The radio relic in Abell 2256

18 Pages·2015·1.79 MB·English

A&A575,A45(2015) Astronomy
DOI:10.1051/0004-6361/201423972 &
(cid:2)c ESO2015 Astrophysics
The radio relic in Abell 2256:
(cid:2)
overall spectrum and implications for electron acceleration
M.Trasatti1,H.Akamatsu2,L.Lovisari1,U.Klein1,A.Bonafede3,M.Brüggen3,D.Dallacasa4,5,andT.Clarke6
1 ArgelanderInstitutfürAstronomie,UniversitätBonn,AufdemHügel71,53121Bonn,Germany
e-mail:[email protected]
2 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands
3 HamburgerSternwarte,UniversitätHamburg,Gojenbergsweg112,21029Hamburg,Germany
4 DipartimentodiAstronomia,UniversitàdiBologna,viaRanzani1,40127Bologna,Italy
5 INAF–IstitutodiRadioastronomia,viaGobetti101,40129Bologna,Italy
6 NavalResearchLaboratory,4555OverlookAve,SW,DC20375Washington,USA
Received10April2014/Accepted1November2014
ABSTRACT
Context.Radiorelicsareextendedsynchrotronsourcesthoughttobeproducedbyshocksintheoutskirtsofmerginggalaxyclusters.
TheclusterAbell2256hostsoneofthemostintriguingexamplesinthisclassofsources.Ithasbeenfoundthatthisradiorelichas
aratherflatintegratedspectrumatlowfrequenciesthatwouldimplyaninjectionspectralindexfortheelectronsthatisinconsistent
withtheflattestallowedbythetestparticlediffusiveshockacceleration(DSA).
Aims.WeaimattestingtheoriginsoftheradiorelicinAbell2256.
Methods.Weperformednewhigh-frequencyobservations at2273, 2640, and4850MHz.Combiningthesenewobservationswith
imagesavailableintheliterature,weconstraintheradio-integratedspectrumoftheradiorelicinAbell2256overthewidestsampled
frequencyrangecollectedsofarforthisclassofobjects(63−10450MHz).Moreover,weusedX-rayobservationsoftheclusterto
checkthetemperaturestructureintheregionsaroundtheradiorelic.
Results.Wefindthattherelickeepsanunusually flatbehavioruptohighfrequencies. Althoughtherelicintegratedspectrumbe-
tween 63 and 10450 MHz is not inconsistent with a single power law with α10450 = 0.92±0.02, we find hints of a steepening
atfrequencies>1400MHz.Thetwofrequencyranges63−1369MHzand13696−310450MHzare,indeed, bestrepresentedbytwo
differentpowerlaws,withα1369 = 0.85±0.01andα10450 = 1.00±0.02.Thisbrokenpowerlawwouldrequirespecialconditions
63 1369
tobeexplainedintermsoftest-particleDSA,e.g.,non-stationarityofthespectrum,whichwouldmaketherelicinA2256arather
youngsystem,and/ornon-stationarityoftheshock.Ontheotherhand,thesinglepowerlawwouldmakeofthisrelictheonewiththe
flattestintegratedspectrumknownsofar,evenflatterthanwhatisallowedinthetest-particleapproachtoDSA.Wefindaratherlow
temperatureratioofT /T ∼1.7acrosstheGregionoftheradiorelicandnotemperaturejumpacrosstheHregion.However,inboth
2 1
regionsprojectioneffectsmighthaveaffectedthemeasurements,therebyreducingthecontrast.
Keywords.galaxies:clusters:general–galaxies:clusters:individual:Abell2256–accelerationofparticles
1. Introduction lifetimeoftheemittingelectronsimpliestheneedforsomeform
of in situ productionor (re-)accelerationof the electrons in all
A fraction of galaxy clusters exhibit diffuse Mpc-scale syn- thesesources,eventhoughtheunderlyingphysicalmechanisms
chrotronemission(referredtoasradiohalosandradiorelics)not are thought to be different for the different classes of sources.
related to any particular cluster galaxy (for reviews see Feretti Moreover,thesediffuseradioemittingregionsaremostlyfound
etal.2012;Brüggenetal.2012).Thisemissionmanifestsitself inunrelaxedclusters,suggestingthatclustermergersplayakey
bythepresenceofrelativisticelectrons(∼GeV)andweakmag- roleinproducingthem.
netic fields (∼μG) in the intracluster medium (ICM), together
Radio gischt are large, extended arc-like sources, believed
with the hot thermal plasma emitting X-rays. Radio halos per-
to be synchrotron emission from electrons accelerated or re-
meate the central Mpc3 of galaxy clusters and the radio emis- accelerated in merger or accretion shocks through diffusive
sionusuallyfollowstheroundishX-rayemissionfromthether-
shock acceleration (DSA, Fermi-I process; see Ensslin et al.
mal gas. Radio relics are more irregularly shaped and are lo-
1998; Kang & Ryu 2011). A textbook example of such gi-
cated at the clusters periphery. They are usually further subdi-
ant radio relic has been observed in the galaxy cluster CIZA
videdintothreeclasses:radiogischt,radiophoenices,andAGN J2242.8+5301(vanWeerenetal.2010;Stroeetal.2013).Radio
relics(seeKempneretal.2004),dependingontheircharacteris-
phoenices are believed to be the result of the re-energization
ticsandproposedorigin(asdescribedbelow).Thecombination
viaadiabaticcompression,triggeredbyshocks,offossilplasma
oftheMpcsizeofsuchsourcesandtherelativelyshortradiative fromswitched-offAGNradiogalaxies(Enßlin&Gopal-Krishna
2001;Enßlin&Brüggen2002).TherelativisticplasmaofAGN
(cid:3) AppendixAisavailableinelectronicformat origin had the time to age and without the re-energization
http://www.aanda.org would not longer be visible at the currently observable radio
ArticlepublishedbyEDPSciences A45,page1of18
A&A575,A45(2015)
frequencies. An example of a radio phoenixhas been found in spectrumathighenergiesupward,withoutmodifyingthespec-
the galaxy cluster A2443 (Cohen & Clarke 2011). AGN relics tral slope. Even steeper spectra are expected in case of AGN
are indeed such fossil radio galaxies where the AGN switched relics.TheaveragespectralindicesreportedinTable4ofFeretti
off more recently and no re-energizationoccurred.The plasma et al. (2012) for integrated spectra with measured steepening,
is still emitting at observable radio frequencies, and it simply rangefrom1.7to2.9.
evolvespassivelyuntilitbecomesinvisibleintheradiowindow With these ingredients, detailed studies of the integrated
(Komissarov&Gubanov1994). spectrum and of the spectral-index distribution across the
sources,allowustotestthecurrentmodelsandstudytheshock
propertiesincaseofDSA.Thisisaccomplishedbyobservations
Modelsanddiagnostics
made over a broad range of frequencies. However,an accurate
The proposed formation mechanisms differ in the predictions measurementoftheintegratedspectraofradiorelicsisadifficult
of the morphological and spectral characteristics of the differ- task.Thesesourcesusuallycontainanumberofdiscretesources,
ent classes of relics. DSA of both thermal and pre-accelerated whosefluxdensityneedstobecarefullysubtractedfromthetotal
electrons,shouldproducelargerandmoreperipheralstructures, diffuseemission.Thisrequireshigh-resolutionimagingatmany
with strong polarization, and pure power-law integrated syn- frequenciesusingradiointerferometers.However,increasingthe
chrotron spectra (Brüggen et al. 2012). Fermi processes natu- observing frequencies, interferometers encounter the technical
rally predict an injection power-law energydistribution for the problemofthemissingshortspacingsthatmakesthem“blind”
acceleratedelectronpopulationoftheform1 f(E)∝ E−δinj.From toveryextendedstructures.Ontheotherhand,singledishesare
synchrotrontheory,theemissionproducedbythispopulationof optimalto catch all the emission from a field but they lack an-
electronsisalsodescribedbyapowerlaw2, gularresolution.Indeed,integratedspectraoverawiderangeof
frequenciesare available in the literature only for few of these
δ −1 objects(seeFerettietal.2012).
S(ν)∝ν−αinj with αinj = inj2 · (1) An independentmeasureofthepropertiesofshocksispro-
videdbydeepX-rayobservations.Throughthemeasurementsof
Emitting particles are naturally subject to energylosses. These temperatureand/orpressurejumpsatthelocationoftheshock,
lossesaregovernedbymanyphysicalfactorssuchastheprop- properties such as the shock Mach number M and the shock
ertiesofthemagneticfield(seeKardashev1962;Jaffe&Perola compression ratio C can be inferred (see review by Brüggen
1973;Komissarov&Gubanov1994,foradescriptionofthedif- etal. 2012).Inthe testparticlesapproximation3ofDSA, if the
ferentmodelsofelectronaging).Theabsenceofanyconstantin- particlediffusionisspecified,theshockMachnumberisthepri-
jectionofnewelectronswouldleadtoacutoffinthehigh-energy maryparameterthatdeterminestheefficiencyoftheacceleration
regionoftheintegratedspectrum,movingtowardlowerfrequen- mechanismandtheenergydistributionoftheparticlesatinjec-
cies in the course of time. The presence of constant injection tion (Kang & Ryu 2010). In this case, a simple direct relation
of particles with the same energyspectrum, on the other hand, betweentheshockMachnumber M andtheinjectionindexδ
wouldeventuallymaskthecutoff,leadinginsteadtoabreakwith oftheenergyelectronsdistributionexists: inj
a changeof0.5inthe spectralindexoftheintegratedemission
(continuousinjectionmodel, Kardashev1962): 2(M2+1)
δ = · (3)
inj (M2−1)
α =α +0.5. (2)
obs inj
However, radio relics are usually observed in the outskirts
Thisconditiontranslates,incaseofDSA,intheassumptionthat of clusters where the very low density of electrons (ne <
the properties of the shock remain unchanged (stationarity for 10−4 cm−3) make the detection of shocks in the X-ray very
theshock).IftheshockhasbeenpresentintheICMfora time challenging(Akamatsu&Kawahara2013).Indeed,afewclear
exceeding the electron cooling time, a single power law with X-ray shock detections are known in the literature (see review
spectral index α is expected for the integrated radio spec- by Brüggen et al. 2012). In conclusion, multifrequency radio
obs
trum (stationarity for the spectrum). The observed spectral in- measurements, combined with deep X-ray observations, allow
dicesreportedin Table 4of Ferettietal.(2012)forstraightin- asearchforandaproperstudyoftheseshockstotesttheshock-
tegratedspectrarangefrom1.1to1.6.Agradientisexpectedin originmodelforrelics.
thespectral-indexdistributionacrossthesource,withtheflattest
valuesmarkingthepositionoftheshockfrontwheretheparticle
ThecaseofAbell2256
getaccelerated,andthesteepeningshowingtheradiativelosses
astheelectronsareadvectedawayfromtheshock.Suchagradi- One of the most intriguing clusters hosting both a radio relic
entisclearlyobservedintheradiorelicinCIZAJ2242.8+5301 and a radio halo is the galaxy cluster A2256 (z = 0.058).
(vanWeerenetal.2010;Stroeetal.2013). The radio relic emission in this cluster differs in many aspects
In case of revival via adiabatic compression of old radio from the textbook examples of radio gischt in merging clus-
plasma left behind by radio galaxies and pushed towards the ters, e.g.,in CIZA J2242.8+5301(vanWeeren etal. 2010)and
cluster outskirtsby buoyancy,we expectinsteadmore filamen- in A3376 (Bagchi et al. 2006). The A2256 relic emission is,
tary and smaller radio structures(<50 kpc), again strongly po- indeed, dominated by a complex filamentary structure as con-
larized,butwithsteeperandcurvedintegratedspectraduetothe firmed by new wide-band VLA observations published during
already aged populationof electrons that are re-accelerated.In the reviewing process of the present paper (Owen et al. 2014).
facttheadiabaticcompressionwouldjustshiftthealreadyaged Itis moreovercharacterizedby an unusuallylargeaspectratio,
being nearly as wide as it is long, and by an unusual proxim-
1 The distribution is truncated at high energy by the existence of a ity to the cluster center respect to the majority of giant relics
maximumenergytowhichelectronscanbeaccelerated.
2 The synchrotron spectrum has an exponential cutoff at high fre- 3 WhenthedynamicalfeedbackoftheCosmicRayselectronspressure
quency,reflectingthetruncationintheparticledistribution. isignored.
A45,page2of18
M.Trasattietal.:TheradiorelicinAbell2256:overallspectrumandimplicationsforelectronacceleration
known in the literature. It also shows all typical signatures of arepotentiallysensitivetoemissionsonscaleupto∼13(cid:6)witha
a merging cluster system although its dynamical state is not fullresolutionof∼9(cid:6)(cid:6).
yet completely understood. This cluster has been the first ob- The main limiting factor of the field of view is the effect
served with LOFAR at very low frequencies (20−63 MHz) by of the primary beam attenuation. For the WSRT this can be
vanWeeren etal. (2012).Theycollecteddata upto 1400MHz described by the function cos6(c · ν · r) where r is the dis-
and found a radio-integratedspectrum for the relic that can be tance from the pointing center in degrees, ν is the observing
described by a power law with an unusual flat spectral index frequencyinGHzandtheconstantc=68is,tofirstorder,wave-
α1369 =0.81±0.03.Theoccurrenceofsimilarflatspectralinde- length independentat GHz frequencies(decliningto c = 66 at
63
ceshavebeenreportedbyKale&Dwarakanath(2010)inthefre- 325MHzandc = 63at4995MHz).Theresultingfieldofview
quencyrange 150−1369MHz. Assuming stationaryconditions at 2273 MHz is 0.37◦. In order to image a field big enough to
inthetest-particlecaseofDSA,thiswouldrequireaninjection recovertheextendedemissioninA2256,theobservationswere
spectral index which is not consistent with the flattest possible carried out in the mosaic mode. Three different pointing cen-
injectionspectralindexfromDSA.Indeed,adirectconsequence ters were chosen (details in Table 1). In order to have a good
ofthetest-particleapproachtoDSAisthatinthelimitofstrong uvcoverageforeachpointing,theobservationswereperformed
shocks(M (cid:5)1)theparticleindexδ approachesanasymptotic switchingthetelescopefromonepointingtoanothereveryfive
inj
valueof2.Thismeansthatparticleenergydistributionproduced minutes,havingfourhoursofobservationsforeachpointingfor
bytest-particleDSAcannotbeflatterthan2(itmustbeδ >∼2). a total of twelve hours for the entire cluster. The observations
inj
Asaconsequencethesynchrotronspectraatinjectioncannotbe werecarriedoutonthe25thJanuary2003.Theexcellentphase
flatter than 0.5 (α >∼ 0.5). So, we should not observe relics stability of the system allow us to observe primary calibrators
inj
with spectra α <∼ 1. The flat spectrum could be reconciled onlyatthebeginningandtheendofanobservationtocalibrate
obs
withshockaccelerationiftheshockhasbeenproducedveryre- WSRTdata.3C286and3C48wereobservedforthispurpose.
cently(∼0.1Gyrago)andstationarityhasnotbeenreachedyet. Flagging,calibration,imagingandself-calibrationwereper-
Inthiscasea steepeningoftheintegratedspectrumisexpected formedwiththeAIPS(AstronomicalImageProcessingSystem)
atfrequencies(cid:2)2000MHz. package,withstandardproceduresfollowingtheguidelinepro-
Inthispaperwepresentnewhigh-frequencyradioobserva- vided on the ASTRON web-page5. All the antennas were suc-
tions(Sect.2)ofA2256at2273,2640and4850MHzMHzper- cessful,withsomeoccasionalRFI,flaggedoutintheearlystages
formed both with an interferometer (the Westerbork Synthesis ofdatacalibration.3C286wasusedasthemainfluxdensitycal-
RadioTelescope,WSRT)andasingledish(theEffelsberg100m ibratorusingtheBaarsetal.(1977)scale(taskSETJYinAIPS),
Telescope),complementedbyX-rayobservations(Sect.3)per- whichprovidesfluxdensitiesrangingfrom11.74Jyinthefirst
formedwiththe SuzakuandXMM-Newtonsatellites. InSect. 4 IFto11.39JyintheeighthIF.Thethreepointingswereimaged
wepresentanewdeterminationoftherelicradiospectrumover and self-calibrated separately. For each pointing we performed
thewidestsampledfrequencyrangecollectedsofarforthiskind three phase-only cycles of self-calibration, followed by a final
ofobject(63MHz−10450MHz)4.InSect.5weshowtheICM amplitude and phase self-calibration cycle. The diffuse emis-
temperatureinregionsacrosstheradiorelicemission.InSect.6 sion flux was included in the model for the self-calibration. A
we consider the effect of the thermal Sunyaev-Zeldovich(SZ) multiresolutioncleanwasperformedwithintheIMAGRtaskin
decrement on our flux density measurements at high frequen- AIPStobetterreconstructthecomplexdiffuseemissionpresent
cies.DiscussionandconclusionsarepresentedinSects.7and8. intheclusterinthefinalimagesofthepointings.Imagesofthe
We adopted the cosmological parameters H0 = Stokes parameter I, U and Q were obtained for each pointing
71 kms−1 Mpc−1, ΩΛ = 0.73 and Ωm = 0.27 (Bennett and were then combined together (separately for I, U and Q)
etal.2003),whichprovidealinearscaleof1.13kpcarcsec−1at andcorrectedfortheprimarybeamattenuationwiththeFLATN
theredshiftofA2256. taskinAIPSprovidingacentralregionwithauniformσnoise
distributionof∼0.027mJy/beam.Theprimarybeamcorrection
determinesanincreaseofthenoiseintheouterregions.
2. Radioobservationsanddatareduction
A2256wasobservedwiththeWSRTat2273MHzandwiththe
2.2.EFFELSBERGobservations
Effelsberg100mTelescopeat2604and4850MHz.
Inthissectionwepresenttheobservationsandthemainsteps Part of the observations were performed with the Effelsberg
of the calibration and image-making process. All the observa- 100 m Telescope. We used the 11 cm (=2640 MHz) and 6 cm
tionsincludefullpolarizationinformation.Inthispaperwefo- (=4850MHz) receivers. Single-dish observationsdo not suffer
cusonthetotalintensitypropertiesofthecluster.Wepostponea from the zero-spacing problem, and can trace large scale fea-
detailedlocalanalysisbasedonpolarizationpropertiesandspec- tures,althoughwithmodestresolution.
tralindexmapstoaforthcomingpaper(Trasattietal.,inprep.). The data reduction of Effelsberg data was performed with
theNOD2softwarepackage,followingthestandardprocedures
provided on the MPIfR web-page6. The raw images of both
2.1.WSRTobservations
A2256 and the calibrators were partly processed using dedi-
For this project we choose for the WSRT the maxi-short con- catedpipelinesavailableforeach receiver.The defaultstrategy
figuration which has optimized imaging performance for very tocalibrateEffelsbergdataistoobserveprimarycalibratorsdur-
extendedsources.Thereceivercoversthefrequencyrangefrom ingthesessionandthenuseautomatic2DGaussfitpipelinesto
2193MHzto2353MHzwitheightcontiguousintermediatefre-
quencies(IFs)of20MHzwidtheach;theresultingcentralfre- 5 http://www.astron.nl/radio-observatory/
quencyis 2273 MHz and the total bandwidthis 160 MHz. We astronomers/analysis-wsrt-data/
analysis-wsrt-dzb-data-classic-aips/analysis-wsrt-d
4 A very recent paper reports the first observation of a radio relicat 6 https://eff100mwiki.mpifr-bonn.mpg.de/doku.php?
16GHz(Stroeetal.2014). id=information_for_astronomers:user_guide:reduc_maps
A45,page3of18
A&A575,A45(2015)
Table1.WSRTobservationalparameters.
Pointingcenter(J2000) Frequency Bandwidth Exposuretime Telescopeconfiguration
RA Dec (MHz) (MHz) (h)
170107.998 +784503.701 2273 160 4 maxi-short
170425.000 +784503.701 2273 160 4 maxi-short
170242.300 +783459.988 2273 160 4 maxi-short
Table2. Effelsbergobservationalparameters.
Mapcenter(J2000) Frequency Bandwidth Mapsize
RA Dec (MHz) (MHz) (‘×’)
170400 [phone]×48
170400 [phone]×35
calculate the factor to scale the final image converting it from the range 4600−5100 MHz. The resulting central frequency is
mapunit/beamtoJy/beam(taskRESCALEinAIPS). 4850MHzandthetotalbandwidthis500MHz.Theresolution
oftheobservationis2.(cid:6)43 ×2.(cid:6)43.
Multihornsystemsuseadifferenttechniquetoovercomethe
2.2.1. Observationsat2640MHz
scanning effect problem. The scanning is done in an azimuth-
TheEffelsberg11cmreceiverisasingle-hornsystemequipped elevation coordinate system, and must be done only in az-
with a polarimeter with eight small-band frequency chan- imuth direction so that all horns will cover the same sky area
nels, each 10 MHz wide, covering the frequency range subsequently. At any instant each feed receives the emission
2600−2680MHz,plusonebroad-bandchannel,80MHzwide, fromadifferentpartoftheskybuttheyareaffectedbythesame
overthesamefrequencyrange.Theresultingcentralfrequency atmosphericeffects,whichthencancelouttakingthedifference
is2640MHzandthetotalbandwidthis80MHz.Theresolution signalbetweenthetwofeeds(Emersonetal.1979).Similarlyto
oftheobservationis4.(cid:6)5 ×4.(cid:6)5. the11cmreceiver,datain(R,L,U,Q)areprovidedforeachof
TomaptheA2256fieldweusedthemappingmode,which thetwohorn.
consist in rastering the field of interest by moving the tele- We performed a total of 25 coverages of the A2256 field,
scope, e.g., along longitude (l), back and forth, each subscan 15duringthenightbetweenthe22ndandthe23rdofJuneand
shifted in latitude (b) with respect to the other. At centime- 10onthe26thofJune2011.DuetoRFIproblemsonly22cov-
terwavelengthsatmosphericeffects(e.g.,passingclouds)intro- eragescouldbeused.
duce additional emission/absorption while scanning, leaving a Forthecalibrationweobserved3C286andNGC7027dur-
stripy pattern along the scanning direction (the so-called scan- ing the session. The flux densities used for the two calibrators
ning effects). Rastering the same field along two perpendicular are7.44Jy(fromBaarsetal.1977)and5.48Jy(fromPengetal.
directions(bothalonglongitudeandlatitude)helpsinefficiently 2000)respectively.
suppressing these patterns, leading to a sensitive image of the
region(Emerson&Graeve1988).Thistechnique,calledbasket-
3. X-rayobservationsanddatareduction
weavingtechnique,helpsalsoinsettingthezero-baselevel.The
detailsoftheobservationsaresummarizedinTable2.Foreach InthissectionwepresenttheX-rayobservationsofA2256per-
coverage of the field, the receiver provides four images (R, L, formed with Suzaku and XMM-Newton and the main steps of
U, Q) foreach of the nine channels.As circular polarizationis datareduction.
generallyveryweak,theimagesinRandLareverysimilarand Suzaku observed the radio relic region in A2256 (OBSID:
canbeaveragedinthelaterstepsofdatareductionprovidingthe 801061010, Tamura et al. 2011) with an exposure time
totalintensityimage. of 95.2 ks. The satellite X-ray Imaging Spectrometer (XIS:
We performed a total of 14 coverages in the longitude di- Koyamaetal.2007)hasaverylowdetectorbackground,which
rection and 15 coverages in the latitude direction; due to RFI allows us to investigate weak X-ray emission targets such as
(Radio Frequency Interference) and pointing problems we had clusteroutskirts(seeReiprichetal.2013,forareview).TheXIS
todiscardasmallportionofthedata.Thetimerequiredtocom- wasoperatedinthenormalclocking,3×3and5×5mode.To
plete one coverage is ∼17 min in both direction, so we have a increasethesignal-to-noiseratiowe filteredthedatasetusinga
totalobservingtime on sourceof 8.2 h. The observationswere geomagneticcosmic-raycut-offrigidity(COR)>8GV.Thefil-
carried out in the nightbetween the 15th and 16 August2012. tered exposure time is 89.2 ks. The data were processed using
3C286and3C48wereusedasabsolutefluxdensitycalibrators standard Suzaku pipelines (see Akamatsu et al. 2012, for more
using the Baars et al. (1977) scale that provide flux densities details).
at 2640 MHz of 10.65 Jy and 9.38 Jy respectively for the two We complementedSuzakuobservationswith XMM-Newton
calibrators. observations retrieved from the archive (OBSID: 0141380101
and OBSID:0141380201) and reprocessed with SAS v11.0.1.
The data were heavly affected by soft proton flares. The data
2.2.2. Observationsat4850MHz
werecleanedforperiodsofhighbackgroundduetosoftproton
TheEffelsberg6cm receiverisa double-hornsystem,with the flareswithatwostagefilteringprocess(seeLovisarietal.2009,
twofeedsfixedinthesecondaryfocuswithaseparationof6cm, 2011,formoredetailsonthecleaningprocess).Inthisscreening
each with one broad-band (500 MHz) frequency channel in process bad pixels have been excludedand only eventpatterns
A45,page4of18
M.Trasattietal.:TheradiorelicinAbell2256:overallspectrumandimplicationsforelectronacceleration
0−12fortheMOSdetectorsand0forthepndetectorwerecon- whichlabeledinthispaperasI2,I3,K2,G2,J2.Theradiohalo
sidered.Thefilteredexposuretimeis∼19ksforMOS1,∼20ks emission present in the center of the cluster around source D
forMOS2and∼9ksforpn. (Clarke&Ensslin2006)iscompletelyfilteredoutduetoacom-
Forbothsatellitesthebackgroundemissioncanbedescribed binationof effects: its low surface brightnessat this frequency,
asthesumofaparticlebackgroundcomponentandaskyback- combinedwiththelackofsampledshortspacingsintheWSRT
groundcomponent.Theformerisproducedbytheinteractionof observations,thatdeterminesthelossoftheveryextendedweak
high-energyparticleswith the detectors. The latter can be sub- emission.
divided into at least two thermal components, one unabsorbed Therelicemissionexhibitstworegionsofenhancedsurface
duetotheLocalHotBubble(LHB:kT∼0.08keV)andoneab- brightness:awell-definedarc-likeregion(Gregion)inthenorth-
sorbedduetotheMilkyWayHalo(MWH:kT ∼ 0.3keV),and ernpart,andalessdefinedregion(Hregion)inthewesternpart
a power-law componentdue to the Cosmic X-ray Background (seeFig.1).Thetworegionsareconnectedbyabridgeoflower
(CXB:Γ=1.41). brightnessemission.Atfullresolutionthe entirerelic emission
The particle background has been modeled and subtracted coversanareaofabout10.(cid:6)6 ×5(cid:6). ThesizereportedbyClarke
fromthedataofbothsatellitesbeforethespectralfitspresented &Ensslin(2006)at1.4GHzandataresolutionof52(cid:6)(cid:6)×45(cid:6)(cid:6)is
in Sect. 5.1. For the Suzaku observations its contribution 16.(cid:6)9 ×7.(cid:6)8. ConvolvingourWSRT image tothe same resolu-
has been estimated from the Night-Earth database with the tionweobtainasimilarangularsize(notshown).Nevertheless,
xisnxbgen FTOOLS (Tawa et al. 2008). For XMM-Newton the as we might be anyway missing some of the flux on the most
particle component spectra have been extracted from the filter extendedscales,wedonotusethisimageforthecomputationof
wheel closed (FWC) observations and renormalized by using therelicintegratedspectrum.
the out-of-field-of-viewevents. These spectra were supplied as
backgroundspectratotheXSPECfittingroutine. 4.1.2. EFFELSBERGimages
Unliketheparticlebackground,theskybackgroundwasnot In Fig. 2 we show the 2640 MHz (left panel) and 4850 MHz
subtractedfromthedatabutitsdifferentcomponentsweremod- (right panel) Effelsberg images of A2256. In both images, the
eledtogetherwiththeICMemissionduringthespectralfitting. diffuse emission from the relic is mixed up with the emission
To fix the model parameters for the different components in from the more compact sources present in the field due to the
Suzakuobservations,weusedspectraextractedfroma1degree lowresolutionoftheobservations.Inthe2640MHzimagethe
offsetobservationperformedwiththesatellite(PI:Kawaharada, relic emission is blended with the emission from the A+B+C
OBSID: 807025010).For XMM-Newton data, we followed the complexandfromsourceF.At4850MHzitiseasiertoseparate
methodpresentedinSnowdenetal. (2008)inwhichthediffer- the relic emission from the emission of the A+B complex, but
ent componentsare estimated using a spectrum extracted from
still part of the tail of source C is inevitably superimposed on
ROSATdatainanannulusbeyondthevirialradiusofthecluster. thewesternpartoftherelic.Moreover,Brentjens(2008)derived
Theoffsetspectrawerefittedwithaskybackgroundmodelcon- a spectrum α = 1.5± 0.2 for the radio halo between 351 and
sidering the LHB, MWH and CXB components. In the fitting,
1369MHz.Ifthereisnosteepeninginthespectrumofthehalo,
we fixed the temperature of the LHB component to 0.08 keV. we expect a flux density of ∼37 mJy at 2640 MHz and a flux
Abundance(Anders&Grevesse1989)andredshiftofLHBand densityof∼15mJyat4850MHz.Sinceasingledishissensitive
MWHwerefixedto1and0,respectively.Thetemperatureofthe
toalltheemissioninthefield,itsemissionissmoothedwiththe
MWHdeterminedinthefitis0.21±0.03keV.Wealsochecked othersourcesinourEffelsbergimagesifthehalospectrumkeeps
forthepossibilityofanadditional“hotforeground”component straightat these frequencies.Constrainingthehalo spectrumat
withkT ∼0.6−0.8keV(Simionescuetal.2010)addinganother
highfrequencieswouldrequireadedicatedcarefulanalysisthat
thermal component to the background model described above. isbeyondtheaimsofthispaper.
However,theintensityofthisadditionalcomponentresultednot
significantin the offset field and was notincludedin the back-
4.2.Spectralanalysis
groundmodeling.
For the spectral analysis of the radio relic in A2256 we com-
binedourhigh-frequencyobservationswith imagesobtainedat
4. Radioanalysisandresults
otherfrequenciesprovidedbytheauthors:the351MHzWSRT
image (Brentjens 2008), the 1369 MHz VLA C and D con-
4.1.Radioimages
figuration images7 (Clarke & Ensslin 2006) and the Effelsberg
4.1.1. WSRTimage 10450 MHz image (Thierbach 2000). We moreover got infor-
mation on the flux densities in the LOFAR image at 63 MHz
In Fig. 1 we present the 2273 MHz total intensity WSRT im-
published by van Weeren et al. (2012) by the author. All the
age of the central region of A2256. The image has been pro-
different images were calibrated according to the flux scale of
duced with natural weighting of the visibilities in the range
[u −u ] = [260−21035λ].Theshortestspatialfrequency Baars et al. (1977) or to its extension to lower frequencies
min max (below 408 MHz; Perley & Taylor 1999)8. In this way
sampled u determines the largest spatial scale recovered by
min
this observationsLSS (cid:8) 1/u = 13.(cid:6)22 . The image has been we were able to cover the widest range of frequencies
min
corrected for the primary beam attenuation that determines an
7 Weusedthehigh-resolutionCconfigurationdatatomeasuretheflux
increase of the noise in the edge of the image. The high reso-
lution (9(cid:6).(cid:6)84 ×9(cid:6).(cid:6)44 ) allowed us to analyze the substructures densityofdiscretesourceswhileforthedeterminationoftherelicflux
densityweusedtheDconfigurationimage.
of the diffuse relic emission in detail. The map shows several
8 TheoverallfluxscalefortheLOFARobservationsat63MHzwas
ofthewell-knownradiofeaturespresentinthecluster(notation
obtainedcomparingthemeasuredintegratedfluxdensitiesoffivebright
from Bridle et al. 1979;Rottgeringet al. 1994):the radio relic
sourcesinthefieldofview withpredictedfluxespartlybasedonflux
emission(sourcesGandH),thehead-tailsourcesA,B,CandI, densitiesmeasurementsfromthe1.4GHzNVSS(Condonetal.1998)
the complex source F (here resolved in the three components and the 74 MHz VLSS (Cohen et al. 2007), both based on the Baars
F1,F2andF3),aswellasmanyotherdiscretesources,someof etal.(1977)scale.
A45,page5of18
A&A575,A45(2015)
Fig.1.WSRT2273MHztotalintensityradioimage.Contoursaredrawnat[1,2,4,8]×3σ,with3σ=8×10−5Jy/beamandthecolorscalestarts
atthesamelevel.Thebeamsizeis9(cid:6).(cid:6)84×9(cid:6).(cid:6)44.Theimageiscorrectedfortheprimarybeamattenuation.
(63 MHz−10.45GHz) used so far for the determination of the were finally averaged, separately at the two frequencies. The
spectrumofaradiorelic. standarddeviationof these values(at the two frequencies)was
We estimated the uncertaintiesσ on the fluxdensitymea- used as the term (cid:8) . For the WSRT data we used, instead, the
S cal
surementsS withthefollowingformula dispersion of antenna gains. This translates into (cid:8) values of
cal
(cid:2) 1.6%for the Effelsberg 11 cm data, of 1.2%for the Effelsberg
σ = σ 2+σ 2, (4) 6 cm data and of 2% for the WSRT 13 cm data. Combining
S rms cal
this with the uncertainties on the Baars et al. (1977) scale, we
where end up with E (11 cm) = 5.3%, E (6 cm) = 5.2% and
cal cal
√ E (13 cm) = 5.4%. For the other images from the literature
– σrms =σ× Nbeamistheerrorduetotheimagenoise,withσ wcealassumed a similar value E = 6%. It shouldbe noted that
cal
beingtheimagenoiselevel(quotedintheimage’scaptions) theself-calibrationprocessmightaffectthemeasuredfluxden-
andN thenumberofbeamscoveredbythesource;
beam sitiesoninterferometricimages.Theuncertaintiesaretherefore
– σ = E ×S is theerrordueto calibrationuncertainties,
cal cal possiblylargerthanestimatedhere.
determinedinturnbytwofactors:theaccuracyoftheabso-
lute flux density scale adopted ((cid:8) ) and the uncertainties Where nototherwise specified,we estimated the uncertain-
scale
related to the applicationof such scale to our data (the cal- tiesσf onthequantities f calculatedfrommeasuredquantities
ibration method(cid:3), (cid:8)cal); being these two factors uncorrelated (e.g.,spectralindicesandextrapolatedfluxdensitiesofdiscrete
sources) applying the standard error propagation formula. The
weusedEcal = (cid:8)s2cale+(cid:8)cal2. spectraofthetotalcluster,relic+sources,relic,GandHregions
presentedinthenextsectionshavebeendeterminedcalculating
The spectral data provided by Baars et al. (1977) for the flux the flux densities at different frequencieson images convolved
density calibratorshave an absoluteuncertaintyof 5%. For the to the same lowest resolution available (4.(cid:6)4 × 4.(cid:6)4) and using
calibration of the Effelsberg images we performed four cover-
thesameregionsfortheintegration.Theerrorsassociatedtothe
agesof3C286andthreecoveragesof3C48at2640MHzand
spectralindicesaretheerrorsfromthefitsofthedatatakinginto
four coverages of 3C286 and three coveragesof NGC7027 at
accounttheuncertaintiesinthefluxdensitymeasurements.
4850MHz.AGaussianfitoftheimageofacalibratorprovides
its flux density in map unit. A comparison of this value with All the spectra, including those of the discrete sources
theknowncalibratorfluxdensityallowsustocalculatethefac- embedded in the relic emission, are plotted over the same
tor to translate map unit in physicalunit. The slightly different fixed x-axis range (frequency range 40−14000 MHz) for easy
factors deriving from the different coverages of the calibrators comparison.
A45,page6of18
M.Trasattietal.:TheradiorelicinAbell2256:overallspectrumandimplicationsforelectronacceleration
Fig.2.Effelsbergtotalintensityradioimages.Leftpanel:Effelsberg2640MHztotalintensityradioimageincolorscaleandblackcontoursdrawn
at[–1,1,2,4,8,16]×3σ,with3σ=4×10−3Jy/beam.Thebeamsizeis4.(cid:6)4 ×4.(cid:6)4.Rightpanel:Effelsberg4850MHztotalintensityradioimage
incolorscaleandblackcontoursdrawnat[–1,1,2,4,8]×3σ,with3σ=1.86×10−3Jy/beam.Thebeamsizeis2.(cid:6)43 ×2.(cid:6)43.Inbothpanelsthe
bluecontoursarefromtheWSRT2273MHzradioimagenotcorrectedfortheprimarybeamattenuationandaredrawnat[1, 2]×8×10−5Jy/beam.
4.2.1. Totalclusteremission
Wefirstconsideredtheintegratedradiofluxdensityoftheentire
cluster (halo, relic and discrete sources combined) measuring
the flux density in the circular region centered at J2000 posi-
tion α = 17 03 45 δ = +78 43 00 with 10(cid:6) radius, as de-
scribedbyBrentjens(2008).Theclusterradiuswasdetermined
by Brentjens (2008) as the one at which the derivative of the
integratedfluxwithin thecircle respectto the radiusofthe cir-
cle settles to a constant value. The measured flux densities at
351, 1369 (D configuration), 2640, 4850 and 10450 MHz are
summarizedinTable4.Thetotalclusterradioemissionbetween
351and10450MHzcanbemodeledbyasinglepowerlawwith
spectralindexα=1.01 ± 0.02(Fig.10a).
Brentjens(2008)modeledtheclusterfluxdensityasthesum
oftwospectralcomponents,oneduetothehaloandtheotherdue
to the relic anddiscrete sources combined.He showedthat the
secondtermbecomesdominantatfrequenciesabove100MHz. Fig.3.Regionsusedforthespectracomputation.Theredregionmarks
Beingtheradiorelicthedominantcontributortothefluxdensity the region considered for the relic spectrum computation. The to-
at high frequency,its spectrum at the same frequencies cannot tal region is further subdivided into regions G and H. In color scale
be flatter than the total cluster emission spectrum. This shows the WSRT high-resolution image with the green contours from the
qualitativelythatathighfrequenciestherelicspectrumdoesnot 2640 MHz Effelsberg image overplotted. The white region mark the
Csource.
keep the 0.85 slope observed at low frequency by van Weeren
etal.(2012).InSect.4.2.3wewillquantifythissteepening.
differentcomponentsof source C and for the sourcesJ and I2.
Whereavailable,wecombinedourdatawithdatacollectedfrom
4.2.2. Discretesources
the literature andwe modeledthe integratedspectra of the dis-
Figure 3 shows the area selected for measuring the radio relic cretesourcesoverthefrequencyrange63−10450MHz.Allthe
flux densities. From the high-resolution image it is possible to measuredandextrapolatedfluxdensities,aswellasthespectral
see which are the discrete sources included in such area: the indices of the source C components with references are listed
tail of source C C , K, J, I, G2, K2, J2, I2 and I3. To esti- in Table 3. Details on the flux densities derivation are given in
tail
matethefluxdensityfromthediscretesourcesthatneedstobe Appendix A. The spectra of the sources C, K, J, I and I2, for
subtractedfromthe totalemission, we producedtwo imagesat whichmorethantwofluxdensitymeasurementswereavailable,
1369MHz(VLAC configuration)and2273MHz(WSRT) us- are plotted individually in figures from 4 to 8 and all together
ingthesameuv-range(262−15460λ),pixel-sizeandrestoring inredinFig.9.ForthesourcesK2,J2,I3andG2onlyourflux
beamandwecalculatedthespectralindexofthediscretesources densitymeasurementsat1369and2273MHzwereavailableand
embeddedinthe relicemissionbetweenthesetwo frequencies. we simplyassumed straightspectra.Thisassumptionmaylead
Moreoverwe measured the flux density at 10450 MHz for the toaslightoverestimateofthefluxdensitiesatbothlowandhigh
A45,page7of18
A&A575,A45(2015)
sa
em
c
ne
10450MHzS(mJy) (cid:10)a1<d1±a0.80.2<d1(cid:10)a1(cid:10)a1(cid:10)a1(cid:10)a1 onsultrefereRightsite,Int
c
10450MHzS(mJy) <±d4.00.9±d2.10.6<±d1.90.7 8500MHzgS(mJy) (...)(...)±1.20.4(...)(...)(...)(...)(...) FormoredetailscedfromFig.5.4
4850MHzS(mJy) ±a11.71.3±a6.00.9±a5.71.5 4900MHzgS(mJy) (...)(...)±2.10.5(...)(...)(...)(...)(...) heliterature.e()ork.Dedu
tw
2640MHzS(mJy) ±a27.31.4±a14.00.8±a13.31.6 10450α2273 ±d1.400.14±d1.400.19±d1.420.25 4850MHzaS(mJy) ±0.80.1<1.2±2.40.3<1.4±1.00.1±0.70.1±1.10.1±0.60.1 orpublishedineasuredinthis
kM
2273MHzS(mJy) ±d33.71.8±d17.30.9±d16.40.9 2273α1369 ±d1.020.16±d0.720.15±d1.300.16 2640MHzaS(mJy) ±1.60.2<1.5±5.50.3<1.8±1.70.1±1.00.1±1.80.1±1.00.1 herinthisword()al.(1994).
Cfluxdensities. 351MHz1369MHzS(mJy)S(mJy) ±±ad228.717.756.53.4±±ad60.65.125.01.5±±ad168.118.431.51.9 (b)SourceCspectralindices. 3271369αα327153 ±±dd0.880.171.030.07±±ef0.500.100.650.10±±aa1.000.221.230.10 (c)Othersourcesfluxdensities. 1446MHz2273MHzcdSS(mJy)(mJy) ±±3.40.31.90.2±±2.10.31.70.2±±8.21.86.80.4±(...)1.90.1±(...)2.00.1±(...)1.10.2±(...)2.00.1±(...)1.10.1 normalfontaremeasuredvalueseitc()2012).MeasuredbyRottgeringetdbyLinetal.(2009).
Table3.Propertiesoftheradiosourcesembeddedintheradiorelicemission. (a)Source Component63MHz153MHz327MHzS(mJy)S(mJy)S(mJy) ±±±abc1048256ALL4805024620±±±aaa144.526.992.710.763.45.5HEAD±±±aaa903.5257.0387.351.1182.620.7TAIL 153αComponent63 ±a0.880.17ALL±a0.500.10HEAD±a0.950.35TAIL Source63MHz327MHz351MHz1369MHzaadSSSS(mJy)(mJy)(mJy)(mJy) ±±±±c16.33.06.70.1K7.01.03.30.2<<±±c0.651.00.1J11.20.1±±±±a32.220.317.08.916.48.7I9.30.8(cid:10)(cid:10)(cid:10)±a111I21.00.07±±±56.027.811.32.6K2(...)3.20.19±±±7.963.01.0J2(...)1.40.2±±±35.218.68.92.1I3(...)3.00.2±±±23.913.75.51.4G2(...)1.70.1 Notes.italicValuesinareassumedorextrapolatedinthiswork,whilevaluesinab()()belowandtext.Extrapolatedinthiswork.MeasuredbyvanWeerenetal.(fg()()Rightsite(2009).DeducedfromFig.7,Rottgeringetal.(1994).Measure
A45,page8of18
M.Trasattietal.:TheradiorelicinAbell2256:overallspectrumandimplicationsforelectronacceleration
0.01
C all
1 C head ]
C tail y
J
[
y
sit
n
e
d
x α ≥ 0.35
u α = - 0.25 ± 0.15
Fl 0.001
0.1
]
y
J 100 1000 10000
[
y Frequency [MHz]
sit
n
e Fig.6.IntegratedradiospectrumofsourceJ.Filledcirclesaremeasured
d
x fluxdensitieswhileopencirclesareextrapolatedfluxdensities.
u
Fl
0.01
α = 0.39 ± 0.21
]
y
J
[
0.001 sity 0.01 α = 0.69 ± 0.10
n
e
d
100 1000 10000 x
Frequency [MHz] Flu α = 1.38 ± 0.12
Fig.4. Integrated radio spectrum of source C. Filled circles are mea-
suredfluxdensitieswhileopen circlesareextrapolated fluxdensities.
Theblacksolidlineisthespectrumoftheentiresource;thereddashed 0.001
line is the source’s head spectrum; the blue dotted-dashed line is the
spectrumofthetail.Seetextformoredetails.
100 1000 10000
Frequency [MHz]
Fig.7.IntegratedradiospectrumofsourceI.Filledcirclesaremeasured
fluxdensitieswhileopencirclesareextrapolatedfluxdensities.
α = 0.52 ± 0.07
]
y 0.01
J
[
y 0.01
sit ]
n y
e J
Flux d α = 1.09 ± 0.02 ensity [
d
0.001 x α ≥ 0.42
u α = -1.27 ± 0.20
Fl 0.001
100 1000 10000
Frequency [MHz]
100 1000 10000
Frequency [MHz]
Fig.5. Integrated radio spectrum of source K. Filled circles are mea-
suredfluxdensitieswhileopencirclesareextrapolatedfluxdensities. Fig.8. Integrated radio spectrum of source I2. Filledcircles are mea-
suredfluxdensitieswhileopencirclesareextrapolatedfluxdensities.
frequencies(asforstandardradiosourcesynchrotronspectrawe long at high resolution, ∼410(cid:6)(cid:6) in total in our 2273 MHz im-
expect a flattening at low frequenciesand a steepening at high age (see Fig. 3). The published flux densities in the literature
frequencies).Ontheotherhand,theyareweaksourcesandtheir refer to the source as a whole. We modeled the source distin-
flux densities are not crucial for the relic spectrum determina- guishingbetweenthehead(long76(cid:6)(cid:6))andthetailaswearein-
tion.TheirspectraareplottedinblueinFig.9.Thisfigureshows terested only on the contribution from the latter. In Fig. 4 the
thespectraofthediscretesourcesincludedintheregionselected, spectrum over the range 63−10450 MHz is plotted for the en-
comparedtothetotalfluxdensityintheregion(relic+sources). tiresource,thehead,andthe tail.Ascommonamonghead-tail
The main flux density contribution among the discrete sources sources,theheadisflatterthanthetailandthespectrumsteepens
in the relic area come from the tail of source C, especially at at high frequency for both components. At lower frequencies,
lower frequencies. The source appears noticeably narrow and thetailcontainsalmostalltheemissionofthesources.Athigh
A45,page9of18
A&A575,A45(2015)
relic+sources Table4.Totalclusterfluxdensities.
relic+sources
10 relic+sources
C tail ν(MHz) S(mJy)
K
J 351 3320.0±200.0
I 1369 928.6±57.0
I2 2640 459.0±24.8
G2
K2 4850 246.3±13.3
J 10450 107.8±7.5
1 I3
selected area is shown in Fig. 3. The region is further divided
intotworegionsofenhancedradiobrightness(regionsGandH)
] discussedinthenextsection.Asdiscussedintheprevioussec-
y
y [J tion,thisregionincludetheradiorelicandsourcesCtail,K,J,I,
sit G2,K2,J2,I2andI3.
n 0.1 We first considered the total emission from the region.
e
x d Althoughthefluxdensitymeasurementsoftherelic+sourcesin
Flu therange63−10450MHzcanbefittedwithasinglepowerlaw
withα10450 = 0.93±0.02(Fig.9),hintsofasteepeningatfre-
quenci6e3s >1400 MHz are present. A separate fit of the spectra
between63and1369MHzandbetween1369and10450MHz
shows indeed that these two frequency ranges are best repre-
0.01
sented by two different power laws, with α1369 = 0.86± 0.01
63
and α10450 = 1.02±0.02 (Fig. 9). All the fits are plotted over
theen1t3ir6e9range63−10450MHztohighlightdifferences.Since
the relic is the major contributor in the region both at low and
highfrequency,thissuggeststhatasteepeningmightbepresent
in its spectrum as well. Moreover the relic spectrum between
0.001 63and1369MHzmustbeα(relic)1369 ≤ 0.86±0.01asitcan-
63
notbesteeperthantherelic+sourcesspectrum.Similarlyitmust
100 1000 10000 be α(relic)10450 ≥ 1.02 ± 0.02 as it cannot be flatter than the
Frequency [MHz] relic+sourc1e3s69spectrum. Indeed, after discrete sources subtrac-
Fig.9.Integratedradiospectraofthedifferentcomponentsintherelic tion we find that, although the relic spectrum between 63 and
10450 MHz is not inconsistent with a single power law with
regionshowninFig.3.Inblackweshowthefluxdensitiesandspec-
traoftheemissionfromtheentireregion(relic+sources).Thedashed α(relic)10450 =0.92±0.02,itisbestrepresentedbytwodifferent
63
(α10450 = 1.02 ± 0.02) and dot-dashed (α1369 = 0.86 ± 0.01) lines powerlaws,withα(relic)1369 = 0.85±0.01andα(relic)10450 =
rep13r6e9sent a double power-law fit to the data6.3The solid line (α10450 = 1.00±0.02 (Fig. 10b). T6h3is is supported by a lower va1l3u6e9s of
0.93±0.02)isasinglepower-lawfit.Inredandblueweshowth6e3spec- thereducedχ2inthecaseofthedoublepowerlaw(χ2 =0.19)
traofthediscretesourcesincludedintheregion.Redspectraareplotted respecttothesinglepowerlaw(χ2 =0.92).Themearseudredrelic
individuallyinfiguresfrom4to8.Bluespectraarestraightpower-law red
fluxdensities(beforeandafterdiscretesourcessubtraction)are
fitstothemeasuredfluxdensitiesat1369and2273MHz.Seetextfor
summarizedinTable5.Thefluxdensitiesmeasuredat63MHz,
moredetails.
351MHzand1369MHzareinagreementwithintheerrorbars
withthealreadypublishedfluxdensitiesatthesamefrequencies.
Thelow-frequencyspectralindexisinagreementwithwhatwas
frequenciesthe electrons in the tail are older while the head is foundbyvanWeerenetal.(2012).
moreclearlyvisible. Clarke & Ensslin (2006) reporta relic mean spectral index
Sources J and I2 have an inverted-spectrum. Their spectra between1369MHzand1703MHzof1.2.However,thisvalue
(plottedinFigs.6and8)haveaconvexshapeatGHzfrequen- hasabiguncertainty(notquotedbyauthor)sinceitisderivedas
cies,typicalofinverted-spectrumsources,andarelikelyyoung themeanvaluefromthespectralindeximagebetweentwovery
radioobjects. closefrequencies.Wecannotthereforeexcludeitisinagreement
with our value. Moreover, the very recent JVLA observations
by Owen et al. (2014) reports for the relic an overall intensity
4.2.3. Radiorelicspectrum weighted spectral index in the L-band of ∼0.94, in agreement
To avoid thatresolutioneffectsat differentfrequenciesmay al- withourfinding.
ter the determination of the relic integrated spectrum, we con-
volvedalltheimagesusedforthefluxdensitiescomputationto
4.2.4. RegionsGandH
thesameresolutionof4.(cid:6)4×4.(cid:6)4.Atsuchlowresolution,itisnot
easytoidentifytheregionwheretomeasuretherelicfluxden- Our high-resolution image shows that the relic can be divided
sity. We adopted the following strategy: we first computed the into two separate parts: regions G and H in Fig. 3. The cases
relic flux density from the 4850 MHz image at full resolution, of double relics in the same cluster are getting more and more
whereitiseasiertoseparatetherelicemissionfromthecomplex commonsincethefirstdiscoveryoftwoalmostsymmetricrelics
A+Bemission.Wethenchoosetherelicregioninthe4850MHz located on opposite sides in A3667. Since then, several other
imageconvolvedtotheresolutionof4.(cid:6)4×4.(cid:6)4,matchingtheflux doublerelicssystemshavebeenfound(seeFerettietal.2012).
density measured at full resolution at the same frequency.The We haveinvestigatedthepossibility thatthe twodifferentparts
A45,page10of18