Another shock for the Bullet cluster, and the source of seed electrons for radio relics

2.9 MB·English

by  Timothy W. Shimwell

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
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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.
MarkevitchM.,VikhlininA.,2007,PhysRep,443,1
acknowledgessupportfromtheMarsdenFund.WethankDouglas
MatthewsT.A.,MorganW.W.,SchmidtM.,1964,ApJ,140,35
Clowe for kindly providing the R-band image and we thank the
Nuza S. E., Hoeft M., van Weeren R. J., Gottlo¨ber S., Yepes G.,
anonymousrefereeforcomments.
2012,MNRAS,420,2006
OgreanG.,Bru¨ggenM.,SimionescuA.,Ro¨ttgeringH.,vanWeeren
R.J.,CrostonJ.H.,HoeftM.,2013,AstronNachrichten,334,
342
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