1934-638 Phasecalibrator PMNJ0742-56 al.2013b)whichraisesquestionsregardingtheformationscenario ImageRMS 15µJy/beam ofradiorelics.Itisthereforeessentialthattherelationshipbetween On-sourcetime 32hours theradiorelicsandX-raypropertiesoftheICMisfurtherstudied, Frequencyrange 1.1-3.1GHz asthiscanenhanceourunderstandingofradiorelicformation. Spectralresolution 1MHz The Bullet cluster 1E0657-55.8 is a well known, extremely SynthesisedbeamFWHM 6.5(cid:48)(cid:48)naturalresolution hot and massive merging galaxy cluster with a prominent bow PrimarybeamFWHM 42(cid:48)-15(cid:48) shock,richgravitationallensingdatasetandapowerfulgiantradio Polarisationsmeasured XX,YY,XYandYX halo(seee.gTucker,Tananbaum,&Remillard1995,Tuckeretal. 1998,Markevitchetal.2002,Cloweetal.2006,Liangetal.2000 andShimwelletal.2014).Liangetal.(2000)suggestedthatalarge etal.(2000).Differentpointings,observedwithsmallrelativeoff- radio relic is present in the peripheral region of this cluster. This sets to minimise the effect of detector response variations, were proposedradiorelichasnotbeenconfirmeduntilourrecentdeep coadded,imagesofthebackgroundcomponentssubtracted,andthe 1.1-3.1GHzAustraliaTelescopeCompactArray(ATCA)observa- resultdividedbytheexposuremap.Thefinalimagewasextracted tions(Shimwelletal.2014)whichweusehere.Wealsoexplorethe in the 0.8−4 keV band and binned by 8 to a pixel size of 3.9(cid:48)(cid:48). relationshipbetweentheradioemissionandX-rayemissionusing GiventhehighgastemperatureandthepeakACIS-Isensitivityat ourradiodatatogetherwithadeepChandraX-raydataset. E=1−2keV,theimageinthisenergybandessentiallygivesthe HereafterweassumeaconcordanceΛCDMcosmology,with line-of-sightintegralofthesquareofthegasdensity. Ωm = 0.3, ΩΛ = 0.7 and H0 = 70 kms−1Mpc−1. At the red- PointsourceswereleftintheX-rayimagesshowninthispa- shift of the Bullet cluster (z=0.296) the luminosity distance is pertoillustratetheangularresolutionofthedata,butexcisedfrom 1529Mpc and 1(cid:48)(cid:48) corresponds to 4.413kpc. All coordinates are further analysis. The spectral analysis followed standard steps of giveninJ2000. extracting the spectra of the source and of the background com- ponents(sky+detectorandreadout)fromaregionofinterest,cre- ating the instrument response files, and fitting a thermal plasma model (APEC) in XSPEC, fixing the Galactic absorption column 2 OBSERVATIONSANDDATAREDUCTION atNH =4.6×1020cm−2,andincludingtheadditionalcomponent toaccountforthedifferencebetweentheblank-skymodelandthe WeusedtheATCAtoobserve1E0657-55.8between2012Decem- actual source-free background spectrum as described above, nor- ber 17 and 2013 February 17 (project C2756). The observations malisedaccordingtotheregionarea.Theuncertaintyofthiscom- werecarriedoutinfourconfigurationsoftheATCAantennas;the ponentwasnegligible—omittingthiscomponententirelychanged targetwasobservedfor8,5,9and10hoursinthe1.5B,1.5D,6A thebest-fittemperaturesbyasmallfractionoftheirstatisticalun- and6Barrays,respectively.Theseobservationsaresummarisedin certainty.Toincludetheeffectofthebackgrounduncertaintiesin Table1 andafull descriptionofthesedata isgivenby Shimwell thederivedgastemperaturevalues,wevariedthebackgroundnor- et al. (2014), in which we concentrated on the cluster’s giant ra- malization by ±3% (a 90% scatter; Hickox & Markevitch 2006) diohalo.Theflagging,calibrationandimagingtechniquesareall andaddedthedifferenceinquadraturetothestatisticaluncertainty. describedbyShimwelletal.(2014). FortheX-rayanalysisweuseasetofarchivalChandraACIS- Ipointingswithatotalcleanexposureof522ks,resultsfromwhich werepresentedbyMarkevitch(2006),Cloweetal.(2006),Owers 3 RESULTS etal.(2009)andinotherworks.Fulltechnicaldescriptionofthe In Figure 1 we present a medium resolution image of the 1.1- X-ray analysis will be presented by Markevitch et al., in prepa- 3.1GHzradioemissionfromtheBulletcluster.Weobservediffuse ration;herewegivethedetailsrelevantforourinterestingregion emissionintheeasternperipheryoftheBulletcluster.Thedetected of low X-ray surface brightness, for which background modeling object peaks at RA (J2000) 06:58:51.2 DEC (J2000) -55:57:13.5 isimportant.Lightcurvesoftheindividualpointingswereexam- whichis≈170(cid:48)(cid:48)(750kpc)fromtheclustercentre.Aroundthispeak inedforperiodsofevelatedbackground;onlyamininalnumberof thereisabulbofbrightdiffuseemissionthatisconnectedtoafaint, suchtimeintervalshadtobeexcluded.Thedetectorplusskyback- linearstructurethatextendssouthwards.Hereafter,werefertothe groundwasthenmodeledusingtheblank-skydatasetnormalised brightnorthernbulbasregionA(declinationrange-55:56:09.1to- bycountsinthe9.5–12keVenergyband.Afterthat,wechecked 55:57:57.1),thefaintersouthernpartasregionB(declinationrange thebackgroundinthesource-freeregionsofthefieldofviewand -55:57:57.1to55:59:42.0)andtheentirestructureasregionA+B. detectedaresidualfaintflarecomponent,whichwemodeledwith We have measured the integrated flux of the diffuse radio a power law spectrum (without the application of the mirror en- emissionasafunctionoffrequencyforregionsAandB(seeFig- ergyresponse)andaspatialdistributionsimilartothatofthequi- ure 2) and characterised the 1.1GHz to 3.1GHz emission with escent background. This model described the difference between (cid:16) (cid:17)α the blank-sky background and the real background in our obser- Iν=Iν0 νν0 ,whereν0is1.4GHz.Wecalculatedtheuncertainty vations well at all energies except for residuals below our lower onourintegratedfluxdensitymeasurementsbyaddinginquadra- energy bound, which we ignored. The ACIS readout artifact was ture the ATCA absolute flux calibration error of 2% (Reynolds modeledasabackgroundcomponentasdescribedbyMarkevitch 1994)withtheerrorontheintegratedfluxdensityderivedfromthe (cid:13)c 0000RAS,MNRAS000,000–000 3 -55°55'00.0" 2.56 102 HALO 1.28 56'00.0" 0.64 y) 57'00.0"Region A mJ Dec (J2000) 58'00.0" 00..1362mJy/beam ated flux (101 gr Region B e 59'00.0" 0.08 Int J06587-5558 0.04 -56°00'00.0" 0.02 01'00.0" 100 2 3 Frequency (GHz) 59m00.00s 50.00s 40.00s 30.00s 6h58m20.00s 0.01 RA (J2000) Figure2.Theintegratedfluxoftheproposedradiorelicasafunctionof Figure1.Amediumresolution(FWHM≈7(cid:48)(cid:48))StokesIimagetowards1E frequency.ThegreenandredpointsshowtheintegratedfluxforregionsA 0657-55.8.This1.1-3.1GHzimagehasbeenprimarybeamcorrected,the andB,respectively.Theblacksolidlinesshowthebestfittingpowerlaws greyscaleisinmJy/beamandthemeasurednoiseis15µJy/beam.Theradio tothemeasurementsfromregionsAandBandthedashedlinesshowthe halo(Shimwelletal.2014),LEH2001J06587-5558andtheproposedradio errorsonthesefits. relic(regionsAandB)arelabelled. imagenoise.WedeterminethatIν0 isequalto77.8±3.1mJyand 0.14 4.8±0.6mJyandthatαisequalto−1.07±0.03and−1.66±0.14 forregionsA,andB,respectively.Awiderfrequencyrange(1.4- 0.12 6.2GHz)offluxmeasurementswillbepresentedbySrinivasanet )% al.,inpreparation.IntheleftpanelofFigure3weshowthevaria- n (0.10 o tionsinthespectralindexacrossthediffuseemission.InregionA ati weareabletoaccuratelyconstrainthespectralindexandobserve aris0.08 ol an East-West gradient, with a spectral index of ≈-0.8 in the East al p0.06 and≈-1.5intheWest.InregionBthediffuseemissionisdetected on atlowersignificance,whichpreventedusfromcharacterisingspa- acti0.04 Fr tialtrendsinitsspectralindex. 0.02 InFigure1andShimwelletal.(2014)weseeaverylowsig- nificance bridge of emission between region B of the radio relic, 0.00 J06587-5558, and the south eastern region of the radio halo. The 1.5 2.0 2.5 3.0 Frequency (GHz) bridge may physically connect the radio halo and radio relic but J06587-5558–whichLiangetal.(2001)speculatedmaybeagrav- Figure4.Thefractionalpolarisedfluxasafunctionoffrequencyforregion itationallens,ahighredshiftradiogalaxy,oraradiorelic–isnow Aoftheproposedradiorelic.Theredpointsshowthefractionalpolarised thoughttobeahighredshift(z∼2.8)galaxy(Johanssonetal.2012). emission,wheretheStokesQandUdatawereimagedin100MHzsections (cid:112) Furtherdataisrequiredtoproperlycharacterisethisbridgeofemis- andP= U2+Q2.Thebluepointsshowthefractionalpolarisedemission fromtherelicbutwheretheStokesQandUdatawereimagedin200MHz sionbutstructuresconnectingrelicsandhalosareobservedinother sections. clusters(seee.g.Venturietal.2013). tionanglesshownonthe2.3-2.5GHzimagepresentedinFigure3 3.1 Polarisationresults by14degreesanticlockwise.Figure3showsthatinthenorthern- mostregionoftherelic,wherethepolarisedemissionisstrongest, RegionAoftherelicispolarisedandinFigure4wepresentthe the B-vectors are aligned with the major axis of the relic. South- fractional polarised emission as a function of frequency for this wardsofthistheB-vectorsgraduallybecomelessalignedwiththe region. The low significance of the emission from region B pre- majoraxisoftherelicbutthistrendcouldbeduetorotationmea- ventedusfrommakingsimilarmeasurementsinthatregion.Polar- surevariationsacrosstheobjectwhichwehavenottakenintoac- isationmeasurementswereperformedon100MHzand200MHz count.WhilsttheboundarybetweenregionsAandBoftherelic sub-bands.Theconsistentresultsfromthesemeasurementsdemon- cannotbepreciselydefined,thegradualtransitionintherotationof stratethatourintegratedpolarisedfluxmeasurementsarenotsig- thepolarisationvectorsgivestheimpressionthatregionsAandB nificantly effected by bandwidth depolarisation. From 1.5GHz maybephysicallyconnectedandnotmerelyachanceprojection. to 2.2GHz the fractional polarisation increases rapidly with fre- quency.InthecentrepanelofFigure3weshowapolarisationim- age of the diffuse emission. Additionally, we have measured that 3.2 RegionA:noopticalcounterpart theFaradaydispersionspectrum,F(φ),peaksatφ=−16rad/m2at theregionofthepeakoftheStokesIemissionbutissimilaracross ThesurfacebrightnessofregionAisanomalouslyhighforarelic, thediffuseemission.Thiscorrespondstoarotationofthepolarisa- which suggests that it may be a remnant of a radio galaxy. Un- (cid:13)c 0000RAS,MNRAS000,000–000 4 Shimwelletal. 0.30 50000 0.30 -55°56'00.0" 45000 0.27 40000 35000 -55°57'00.0" 0.45 -55°57'00.0" 0.24 30.0" 30000 0.60 25000 0.21 Dec (J2000) 58'00.0" 00..9705αSpectral Index () Dec (J2000) 58'00.0" 00..1158mJy/beam Dec (J2000) 57'3000..00"" 112050000000000Counts/pixel 1.05 0.12 5000 59'00.0" 59'00.0" 1.20 0.09 58'00.0" 1.35 0.06 -56°00'00.0"59m00s 57s 54s 51s6h58m48s -56°00'00.0"59m00s 57s 54s 51s6h58m48s 30.0" 59m00s 56s 52s 6h58m48s 0 RA (J2000) RA (J2000) RA (J2000) Figure3.Left:thecontoursshowthe1.1-3.1GHzprimarybeamcorrectedcontinuumemissionfromtheproposedradiorelicataresolutionof10(cid:48)(cid:48) with √ contourlevelsat±5× 1,2,4,8,...×15µJy;positivecontoursaresolidlinesandnegativecontoursaredashed.Thecolourscaleshowsthe1.1-3.1GHz spectralindeximageatthesameresolution,whereregionsinwhichthespectralindexerrorexceeds0.3havebeenblanked.Centre:Thecolourscaleshows the2.3-2.5GHzpolarisationimage(P=(cid:112)U2+Q2)ataresolution√of≈10(cid:48)(cid:48),thescaleisinJyandpixelswithP/σQ,U<2havebeenblanked,whereσQ,Uisthe standarddeviationoftheQandUimages.Thecontoursshowthe 1,2,4,8,...×100µJylevelontheimage.ThevectorsrepresenttheobservedB-vectors(the E-vectorsorpositionangle,θ,rotatedby90◦,wheretan(2θ)=U/Q),andwerederivedfromthe2.3-2.5GHzdataset.Thevectorshavenotbeenrotatedback tothezero-wavelengthpositionanglebutmeasurementsoftheFaradaydispersionspectrum,whichpeaksatφ=−16rad/m2,indicatethatthiscorrespondsto arotationofonly14degreesanticlockwise.Right:ThecolourscaleshowstheR-bandimagepresentedbyCloweetal.(2006).Theoverlaidcontoursarethe √ 5× 1,4,16,...×15µJylevelsofahighresolution(FWHM≈3(cid:48)(cid:48))1.1-3.1GHzATCAimage,showingthestructureinregionAoftherelic.Theredcircles showthecolour-selectedlikelyclustergalaxiesfromCloweetal.(2006).ForallimagestheellipseinthebottomleftcornershowstheATCAsynthesised beam. fortunately,inthisregionontheedgeofthecluster,wehavenot 100000 beenabletofindspectroscopicallyconfirmedclustermembersin 90000 -55°55'00.0" 80000 the literature. However, in Clowe et al. (2006) they used R, B 70000 andVbandMagellanIMACSimagestodeterminecolor-selected 60000 56'00.0" likely cluster galaxies, a catalog of these sources is available 50000 from http://flamingos.astro.ufl.edu/1e0657/public.html. Although 40000 ttahhgeiesrpceradetssahelonifgttedteorigrnoertChsleoorwnwetihtehetsatehl.es(o2du0er0ecp6e)sRtow-bsialelnadbrceMhlfaaogrgreeltlh,aenwseIoMuhrAacveCeoSufistmehde- Dec (J2000) 5587''0000..00"" 2300000000Counts/pixel diffuseradioemission(seetherightpanelofFigure3).TheR-band imagereachesalimitingmagnitudeof25.1andforalmostallofthe 59'00.0" 10000 otherradiosourcesthatwehavedetectedaclearcounterpartisseen intheR-bandmaps–thisisdemonstratedinthewidefieldR-band -56°00'00.0" image presented in Figure 5. There is no obvious counterpart to thediffuseradioemissionbuttherearemanycolour-selectedlikely 01'00.0" clustermembersclosetotheregionofradioemission,particularly 59m00s 50s 40s 30s 6h58m20s 0 RA (J2000) thosenearthesoutheasternareaofregionA,whereinhighreso- lutionradioimagesweobserveatiptotheemission(seetheright Figure5.ThesameR-bandandradioimagethatarepresentedintheright panelofFigure3,butoverawiderfieldofviewtodemonstratethatthereis panelofFigure3).Wewouldexpectthattheproposeddeadradio aR-bandcounterpartformostoftheradiosourcesdetectedinourfield. galaxyishostedbyagiantellipticalgalaxy(Matthews,Morgan,& Schmidt1964)andafuturestudywouldbeusefultoconclusively identifynearbygiantellipticalsandstudytheirproperties. radio brightness profile, extracted in the same strip from the im- age shown on the left of Figure 6 (after excluding several bright pointsources),isshownbyadashedline.Itconfirmstheexactco- 3.3 RegionB:asecondshockfrontintheBulletcluster incidenceoftheX-rayedgeandtheradiorelic.TheX-raybright- WehaveexaminedtheX-rayimage(Figure6)intheregionofthe nessedgeismostprominentinthisstrip,althoughtheX-rayimage relic and discovered a clear X-ray brightness edge. The 2(cid:48)-wide suggestsitmayextendbyanother2−3(cid:48) tothenorthofthestrip, white rectangle in Figure 6 is oriented perpendicular to region B althoughitislesssharpthereandwouldnotqualifyasan“edge”. of the relic and has the relic emission positioned in the middle; TheX-rayedgeinregionBoftherelicisdefinedwellenoughforus itisclearfromtheoverlayofthisrectangle(aswellasofthera- toattempttoderivetheX-raygasdensityandtemperaturejumps. dio contours) on the X-ray image that the X-ray edge coincides TheX-rayprofilehastheexpectedshapeofaprojectedgasdensity withtherelic.Toshowthisbetter,wehaveextractedanX-raysur- discontinuity,asseeninmanycoldfrontsandshockfronts(Marke- facebrightnessprofileinthatrectangle,presentedinFigure7with vitch&Vikhlinin2007),andshouldallowustoderivethedensity x=0” corresponding to the position of region B of the relic. A jump.However,forthatweneedtoknowtheradiusofcurvature (cid:13)c 0000RAS,MNRAS000,000–000 5 ofthisdiscontinuityalongthelineofsight.Theusualapproachis toestimatethecurvatureofthebrightnessedgeintheplaneofthe skyandassumeitisthesameintheline-of-sightdirection.How- ever, our relic is a straight line. Thus, we assume (guided by the shapeoftheclusteroutsidethisregion)thatthecurvaturealongthe lineofsightshouldnotbeverydifferentfromthedistancetothe clustercentre.Wethususethisdistance(1.0Mpcfromthecluster centre to the middle of the relic tail) and change it by factors of 0.5and2tocoveraconservativerangeofpossibilities.Wethenfit thebrightnessprofileintheimmediatevicinityoftheedgewitha modelconsistingofapower-lawdensityprofile(withafreeslope) insidetheedgeandaβ-model(Cavaliere&Fusco-Femiano1978) outside (with a free slope and a fixed core radius for simplicity), withtheamplitudeandpositionofthedensitydiscontinuitybeing freeparameters.Thismodelprovidesanexcellentfit(redhistogram inFigure7);obviously,itisdegeneratewithrespecttotheradius ofcurvature.Forthenominalradius,thedensityjumpisafactor of2.67(therestofthebest-fitparametersare:outerβ =0.57for a fixed core radius of 450 kpc, inner density power-law slope of −1.6, and the density jump position x=+4(cid:48)(cid:48)). A 90% statistical interval(one-parameter,i.e.,leavingallotherparametersfree)for the density jump is 2.19–3.19, and that for the jump position is Figure7.TheX-raysurfacebrightnessprofileintheregionoftheradio ±2(cid:48)(cid:48).However,thesystematicuncertaintyonthejumpamplitude relicisshownbydatapoints.Theprofilewasextractedfromthe2(cid:48)-wide ismuchgreater—whentheradiusofcurvatureischangedbyfac- stripshowninFigure6.Thedashedlineshowsaradiobrightnessprofile torsof2and0.5,thebest-fitdensityjumpsarefactorsof1.92and (arbitraryunits)extractedinthesamestrip,aftertheexclusionofbright 3.64,respectively.(Wechosenottoco-addtheseuncertainties,be- pointsources.Atthepositionoftheradiorelic(x≈0(cid:48)(cid:48))weseeasharp causethestatisticalerrorismeasured,whilethesystematiconeis breakintheX-raysurfacebrightness,whichistypicalofaprojectedabrupt justaguess.)Ifthisedgewereashockfront,usingtheRankine- gasdensitydiscontinuity.Theredlineshowsabest-fitsurfacebrightness Hugoniot adiabat, the nominal best-fit density jump corresponds modelthatincludessuchadiscontinuity(Section3.3);thebest-fitposition ofthediscontinuityisx=+4(cid:48)(cid:48)±2(cid:48)(cid:48). toaMachnumberof2.45,anditssystematicuncertaintygivesan intervalM=1.66−5.5(butseebelowfortheupperbound). 4 DISCUSSION Radiorelicsarebelievedtobeformedbyshock-frontsinthepe- ripheralregionsofclusters(e.g.Ensslinetal.1998andRoettiger, Burns, & Stone 1999). This theory predicts that at radio wave- We have derived the gas temperature inside and outside the lengths there should be spectral steepening across the relic, high edgetoconfirmthatthisisashockfront.Forashock,weexpectthe polarisationlevels,anddiffuseemissionassociatedwiththeICM. temperaturetobehigheronthedenseside,whileforacoldfront Furthermore,thereshouldbeshocksignaturesintheX-rayemis- (another type of the density discontinuity commonly observed in sionfrom theICM. Weassert thatregions AandB ofthe Bullet clusters),thetemperaturejumpwouldhavetheoppositesign.We cluster,whichhavetheseproperties,arearadiorelic.Asaconse- fitthespectrafortheregionsinsideandoutsidetheedge,asshown quencetheBulletclusterbecomesoneofonly≈15objectsknown intheleftpanelofFigure8(asubregionofthestripusedforthe tohostbotharadiorelicandaradiohalo.Unlikeotherrelics,how- brightnessprofile).TherightpanelofFigure8showsthebest-fit ever,thisrelichastwodistinct,butphysicallyconnectedregions, values and 90% uncertainties, which include the background un- which may provide interesting clues on the nature of peripheral certainty(Section2).Whilethetemperatureinthedenserregionis relicsingeneral,aswewilldiscussbelow. high,thetemperatureoutsidetheedgeisessentiallyunconstrained, Theentirerelicstructurehasamaximumlengthof≈930kpc sowecannotunambiguouslyconfirmthatthisisashockbasedon (210(cid:48)(cid:48)),amaximumwidthof≈265kpc(60(cid:48)(cid:48))andapowerof2.3± thesignofthetemperaturejump.However,withsuchahightem- 0.1×1025W/Hz(log10P1.4=25.36±0.02W/Hz)whichmakesthis peraturemeasuredonthedenseside,forthisedgetobeacoldfront oneofthemostpowerfulradiorelicsknown.Unliketypicalradio wouldrequirethatthegasoutsidehasT >20keV,whichisquite relics,whichhaveafairlyuniformsurfacebrightnessdistribution implausibleatsuchdistancesfromthecentre.Thus,thefeatureis (e.g.Ferettietal.2012),forthisrelic94%oftheobservedfluxis almost certainly a shock. This shock is opposite the well-known, fromthenorthernbulb(regionA).Furthermore,regionA,witha westernshock(Markevitchetal.2002),similartothepairofshocks largestlinearsizeofjust330kpc(75(cid:48)(cid:48)),hasanexceptionallyhigh ontheoppositesidesoftheclustermergerseeninA2146(Russell surfacebrightnesscomparedtobothknownradiorelicsandtothe etal.2010,2012).Ifthisisindeedashock,a90%upperlimitonthe faintsouthernregionBoftheBulletclusterrelic(seeFigure9).An temperaturejumpacrossthisedgeisafactorof5.3(takingintoac- intriguingpossibilityisthattheexceptionallyhighfluxinregionA counttheasymmetricerrorsonthetwotemperatures).Whilethese isduetoalarge,pre-existing,populationofrelativisticelectronsin are projected temperatures, the brightness contrast is sufficiently thisregion.Thisexistingpopulationcouldbetheremnantsofara- high and the errors sufficiently large for the projection effects to diogalaxythathavebeenreacceleratedbythesameshockthatcre- beunimportant.ThisconstrainstheMachnumberatthehighend atedtherelicinregionB.Iftheremnantsoftheradiogalaxywere betterthanthedensityjumpdoes;ourfinalvalueisM≈2.5+1.3. reacceleratedthentheiroriginalstructureandpropertieswouldbe −0.8 (cid:13)c 0000RAS,MNRAS000,000–000 6 Shimwelletal. -2 -4 -4 -3 -5 -5 -6 -6 -4 -7 -7 1 Mpc 1 Mpc Figure6.Left:the1.1-3.1GHzprimarybeamcorrectedcontinuumemissionintheregionoftheBulletclusterataresolutionof≈13(cid:48)(cid:48) andwithagrey scaleinlog(S/[Jy/beam]).Centre:acolourscaleimagemadefrom500ksofChandraACIS-Idatafromthe0.8-4keVband(seeSection2)binnedtohavea pixelsizeof3.936(cid:48)(cid:48)withunitsinlog(Sx/[counts/s/(cid:48)(cid:48)2]).Right:theX-rayimagefromthecentrepaneloverlaidwiththeATCAimagewithcontourlevelsof (30,60,120,240,480,960)×1µJy.Inallimages,a2(cid:48)-widerectangularstrip,whichpassesoverthesouthernregionoftherelic,showstheregionusedtoextract theX-raysurfacebrightnessprofile.TheX-rayandATCAimagesaredisplayedwiththesameorientationandfieldofview. -4 2 1 -5 -6 -7 1 Mpc Figure8.Left:TheX-rayimagedisplayedinFigure6withaboxshowingthetworegionsinwhichthetemperaturewasdetermined.Right:Temperature measurementseithersideoftherelic(x=0(cid:48)(cid:48))derivedfromthetworegionsgivenintheleftpanel,theerrorsare90%confidenceandincludethebackground uncertainty. disturbed and they may attain the characteristic alignment in the butrelicsthatareobservedtobeelongated,suchasthisproposed magneticfieldaswellasgradientinspectralindexthatisexpected relic, have a mean spectral index of ≈−1.3 (Feretti et al. 2012). forarelic.Theunusualpropertiesofthisradiorelicmayreveala From our measurements of the spectral index, we determine the linkbetweenradiogalaxiesandradiorelics. expectedMachnumberoftheshockthatproducedtherelic(Bland- ford&Eichler1987)as Whilsttheflux-distributionofthisradiorelicisunusual,the linearmorphologyoftheemissionisverysimilartotheToothbrush M2+1 δ =2 +1, (1) relicthatisassociatedwiththegalaxycluster1RXSJ0603.3+4214 M2−1 (seevanWeerenetal.2012).HydrodynamicalN-bodysimulations where δ is the average power law index of the energy spectrum ofthatradiorelicbyBru¨ggen,vanWeeren,Ro¨ttgering(2012)re- ofemittingelectrons,includingtheageingeffectthatgives“+1” vealedthatalinearshockcouldbereproducedwithathree-cluster in the equation (Ginzburg & Syrovatskii 1964), and is related to merger consisting of two equal-sized clusters and a third much the integrated spectral index by −α =(δ−1)/2. For regions A smallercluster.ForthisrelicintheBulletcluster,thelinearshape andBwederiveMachnumbersofM=5.4+1.7 andM=2.0+0.2, does not necessitate the existence of a third cluster, because the −0.9 −0.1 respectively.TheMachnumberforregionBisingoodagreement X-ray image shows that the shock front is likely a segment of a withtheX-rayderivedMachnumberforthesameregionasderived larger-scalesurfacebrightnesscontourwithalargelocalradiusof inSection3.3,consistentwiththeradioemissionbeingtheresult curvature,whichdoesnotdeviatemuchfromthegeneralaxialsym- offirstorderFermiaccelerationontheICMshock. metryofthecluster. We have been unable to measure the polarisation properties Relicshavesteepspectralindicesintherange−0.9to−2.8, of region B, but within region A, we find that the 1.4GHz frac- (cid:13)c 0000RAS,MNRAS000,000–000 7 (2008)andBourdinetal.(2013)detectanX-raybrightnessedge 42.5 and temperature jump in Abell 521. Ogrean et al. (2013b) detect shocksnearthetworelicsin1RXSJ0603.3+4214(noneofwhich, 42.0 however,havebothunambiguoustemperatureandsurfacebright- ness jumps). Akamatsu & Kawahara (2013) found temperature ) (erg/s)41.5 ((bthuetnSoatussuagrfeacreelbicri)ghatnndestsh)edissocuotnhtineuaisttieersninreCliIcZAin2A24b2e.l8l+35636071. 2S Finoguenovetal.(2010)andAkamatsuetal.(2012)detectedboth LL41.0 g( P()/νν40.5 ttehmenpoerrathtuwreeastnedrnsurreflaicceinbrAigbhetlnle3s6s6d7i.scFoinnatilnlyu,itfioerstahtethCeolmocaactilounstoefr o relic,Ogrean&Bru¨ggen(2013c)detectatemperature(butonlya L hint of density) discontinuity using XMM. Using Suzaku for the 40.0 samerelic,Akamatsuetal.(2013b)reportedboththedensityand temperaturejumpsconsistentwithXMM,whileSimionescuetal. 39.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (2013)reportedasteeperdeclineofthesurfacebrightness,which Distance from cluster center (Mpc) made it impossible to detect the pre-relic emission and derive its Figure9.Theradioluminosity(νP(ν),whereν=1.4GHz)perunitrelic temperature.(At2Mpcfromtheclustercenter,theComarelicis surfacearea.KnownrelicsfromFerettietal.(2012)areshownwithblack moredifficulttostudyinX-raysthanothers.)TheseX-rayresults, circles.Theblue,greenandredcrossesshowthepropertiesoftheBullet albeitatdifferentlevelsofconfidence,supporttheideathatX-ray clusterrelicsregionsA+B,AandB,respectively.Forpreviouslyknown shocksinfluenceradiorelicsintheclusterperipheralregions. relicsthelargestlinearsize(LLS),poweranddistancefromtheclustercen- treweretakenfromFerettietal.(2012).Thisplotissimilartothatpre- sentedbyBrunetti&Jones(2014),buttohighlighttheuniquepropertiesof theproposedBulletclusterrelic,wehaveincludedalltherelicsgivenby 4.1 Sourceofseedelectronsforreaccelerationinperipheral Ferettietal.(2012)eventhoughsomedetectionsareambiguous(seeNuza relics? etal.2012). These new observations pose a clear question: what is the differ- encebetweentheeasternandthepreviouslyknownwesternshock tionalpolarisationislowerthanthe10-30%typicallyobservedin intheBulletcluster?Bothshockshavesimilar,ratherlowX-rayde- relics (see e.g. Ferrari et al. 2008). However, similarly to other rivedMachnumbers,M=3.0±0.4forthewesternshock(Marke- relicswefindthattheobservedfractionalpolarisationchangessig- vitch2006)andM≈2.5forregionBofournewrelic.Thewestern nificantlyasafunctionoffrequency(seeFigure4),withthefrac- shock occurs in a higher density environment, and the study by tionalpolarisedintensityincreasingwithfrequency(seee.g.Pizzo Shimwelletal.(2014)showedthattheradiohaloemissioninthe et al. 2011 and van Weeren et al. 2012). At 1.7GHz, our detec- Bullet cluster is certainly influenced by this shock, but the radio tionofpolarisedemissionisatlowsignificance;atthisfrequency emission at the shock front is very weak and polarised emission thefractionalpolarisationis3.0±0.2%butat2.7GHzthefraction was not detected at all (the spectral index has also remained un- is 12.1±0.9%. By comparison, van Weeren et al. (2012) found determinedinthisregionduetothefaintnessoftheemission).By that in the brightest region of the Toothbrush relic the fractional comparison,theradiorelicemissionfromregionAoftherelicis polarisationat4.9GHzis15–30%butat1.4GHzitisbelow1%, fromaregionofverylowdensity,yettheemissionisverystrong, they suggested the depolarisation is caused by the ICM. For sev- has a distinctive spectral index gradient, and is significantly po- eralradiorelics,thede-rotatedpolarisationanglesarealignedto- larised.EventheweakemissionfromregionBoftherelicissig- wardstheclustercentre(seee.g.vanWeerenetal.2010).Weare nificantlybrighterthantheradiohaloemissionatthewesternshock onlyabletomeasurethepolarisationangleinregionAoftherelic, front. where the polarised intensity is highest, and find that the polari- For shocks of similar Mach numbers, the acceleration effi- sation angle is approximately aligned with the axis of the cluster ciencyshouldbesimilar.Therefore,theresultingsynchrotronradio merger(E-W).Giventhattherotationmeasureoftherelicpeaksat brightnessshouldbeproportionaltothedensityoftheseedelec- φ=−16rad/m2,correspondingtoarotationofthepolarisationan- trons that the shock accelerates, thus we may look there for the gleby14degreesanticlockwise,thisindicatesthattheB-vectoris explanation(ignoringforamomentthepossibledifferenceinthe alignedapproximatelyparalleltotheshocksurfaceasexpectedfor magneticfieldstrength).WerecallthatregionAofourreliclooks ashockfrontcompressingarandommagneticfield(seeFigure3). likeanoldradiogalaxyre-energized(itsagedelectronsreacceler- Thisalignmentisbestinthenorthernregionoftherelicwherethe ated) by a shock passage. While the tail B is coincident with the polarisedemissionisbrightest.SouthofthistheB-vectorbeginsto X-rayshockfront,wealsonoteditspossiblephysicalconnection deviatefromtheexpectedanglebutthiscouldbeduetoRMvaria- tothebulbA(Section3.1),withthepolarizedemissionextending tionsacrosstherelicthatwehavenotmeasured.Weareunableto fromthebulbintothetail(Figure3).Thisisastronghintthatseed distinguishtherotationmeasureoftherelicfromtheGalacticro- electrons in the tail could have been supplied by that same radio tationmeasurecontributioninthisdirection(+4.8±44.0rad/m2; galaxy. In this scenario, the radio galaxy remnant may have pol- Oppermannetal.2012),but,inthisregion,theerrorintheGalactic luted a peripheral region of the ICM (regions A+B), which may rotationmeasureisespeciallyhighduetothelowdensityofmea- presentlycontainamixtureofrelativisticandthermalICMgasbut surementsatdeclinationslessthan-40◦inthestudybyOppermann still remain well-defined spatially. The density of the aged rela- etal.(2012). tivisticelectronstherewouldbemuchhigherthanintherestofthe OnlyahandfulofotherexamplesofX-rayshocksatthepo- cluster(andinparticular,intheregionofthewesternshock),anda sitionsofrelicsexist.Macarioetal.(2011)detectsurfacebright- shockpassagewouldgenerateasynchrotronfeaturespatiallytrac- ness and temperature discontinuities in A754. Giacintucci et al. ingthatregion.Theregioncontainingtheoldradiogalaxyplasma (cid:13)c 0000RAS,MNRAS000,000–000 8 Shimwelletal. mayalsohaveahigherthanaveragemagneticfield,furtherincreas- erfulthanexpectedrelicsexistinsomelow-mass,low-luminosity ingthesynchrotronbrightness. clusters(e.g.Brown&Rudnick2009). Most other peripheral relics do not show such a sugges- Thescenariowheretheradiorelicsresultfromashockfront tive connection to a likely old radio galaxy, with notable excep- re-energizingtheagedradioplasmaremainingfromalocalradio tionsincludingtheComarelic(Giovannini,Feretti,&Stanghellini galaxy was proposed before by Ensslin & Gopal-Krishna (2001, 1991 and Enßlin & Bru¨ggen 2002) and the northern relic in hereafter E01) and most recently by Bonafede et al. (2014), us- PLCKG287.0+32.9(Bonafedeetal.2014),whichapparentlycon- ingdifferentphysicalmechanisms.Inthe“phoenix”modelofE01 nects to the lobes of a radio galaxy. However, many relics do (see also Enßlin & Bru¨ggen 2002), a cocoon of aged relativistic show features that are difficult to explain in the simple scenario plasma,nolongeremittingattheobservableradiofrequenciesand where the shock (re-)accelerates seed electrons uniformly dis- inpressureequilibriumwiththesurroundingthermalICM,isadi- tributed throughout the ICM, or seed electrons coming directly abaticallycompressedbyashockfrontthatpropagatesintheICM from the thermal pool. Peripheral relics often have well-defined but skips the cocoon because of much higher sound speed there. boundariesandashapeinconsistentwiththeexpectedlens-likeor This shifts the exponential cutoff of the synchrotron spectrum of “umbrellainprojection”shapeswithtaperedbrightnessattheends theagedelectronstohigherfrequencies.Toexplaintheobserved expected in this scenario. For example, van Weeren et al. (2011) power-law shape of the relic spectra in the usual radio band, this triedtomodeltheSausagerelicwithashockfrontpassingthrough frequency increase must be large, requiring a large compression seedparticlesdistributedproportionallytotheICMdensity.Quot- factor (ν ∝ρ4/3 for simple assumptions about the magnetic ingtheirwork:“Oneofthemostintriguingpropertiesofthenorth- peak field compression, e.g., Markevitch et al. 2005). The “phoenix” ernrelicinCIZAJ2242.8+5301isitsverynarrowwidthandrather modeltakesfulladvantageofthefactthatadiabaticcompression uniformluminosityalongitsextent”,whichtheirsimulationscould factor can be much higher than shock compression for the same notreproduce.If,instead,theshapeoftherelictracestheunder- increaseofpressure;theformerisρ ∝ p3/4 andunlimited,while lying region of excess density of seed electrons (remaining from thelatterislimitedbyfactor4.Still,toexplaintheobservedspec- anoldradiogalaxylobe),thismaymoreeasilyexplaintheradio trum of the Coma relic, E01 considered a shock with a factor 40 morphologyoftheSausage,Toothbrush,andother,moreirregular pressure jump, corresponding to a rather high M ≈6, which is relics,fewofwhichlookliketheclean,smoothICMshockfronts much higher than the recent X-ray results for Coma (e.g Ogrean seen in X-rays. In fact, at one end of the Sausage relic, there is &Bru¨ggen2013cderiveapressurejumpofatmostafactorof5) a radio galaxy, apparently disconnected from the relic at present, andingeneralnotobservedinanyclustersofar.Forourshockat butwhichcouldinthepasthaveproducedtheunderlyingrelativis- thepositionoftheBulletrelic,thepressurejumpisatmostafactor ticmaterialfortherelic.InournewrelicintheBulletcluster,we 15(M<3.8,seeSection3.3);furthermore,adistinctcocoonatthe mayhavebeenluckytocatchthe“smokinggun”—whentheradio position of the shock would create a noticeable X-ray cavity and galaxystillhasarelativelycompactanddensecomponent,whilein makeitdifficulttoobserveacharacteristicsharpX-raybrightness otherclusters,theremaysimplynolongerbeanyvisualcluesofa edgeshowninFigure7. pastradiogalaxybecauseithasbeencompletelydisrupted. Apossibleproblemforthisscenarioishowsuchwell-defined, A more attractive mechanism is shock reacceleration of the megaparsec-sizedregionsofthethermal/relativisiticgasmixture fossil electrons mixed with the ICM (e.g., Ensslin et al. 1998; could survive the merger events. However, consider that clusters Markevitch et al. 2005; Kang & Ryu 2011; Kang, Ryu, & Jones havestrongradialgradientsoftheICMspecificentropy.Insucha 2012;Pinzke,Oh,&Pfrommer2013).Itisefficientevenforrel- stratified,convectivelystableatmosphere,arelativelysmall(com- atively low Mach numbers typically seen in clusters, and it nat- paredtotheclustersize)radiogalaxyintheperipherycanbemixed urally produces power-law synchrotron spectra (with slopes con- with the ICM by a merger. Once the merger induced disturbance sistentwiththeX-rayshockMachnumberforwell-observedand subsides, the polluted region of the ICM is likely to spread as a well-modeled shocks — i.e., those with clear geometry and both pancake, or a sausage, along an equipotential surface that corre- thetemperatureanddensityjumpsmeasuredinX-ray,e.g.,Giac- spondstoitsspecificentropy.(Thisprocessmayalsostretchtheini- intucci et al. 2008; Macario et al. 2011; Bourdin et al. 2013). So tiallyrandommagneticfieldpreferentiallyalongthesamesurface). far,simulationsofrelicformationbyreaccelerationinjectthefos- Attheseoff-centerdistances,suchsurfacesareapproximatelythe silelectronsthroughaseriesofshocksinthecluster(Pinzke,Oh, sphericalshells.Inthispicture,therelicregionBisolderthanthe &Pfrommer2013)butBonafedeetal.(2014)notedaconnection regionAandalreadyhadtimetomixandspread,whileAisrela- oftherelicinPLCKG287.0+32.9withlobesofaradiogalaxyand tivelynew—thatis,AisthesmokinggunandBissmokefrom suggestedthatthatgalaxymightbethesourceoffossilplasmafor thepreviousshot.Theperiodbetweenthedeathoftheradiogalaxy reacceleration.OurscenariofortheBulletclusterrelicissimilar; and its complete disruption should be relatively short in a merg- weextenditbyspeculatinghowsuchregionsofthepollutedICM ingcluster.Ifashockfrontpassesacrossthepollutedregionofthe maynaturallyassumethegeometricshapeoftheobservedrelics. ICM,itwillreaccelerateitsrelativisticparticlesandcreateagiant, arc-like relic. It would be very interesting to check this scenario ComparingthetwoBulletclustershocksingreaterdetailwill withnumericalsimulationswhich,sofar,primarilyassumearela- assistinourunderstandingoftherelationshipsbetweenradiorelics, tivelyuniformdistributionofseedelectrons(e.g.Vazzaetal.2012, radio halos and radio sources. Unfortunately, the present genera- Skillmanetal.2013andPinzke,Oh,&Pfrommer2013).Wealso tionofX-raytelescopesareunlikelytoprovidesignificantlymore notethatlarge,coherentperipheralrelicssuchastheSausageand informationaswearealreadyexploiting500ksofChandradata. ournewrelicarequiterareeveninmergingclusters,whichmight Onepossibilitythatmaybecomeavailableistousehighresolution beexplainedbytherequiredspatialcoincidenceofashockpassing SunyeavZel’dovich(Sunyaev&Zel’dovich1972)observationsto across a region of seed-rich plasma in the cluster periphery. Fur- searchforpressuresubstructuresatthepositionoftheshocks(e.g. thermore,thisexplanationcanjustifywhysignificantlymorepow- PlanckCollaborationetal.2013). (cid:13)c 0000RAS,MNRAS000,000–000 9 5 CONCLUSIONS BourdinH.,MazzottaP.,MarkevitchM.,GiacintucciS.,Brunetti G.,2013,ApJ,764,82 WehaveanalysedbothradioandX-rayobservationsoftheBullet BonafedeA.,IntemaH.T.,Bru¨ggenM.,GirardiM.,NoninoM., clusterandintheperipheralregionofthecluster,wefindapow- KanthariaN.,vanWeerenR.J.,Ro¨ttgeringH.J.A.,2014,ApJ, erfulradiorelic.WehavefoundasharpedgeintheX-raysurface 785,1 brightnessattheregionoftheradiorelicwhichisasignatureofa CavaliereA.,Fusco-FemianoR.,1978,A&A,70,677 likelyshockfromintheICM.Ourobservationssupportthetheory Clowe D., Bradacˇ M., Gonzalez A. H., Markevitch M., Randall thatshocksintheperipheralregionsofgalaxyclusterscancreate S.W.,JonesC.,ZaritskyD.,2006,ApJ,648,L109 radiorelics.Fromourstudyourmainconclusionsarethefollow- EnsslinT.A.,BiermannP.L.,KleinU.,KohleS.,1998,A&A,332, ing: 395 • Thelocation,spectralandpolarimetricpropertiesoftheelon- EnßlinT.A.,Gopal-Krishna,2001,A&A,366,26 gatedradioemissiontotheeastoftheBulletclusterareconsistent EnßlinT.A.,Bru¨ggenM.,2002,MNRAS,331,1011 withthepropertiesexpectedforaradiorelic. FerettiL.,GiovanniniG.,GovoniF.,MurgiaM.,2012,A&ARev, • Therelic1.4GHzpowerof2.3±0.1×1025W/Hzmakesthis 20,54 oneofthemostpowerfulradiorelicsknown. Ferrari C., Govoni F., Schindler S., Bykov A. M., Rephaeli Y., • The morphology of the Bullet cluster relic is reminiscent of 2008,SpaceSciRev,134,93 the “toothbrush-relic” associated with the galaxy cluster 1RXS Finoguenov A., Sarazin C. L., Nakazawa K., Wik D. R., Clarke J0603.3+4214(seevanWeerenetal.2012). T.E.,2010,ApJ,715,1143 • TheX-rayimageshowsanICMdensityedgeexactlytracing GiacintucciS.,etal.,2008,A&A,486,347 thetailregionoftherelic,whichsuggeststhatthereisashockfront Ginzburg,V.L.,&Syrovatskii,S.I.1964,TheOriginofCosmic atthatlocation.Ifthisisindeedashock,itsMachnumberderived Rays(Pergamon) fromtheX-raysisinagreementwiththeradiospectralindexunder JohanssonD.,etal.,2012,A&A,543,A62 theassumptionoffirstorderFermishockacceleration. SkillmanS.W.,XuH.,HallmanE.J.,O’SheaB.W.,BurnsJ.O., • Therelicconsistsofthetail,coincidentwiththeX-rayshock, LiH.,CollinsD.C.,NormanM.L.,2013,ApJ,765,21 andananomalouslybright“bulb”regionthatappearstobephys- GiovanniniG.,FerettiL.,StanghelliniC.,1991,A&A,252,528 icallyconnectedtothetail.Thebulbmaybearemnantofaradio HickoxR.C.,MarkevitchM.,2006,ApJ,645,95 galaxythathassuppliedseedelectronsforshockaccelerationinthe HongS.E.,RyuD.,KangH.,CenR.,2014,ApJ,785,133 relic.Suchascenariomaybeapplicabletootherperipheralrelics, KangH.,RyuD.,2011,Ap&SS,336,263 wheresucha“smokinggun”isnotobviousduetoagreaterageof KangH.,RyuD.,JonesT.W.,2012,ApJ,756,97 thestructureorastrongerclusterdisturbance.Thiswouldexplain LiangH.,HunsteadR.W.,BirkinshawM.,AndreaniP.,2000,ApJ, certainpeculiarfeaturesoftheperipheralrelics. 544,686 LiangH.,EkersR.D.,HunsteadR.W.,FalcoE.E.,ShaverP.,2001, MNRAS,328,L21 MacarioG.,MarkevitchM.,GiacintucciS.,BrunettiG.,VenturiT., 6 ACKNOWLEDGEMENTS MurrayS.S.,2011,ApJ,728,82 MarkevitchM.,etal.,2000,ApJ,541,542 The Australia Telescope Compact Array is part of the Australia MarkevitchM.,etal.,2002,ApJ,567,L27 Telescope National Facility which is funded by the Common- MarkevitchM.,GovoniF.,BrunettiG.,JeriusD.,2005,ApJ,627, wealth of Australia for operation as a National Facility managed 733 byCSIRO.B.M.G.acknowledgesthesupportofAustralianLaure- MarkevitchM.,2006,ProceedingsoftheTheX-rayUniverse2005, ateFellowshipFL100100114fromtheAustralianResearchCoun- 604,723 cil.M.J.-H.acknowledgessupportfromtheMarsdenFund.M.J.-H. 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