Double relics in Abell 2345 and Abell 1240

14 Pages·2009·1.12 MB·English

A&A494,429–442(2009) Astronomy
DOI:10.1051/0004-6361:200810588 &
(cid:2)c ESO2009 Astrophysics
Double relics in Abell 2345 and Abell 1240
Spectral index and polarization analysis
A.Bonafede1,2,G.Giovannini1,2,L.Feretti2,F.Govoni3,andM.Murgia2,3
1 UniversitádiBologna,Dip.diAstronomia,viaRanzani1,40126Bologna,Italy
e-mail:[email protected]
2 INAF,IstitutodiRadioastronomia,viaGobetti101,40129Bologna,Italy
3 INAF–OsservatorioAstronomicodiCagliari,Strada54,Loc.PoggiodeiPini,09012Capoterra(Ca),Italy
Received14July2008/Accepted19September2008
ABSTRACT
Aims.TheaimofthepresentworkistostudytheradiopropertiesofdoublerelicsinAbell1240andAbell2345intheframeworkof
doublerelicformationmodels.
Methods.WepresentnewVeryLargeArrayobservationsat20and90cmforthesetwoclusters.Weperformedspectralindexand
polarizationanalysisandcomparedourresultswithexpectationsfromtheoreticalmodels.
Results.Thepresenceofdoublerelicsinthesetwoclusterisconfirmedbythesenewobservations.DoublerelicsinAbell1240show
radiomorphology,spectralindex,andpolarizationvaluesinagreementwith“outgoingmergershocks”models.Oneoftherelicsof
Abell2345showsapeculiarmorphologyandspectralindexprofile,whicharedifficulttoreconcilewithpresentscenarios.Wesuggest
apossibleoriginforthispeculiarrelic.
Key words. galaxies: clusters: general – galaxies: clusters: individual: Abell 1240 – galaxies: clusters: individual: Abell 2345 –
radiationmechanisms:non-thermal–polarization–magneticfields
1. Introduction induced by shock waves) is not obvious. Feretti & Neumann
(2006) did not find any evidence of a temperature jump near
Thehierarchicalmodelofstructureformationpredictsthatclus- the Coma cluster relic, and only recently a temperature gradi-
ters of galaxies form by subsequent merging of smaller struc- ent has been found near the relic in Abell 548b by Solovyeva
tures. A consistent amount of energy (∼1063−1064 erg) is re- et al. (2008). Here, a precise orientation of the cluster merger
leasedintheintraclustermedium(ICM)asresultofsuchmerger with respect to the line of sight is inferred, and the projected
events. Cosmological numerical simulations have shown that displacementoftherelicfromtheshockisthusexplained.
shocksandturbulenceassociatedwiththese processesnotonly
Particularly interesting would be to explore the connection
heattheICMbutalsoplayanimportantroleinnon-thermalphe-
between merger shock waves and clusters with double relics,
nomenaoccurringintheICM(seee.g.thereviewbyDolagetal.
i.e. clusters hosting two relic radio sources located in the pe-
2008). Althoughthese processes are yet not completely under-
ripheral region and symmetric with respect to the cluster cen-
stood,thisenergyinputissupposedtoaccelerateandinjectrela-
ter. So far, a very small number of clusters with two double
tivisticparticlesonaclusterscaleandamplifythemagneticfield
relics has been found. One of them is Abell 3667 (Röttgering
intheICM.
etal.1997;Johnston-Hollittetal.2002).Here,theclusterX-ray
Extendedradiosourcesonaclusterscalenotassociatedwith emission shows an elongated shape, interpreted as the merger
any optical counterpart but arising from the ICM have been axisoftwosub-clusters,andrelicsaredisplacedsymmetrically
detected in an increasing number of galaxy clusters. They are andperpendiculartothemainaxis.TheX-ray,optical,andradio
calledradiohalosandradiorelicsdependingontheirmorphol- propertieshavebeenreproducedbyanumericalsimulationofa
ogy,location,andradioproperties. mergerbetweenclusterswithmassratioof0.2byRoettigeretal.
Radio relics are extendedradio sources located on the out- (1999). We note, however, that not all of the predictions made
skirtsofgalaxyclusters,andarestronglypolarized,withlinear by such simulations could be tested with available data. Apart
fractionalpolarizationat20cmabove10%,reachingvaluesup fromAbell3667,doublerelicshavebeenobservedinAbell3376
to50%insomeregions(seee.g.Govoni&Feretti2004;Ferrari (Bagchietal.2006)andinterpretedas“Outgoingmergershock
etal.2008).Theiroriginisdebatedandnotwellknown.There waves”.DoublerelicshavealsobeenobservedinRXCJ1314.4-
isageneralconsensusthatitisrelatedtophenomenaoccurring 2515 (Feretti et al. 2005; Venturi et al. 2007), but no detailed
in the ICM during merging events. So far, there are ∼20 clus- study of the relics formation has been performed on this clus-
ters of galaxies where at least one radio relic is present. Their ter so far. Two more candidates for hosting double relics are
radio morphology and location are quite varied and could re- Abell 2345(Giovanniniet al. 1999) and Abell 1240(Kempner
flectdifferentphysicaloriginorICMconditions(Kempneretal. &Sarazin2001).
2004;Giovannini&Feretti2004).DuetolowX-raybrightness WepresentherenewVeryLargeArray(VLA)observations
attheclusterperiphery,acomparisonofrelicpropertieswiththe of these two clusters at 20 and 90 cm to confirm and study
surrounding medium (i.e. temperature and brightness gradient the double relic emission in the framework of relic formation
ArticlepublishedbyEDPSciences
430 A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240
Table1.VLAobservations.
Source RA Dec ν Bandwidth Config. Date Duration
(J2000) (J2000) (MHz) (MHz) (h)
Abell2345 212712.0 –121030.0 325 3.125 B 16-Aug.-2006 2.0
325 3.125 C 08-Dec.-2006 5.4
Abell1240 112337.0 430515.0 325 3.125 B 05-Aug.-2006 2.6
325 3.125 C 08-Dec.-2006 4.7
Abell2345-1 212643.0 –120750.0 1425 50 C 08-Dec.-2006 1.9
212643.0 –120750.0 1425 50 D 09-Apr.-2007 1.0
Abell2345-2 212736.0 –121125.0 1425 50 C 08-Dec.-2006 2.0
212736.0 –121125.0 1425 50 D 09-Apr.-2007 1.0
Abell1240-1 112325.0 431030.0 1425 50 C 08-Dec.-2006 1.8
112325.0 431030.0 1425 50 D 12-Apr.-2007 1.0
Abell1240-2 112350.0 430020.0 1425 50 C 08-Dec.-2006 1.9
112350.0 430020.0 1425 50 D 12-Apr.-2007 1.0
Abell2345 212657.2 –121249 1490 50 AnB 02-Nov.-1991 0.1
Column1:sourcename;Cols.2,3:pointingposition(RA,Dec);Col.4:observingfrequency;Col.5:observingbandwidth;Col.6:VLAconfigu-
ration;Col.7:datesofobservation;Col.8:nettimeonsource.
models.Spectralindexanalysisofbothradiorelicsinthesame solutions,followedbyafinalamplitudeandgainself-calibration
clusterhavenotbeenperformedsofar.InAbell3667,thespec- cycle.
tralindeximagehasbeenobtainedforonlyoneofthetworelics, InadditionwerecoveredfromtheVLAdataarchiveashort
and no spectral index information are available for relics in observation performed with AnB array. The source 3C 48 was
Abell3376.Onlyintegratedspectralindexinformationisavail- usedasprimaryfluxdensitycalibratorandthesource2121+053
able for the relics in RXCJ 1314.4-2515.Studyingthe spectral was used as phase calibrator. We reduced and calibrated these
index and of the polarization properties of relics offers a pow- dataasexplainedabove,anddetailsaregiveninTable1.
erfultoolforinvestigatingtheconnectionbetweendoublerelics
Observations at 90 cm (325 MHz) were performed in the
andoutgoingshockwavesoriginatinginamergerevent.Infact,
spectral line mode, using 32 channels with 3.127 MHz band-
theoreticalmodelsandnumericalsimulationsmakeclearpredic-
width. This observing method avoids part of the VLA in-
tions on the relic spectral index trend and magnetic field prop-
ternal electronics interferences and allows us to remove ra-
erties (see Ensslin et al. 1998; Roettiger et al. 1999; Hoeft &
dio frequencyinterferences(RFI) accurately.This also reduces
Brüggen2007).
bandwidth smearing, which is quite strong at low frequencies.
The paper is organized as follows. In Sect. 2 observations
Primaryfluxdensityandphasecalibratorswerethesamesources
and data reduction are described, in Sects. 3 and 4 we present
asusedin1.4GHz observations.Both3C48and3C286were
the analysis of the cluster Abell 2345 and Abell 1240. Results
alsousedforbandpasscalibration.RFIareparticularlystrongat
are discussed in Sect. 5 in comparison with other observed
lowradiofrequency,sothatanaccurateeditingwasdonechan-
relics and theoretical models, and conclusions are presented in
nelbychannel,resultinginaconsistentflagofdata.Thisincon-
Sect. 6. We assume a ΛCDM cosmological model with H =
0 junctionwithbaddatacomingfromEVLAantennas,resultsin
71kms−1Mpc−1,ΩM =0.27,ΩΛ=0.73. a loss of ∼40% of observing time. We calibrated data follow-
ingthe“SuggestionsforPbanddatareduction”byOwenetal.
(2004).
2. VLAradioobservations
Afterwordstheinitialbandpasscalibrationchannelsfrom1
2.1.Totalintensitydatareduction to 4 and from 28 to 32 were flagged because of the roll-off of
the bandpass. In the imaging procedure data were averaged to
ObservationswereperformedattheVeryLargeArray(VLA)at
8channels.Imagingwasdoneusingthewide-fieldimagingtech-
20 cm in the C and D configurationandat 90 cm in the B and
niquetocorrectfornoncomplanarityeffectsoverawidefieldof
C configuration,to obtain the same spatial frequencycoverage
view. Twenty-fivefacetscoveringthe mainlobe ofthe primary
intheUVplane.ObservationsdetailsaregiveninTable1.
beam were used in the cleaningand phase-self calibrationpro-
Observationsat20cm(1.4GHz)werepointedseparatelyon
cesses. We also searchedin the NVSS data archivefor sources
thetworelicsinbothoftheclustersbecauseofthesmallerfull
strongerthan0.5Jyoveraradiusaslargeas10◦.Thesesources
width at half power of the primary beam. Observations of the
wereincludedintheinitialcleaningandselfcalibrationsteps.
cluster Abell 1240 were calibrated using the source 3C 286 as
the primaryfluxdensitycalibrator1. Thesource1156+314was Each (u,v) data set at the same frequency (but observed
observed at intervals of about 30 min and used as phase cal- with different configurations) was calibrated, reduced, and im-
ibrator. Observations of Abell 2345 were calibrated using the aged separately and then combined to produce the final im-
sources3C48astheprimaryfluxdensitycalibrator.Phasecali- ages. Images resulting from the separate pointed observations
brationwasperformedbyobservingthesource2137-207atin- at 1.4 GHz were then linearly combined with the AIPS task
tervalsof∼30min. LTESS.Wecombinedthedatasetandproducedimagesathigher
We performed standard calibration using the NRAO andlowerresolutions(hereinafterHR imagesandLRimages)
Astronomical Imaging Processing Systems (AIPS). Cycles of giving uniform and natural weight to the data. For the pur-
phase self-calibration were performed to refine antennas phase posesofthespectralanalysis,thefinalimagesat325MHzand
1.4GHz,wererestoredwiththesamebeam(reportedinTables2
1 WerefertothefluxdensityscalebyBaars&Martin(1990). and3)andcorrectedfortheprimarybeameffects.
A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240 431
Table2.Abell2345. andboththelightandgalaxynumberdensitydistributionshave
several peaks close to the central galaxy. The authors find that
Sourcename ν θ σI Fig. the projected mass distribution has the most prominent peak
MHz arcsec mJy/beam displaced from the central cD by ∼1.5(cid:5), although a secondary
Abell2345-1HR 1425 37×20 0.08 peak is closerto the centralcD. No informationaboutthe pos-
Abell2345-1LR 1425 50×38 0.09 2,centralpanel
sible presence of a cooling flow associated with this galaxy is
Abell2345-2HR 1425 37×20 0.09
presentintheliterature.Dahleetal.(2002)concludefromtheir
Abell2345-2LR 1425 50×38 0.09 2,centralpanel
Abell2345HR 325 37×20 1.7 analysis that the cluster may be a dynamically young system.
Abell2345LR 325 50×38 2.0 2,rightpanels Cyprianoetal.(2004)reportthemassdistributionderivedfrom
Abell2345 1490 6×6 0.13 1,centralpanel weaklensinganalysisandfindthatthebestfittotheirdataisa
singular isothermalellipsoid with the main axisorientedin the
Column1:sourcename;Col.2:observationfrequency;Col.3:restoring EWdirection.
beam;Col.4:rmsnoiseofthefinalimages;Col.5:figureofmerit.
The radio emission of Abell 2345 is characterized by the
presence of two relics visible in the NVSS (Giovannini et al.
Table3.Abell1240. 1999).OurnewVLAobservationsconfirmthepresenceoftwo
regionswherenon-thermalemissionispresentattheclusterpe-
Sourcename ν θ σI Fig. riphery, nearly symmetrical with respect to the cluster center.
MHz arcsec mJy/beam
These new observations, together with the archive data, allow
Abell1240-1HR 1425 22×18 0.04 6
studyoftheclusterradioemissioninawiderangeofresolutions
AAbbeellll11224400--12LHRR 11442255 4222××3138 00..0044 7,centr6alpanel goingfrom∼6(cid:5)(cid:5)to∼50(cid:5)(cid:5).Therefore,itispossibletoseparatethe
Abell1240-2LR 1425 42×33 0.05 7,centralpanel contribution of discrete sources whose emission is not related
Abell1240HR 325 22×18 0.9 to the relic’s physical properties. In Fig. 1 the radio emission
Abell1240LR 325 42×33 1.0 7,leftpanels of Abell2345at6(cid:5)(cid:5) resolutionis shownoverlaidontothe opti-
calemission(takenfromtheDigitalizedPalomarSkySurveyII,
Column1:sourcename;Col.2:observationfrequency;Col.3:restoring
redband).Twocentralradio-tailsourcesareassociatedwithop-
beam;Col.4:rmsnoiseofthefinalimages;Col.5:figureofmerit.
tical galaxies in the cluster center. The central cD is visible in
theopticalimage.Relicsarenotvisibleinthisimagebecauseof
Table4.Totalandpolarizationintensityradioimagesat1425MHz. the lack of shortbaselines.This confirmsthat the emission de-
tected in lower resolutionobservationsis indeed extended,and
Sourcename θ σI σQ,U Fig. it is not due to the blending of discrete sources. In the same
arcsec (mJy/beam) (mJy/beam) figure we also report the radio relic emission as detected by
Abell2345-1 23×16 0.05 0.02 5,rightpanel
C array observations. The western relic (Abell 2345-1) is lo-
Abell2345-2 23×16 0.07 0.02 5,leftpanel catedat∼1MpcfromtheclusterX-raycenter,whiletheeastern
Abell1240-1 18×17 0.04 0.02 10,toppanel
Abell1240-2 18×17 0.04 0.01 10,bottompanel relic(Abell2345-2)is∼890kpcfarfromtheclustercenter(see
Table6).
Column 1: source name; Col.2: restoring beam; Col.4: rmsnoise of Thereareseveraldiscretesourcesclosetothewesternrelic,
theIimage;Col.5:rmsnoiseoftheQandU images;Col.6:figureof A2345-1, visible in the 1.4 GHz image. They are labeled with
merit.
lettersfromAtoFintherightpanelofFig.1.ThesourcesA,C,
D,E,andFcouldbeassociatedwiththeopticalgalaxiesvisible
in the DPOSSII image, whereas B does not have any obvious
2.2.Polarizationintensitydatareduction
optical identification. Optical emission is present at 35(cid:5)(cid:5) in the
Observationsat20cm(1.425GHz)includefullpolarizationin- NEdirectionfromtheradiopeak.Thisisgreaterthantheerror
formation. Polarization data observed with the D array are not associated with the beam, that is only 6(cid:5)(cid:5) × 6(cid:5)(cid:5) in the highest
usablebecauseofthebadqualityofdataofthepolarizationcal- resolution image. We can then conclude that no optical coun-
ibrator. The absolutepolarizationposition anglewas calibrated terpartoftheBradiosourceisdetectedintheDPOSSIIimage.
byobserving3C286forbothclustersinCconfiguration.Thein- The sources D, E and F are not visible in the 325 MHz image
strumentalpolarizationoftheantennaswascorrectedusingthe (seeFig.2,topleftpanel).Thisisconsistentwitharadiosource
source 1156+314 for Abell 1240 and the source 2137-207 for havingaspectralindex<1.22.Thereisonlyonediscretesource
Abell2345. neartherelicA2345-2,labeledwithGintheFig.1withoutany
StokesparametersU andQimageswereob(cid:2)tained.Wethen obviousopticalidentification.Thissourceisalsodetectedinthe
derived the polarization intensity image (P = U2+Q2), the higherresolutionimage.
polarizationangleimage(Ψ = 1arctanU)andthefractionalpo- The whole extension of the relics is properly revealed by
2 Q LR images (Fig. 2). The morphologyof the relics is similar at
larizationimage(FPOL = P),with I the totalintensityimage.
I 1.4GHzand325MHz,althoughonlythebrightestregionscan
MoredetailsaregiveninTable4.
be seen at 325 MHz due to the higher rms noise levelof these
observationswithrespecttothe1.4GHzones.Thetotalfluxof
therelicsatthe2frequencies,excludingthecontributionofthe
3. TheclusterAbell2345
discretesources,arereportedinTable6,wherethemainphysi-
Optical information is available for this cluster, while little is calparametersaresummarized.
known about its X-ray emission. General data are reported in The relic A2345-1 shows an elongated shape at high res-
Table5.Weakgravitationallensinganalysishasbeenperformed olution, while at lower resolution it shows a weaker wide
by Dahle et al. (2002) and by Cypriano et al. (2004). Optical emission extending in the western direction, i.e. toward the
datacovertheinnerpartofthecluster(∼3(cid:5)×3(cid:5)).Theyfindthat
this cluster has a well-defined core dominated by a cD galaxy, 2 ThespectralindexαisderivedaccordingtoSν ∝ν−α.
432 A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240
Table5.Abell2345andAbell1240properties.
Sourcename RA Dec z Scale F L
X X
(J2000) (J2000) (kpc/(cid:5)(cid:5)) 10−12erg/s/cm2 1044erg/s
Abell2345 212711.00 –120933.0 0.1765 2.957 5.3 4.3
Abell1240 112332.10 430632 0.1590 2.715 1.3 1.0
Column1:sourcename;Cols.2,3:clusterX-raycentre(RA,Dec);Col.4:clusterredshift;Col.5:arcsectokpcconversionscale;Col.6:fluxin
the0.1−2.4keVband(Abell2345)andinthe0.5−2keV(Abell1240);Col.7:X-rayclusterluminosityinthe0.1−2.4keVband(Abell2345)and
inthe0.5−2keV(Abell1240);datafromBöhringeretal.(2004)forAbell2345andfromDavidetal.(1999)forAbell1240,correctedforthe
adoptedcosmology.
Table6.Abell2345.
Sourcename Proj.dist LLS F F B –B(cid:5) (cid:7)α(cid:8)
20cm 90cm eq eq
kpc kpc mJy mJy μG
Abell2345-1 340(cid:5)(cid:5)=1000 390(cid:5)(cid:5)=1150 30.0±0.5 291±4 1.0–2.9 1.5±0.1
Abell2345-2 300(cid:5)(cid:5)=890 510(cid:5)(cid:5)=1500 29.0±0.4 188±3 0.8–2.2 1.3±0.1
Column1:sourcename;Col.2:projecteddistancefromtheX-raycentroid;Col.3:largestlinearscalemeasuredonthe20cmimages;Cols.4
and5:fluxdensityat20and90cm;Col.6:equipartitionmagneticfieldcomputedatfixedfrequency–fixedenergy(seeSect.3.3);Col.7:mean
spectralindexinregionwhereboth20and90cmsurfacebrightnessis>3σ.
A2345
05:00.0 F
A
E
G on D
nati -12:10:00.0 B
Decli
C
15:00.0
500 kpc
20:00.0
30.0 21:27:00.0 26:30.0
Right ascension
Fig.1. The cluster Abell 2345. Inthe center: DPOSSIIoptical emission (red band) in colors overlaid onto radio contours at 1.490 GHz. First
contours are±0.4mJy/beam, andtheyarethenspacedbyafactor 2.Thebeamin6(cid:5)(cid:5) ×6(cid:5)(cid:5).Topinsetshowsthezoomed imagesofthecentral
sources:thecentralcDgalaxyandtworadiogalaxiesarevisible.Bottominsetshowsthezoomedimageofthesouthernradiosource.Redboxes
mark theregionof therelics, completelyresolved inthehigh-resolution image. Intheleft andright panels zoomed imageof theredboxes is
shown.HerecolorsrepresenttheopticalDPOSSIIemission,whilecontoursrepresenttherelicradioemissionattheresolutionof23(cid:5)(cid:5)×16(cid:5)(cid:5).The
relicA2345-1isvisibleintherightpanel,whileA2345-2isintheleftpanel.Contoursstartat±0.15mJy/beamandarespacedbyafactor2.Red
arrowsindicatethepositionofthediscretesourcesembeddedintherelicemission.
cluster outskirts. We note that this circular filamentary the low-frequencyimage, where the noise level is significantly
morphology is not seen in other double relic sources, as dis- higherthanthethermalnoise.Aconsistentspectralindexanal-
cussedintheintroduction. ysis has to consider the different extension at the two frequen-
cies. Infact,asalreadypointedoutbyOrrúetal. (2007),ifwe
compute spectral index analysis considering only regions that
3.1.Spectralindexanalysis haveasignal-to-noiseratio>3atbothfrequencies,weintroduce
a bias, since we are excluding a priori low spectral index re-
Wederivedthespectralindeximageofthecluster’srelicscom-
gions, whose emission cannotbe detectedat 325MHz. For in-
paringtheLRimagesat1.4GHzand325MHz.Thermsnoise
stance,therelicA2345-1radiobrightnessat1.4GHzdecreases
of the imagesarereportedin Table2. Spectralindexand spec-
asthedistancefromtheclustercenterincreases.Thefainterre-
tralindexnoiseimagesareshowninFig.2.Theywereobtained
byconsideringonlypixelswhosebrightnessis>3σatbothfre- gioncouldbedetectedinthe325MHzimageonlyifitsspectral
index,α,weresteeperthan∼1.8.
quencies. We note that relics are more extended at 1.4 GHz
than at 325 MHz. This can be due to the different sensitivities Inbothoftherelicsthespectralindexispatchy.Thespectral
at 1.4 GHz and 325 MHz. Confusion and RFI strongly affect indexrms is σ ∼ 0.4 while the mean spectralindexnoise is
spix
A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240 433
A2345-1 spectral index A2345-1 spectral index error
DEC DEC
-12:06 -12:06
-12:08 -12:08
A2345
-12:10 -12:10
05:00.0
21h26m50s 21h26m40s 21h26m30s 21h26m50s 21h26m40s 21h26m30s
RA RA
0 (SP1 INDEX) 2 3 Declination -12:10:00.0 0 (SP I1NDEX) 2
A2345-2 spectral index A2345-2 spectral index error
15:00.0
DEC DEC
-12:06 -12:06
500 kpc
20:00.0
-12:08 28:00.0 30.0 21:27:00.0 26:30.0 -12:08
Right ascension
-12:10 -12:10
-12:12 -12:12
-12:14 -12:14
21h27m45s 21h27m30s 21h27m45s 21h27m30s
RA RA
Fig.2. Center: thecluster Abell 2345 radioemission at 1.4GHz.Thebeam is50(cid:5)(cid:5) ×38(cid:5)(cid:5). Contours startat 3σ (0.24mJy/beam) andarethen
spacedbyafactor2.ThecrossmarkstheX-rayclustercenter.Left:colorsrepresentthespectralindexoftherelicA2345-1(top)andA2345-2
(bottom)superimposedovertheradioemissionat325MHz(contours).Thebeamis50(cid:5)(cid:5) ×38(cid:5)(cid:5),contoursstartat3σ(6mJy/beam)andarethen
spacedbyafactor2.Right:spectralindexerrorimage(colors)superimposedontotheemissionat325MHz(contoursareasabove).
(cid:7)Spix Noise(cid:8) ∼ 0.1 for both relics. Thus, by comparing these instead shows a different trend, going from ∼1.4 in the inner
twoquantities,wecanconcludethatspectralindexfeaturesare shellto∼1.1intheouterrim(Fig.3).
statisticallysignificant.
Our aim here is to investigate whether there is a system-
3.2.Radio-X-raycomparison
atic variation in the relic spectral index with distance from the
cluster center as found in other radio relics (e.g. 1253+275by No X-ray studies are present in the literature for this cluster.
Giovannini et al. 1991; Abell 3667 by Röttgering et al. 1997; X-ray observations in the energy band 0.1−2.4 keV were re-
Abell2744byOrrúetal.2007;Abell2255byPizzoetal.2008; trievedbytheROSATall-skysurvey(RASS)dataarchive.The
Abell521byGiacintuccietal.2008). total exposure time is of ∼4 ks. After background subtraction,
Toproperlyobtaintheradialtrendofthespectralindex,we the event file was divided by the exposure map. We smoothed
integratedthe radio brightnessat 325 MHz and 1.4 GHz in ra- the resulting imagewith a Gaussian of σ = 60(cid:5)(cid:5). Theresulting
dial shells of ∼50(cid:5)(cid:5) in width wherever the 1.4 GHz brightness imageisshowninFig.4.
is>3σ,andthenwecomputedthevalueofthespectralindexin The X-ray emission of this cluster is elongated in the
eachshell.Weexcludedtheregionswherediscreteradiosources NW-SE direction. Two bright regions are visible at ∼10(cid:5) and
are embedded in the relic emission (see insets in Fig. 3). The 14(cid:5) in N-Wdirectionfromthe clustercenter (referredto asX1
shells were centeredin the extrapolatedcurvaturecenterof the and X2, respectively). The galaxy J21263466-1207214(RA =
relicA2345-2,thatis2.6(cid:5) souththeclusterX-raycenter.Shells 21h26m34.6s, Dec= −12d07m22s,z= 0.178221)isclosetothe
are thenparallelto therelicsmainaxis.We computedthe inte- first one. Another bright region is present at ∼4(cid:5) south of the
gratedbrightnessin eachshe√llat 20and90cm,andcalculated clustercenter(X3).
the associated error as σ × N , where σ is the image rms Datapresentedhereallowaninterestingcomparisonamong
beam
noise, and N the number of beams sampled in the shell. In clusteremissionatdifferentwavelengths.Wenotethatmassdis-
beam
those shellswherethe brightnessis>3σin the 1.4GHz image tribution from weak lensing studies (Cypriano et al. 2004) is
but<3σinthe325MHzimage,onlyupperlimitsonthemean represented well by an ellipsoid with the major axis directed
spectral index can be derived. The spectral index profiles thus in the EW direction, and relics are found perpendicularto this
obtained are shown in Fig. 3. These plots show that the spec- axis.Consistentlywiththeopticalanalysis,theX-rayemissionis
tralindexintherelicA2345-1increaseswithdistancefromthe elongatedintheNW-SEdirection,indicatingapossiblemerger
cluster center, indicating a spectral steepening of the emitting along that direction, and relics are displaced perpendicular to
particles. The spectral index in each shell is rather high, going thataxis.InFig.4theX-rayemissionissuperimposedontoradio
from∼1.4intheinnerrimto∼1.7inthecentralrimoftherelic. contours.A2345-2islocatedattheedgeoftheX-rayemission,
Thespectralindextrendderivedfortheoutershellsisconsistent as foundin relics of Abell3667and A3376.A2345-1,instead,
withfurthersteepening.ThespectralindexoftherelicA2345-2 islocatedbetweeneasternedgeoftheclusterandtheX1region,
434 A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240
Fig.3.SpectralindexradialtrendofA2345-1 (left)andA2345-2 (right),computedinshellsof∼50(cid:5)(cid:5) inwidth.Itwascomputed excluding the
contributionofthediscretesources.Crossesrefertospectralindexvaluescomputedinshellswherethemeanbrightnessis>3σatboth325MHz
and1.4GHz.Arrowsare3σupperlimitsonthespectralindexmeanvalue(seetext).TheredcrossreferstotheclusterX-raycenter,thebluecross
referstothecenterofthesphericalshells.Intheinsets:displacement oftheshellsoverwhichthemeanspectralindexwerecomputed.Circles
refertothediscretesourcesembeddedintherelicemission.
A2345
X1
X2
05:00.0
n
o
ati-12:10:00.0
n
cli
e
D
15:00.0
X3
28:00.0 30.0 21:27:00.0 26:30.0
Right ascension
Fig.4.Abell2345X-rayemission(colors)intheenergyband0.1−2.4keVfromRASS.TheimagehasbeensmoothedwithaGaussianofσ∼
60(cid:5)(cid:5);contoursrepresenttheradioimageoftheclusterat1.4GHz.Thebeamis50(cid:5)(cid:5)×38(cid:5)(cid:5).Contoursare0.24mJy/beamandarethenspacedbya
factor2.ArrowsmarkthepositionoftheX1−X3regions.
10(cid:5) from Abell 2345 center, and its radio emission extends to- merger with X3 and X1 groups, and this could explain the pe-
wardX1. culiarpropertiesofA2345-1.MoresensitiveandresolvedX-ray
observations in conjunction with optical studies are required
From the same figure, in the X3 region a narrow-angletail
to shed light on the connection between the radio emission of
radio galaxy is visible in radio images at every resolution (see
A2345,X1−X3.
Figs. 1 and 2). Although redshiftis not available for this radio
source,itsstructurefavorsaconnectiontothecluster and/orto
thecloseX-raypeak.OnepossibilityisthattheseX-raymultiple
3.3.Equipartitionmagneticfield
featuresaregalaxyclumpsinteractingwithAbell2345.
A selfconsistentscenarioarisesfromthisanalysis,indicat- Under the assumption that a radio source is in a minimum en-
ing that the cluster Abell 2345 could be undergoing multiple ergy conditions, it is possible to derive an average estimate
A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240 435
A2345
DEC
-12 06 -12:04
07
08
-12 05
-12:08
09
06
DECLINATION (J2000) 111012 -12:12 DECLINATION (J2000) 0078
13 09
14 -12:16 10
15
11
16 21 26 55 50 45 40 35 30 25
21 27 45 40 RIGH3T5 ASCENSIO3N0 (J2000) 25 20 21h27m40s 21h27m20s 21h27m00s 21h26m40s RIGHT ASCENSION (J2000)
RA
0 0.0005 0.001
(JY/BEAM)
Fig.5. Abell 2345: polarized emission of Abell 2345 at 1.4 GHz. In thecentral panel the polarized radio emission at 1.4 GHzisshown. The
restoringbeamis23(cid:5)(cid:5) ×16(cid:5)(cid:5).Leftandrightpanels: contoursrefertotheradioimageAbell2345-2andAbell2345-1 (seeTable4fordetails).
Contoursstartfrom3σandarespacedbyafactor2.Evectorsaresuperimposed:lineorientationindicatesthedirectionoftheEfield,whileline
lengthisproportionaltothepolarizationintensity(Leftpanel:1(cid:5)(cid:5)correspondsto5.5μJy/beam;Rightpanel:1(cid:5)(cid:5)correspondsto10μJy/beam).
of the magnetic field strength in the emitting volume (see e.g. if a spectral curvatureis present. We note that a detailed study
Pacholczyk1970).We assumethatthemagneticfield andrela- of the radio spectrum on a wide frequency range is available
tivisticparticlesfillthewholevolumeoftherelics,andthaten- for three peripheral relics: the one in Abell 786, in the Coma
ergycontentinprotonsandelectronsisequal.Wefurtherassume cluster (see Giovannini& Feretti 2004, and referencestherein)
thatthevolumeoftherelicsiswellrepresentedbyanellipsoid and in Abell 521 (Giacintucci et al. 2008). In these relics a
havingthemajorandminoraxisequaltothelargestandsmallest straight steep radio spectrum is observed. We also note that a
linearscalesvisibleinourimages;weestimatedthethirdaxesto low-frequencycut-offof10MHzandamagneticfieldof∼1μG
bethemeanbetweenthemajorandminorones.Thesynchrotron imply a low-energy cut-off of γ ∼ 1500. Thus, if the spec-
min
luminosityiscalculatedfromalow-frequencycut-offof10MHz trumoftheemittingparticlesistruncatedatγ>1500,bothB(cid:5)
eq
toahighfrequencycut-offof10GHz.Theemittingparticleen- and B could overestimate the magnetic field strength. Future
eq
ergydistributionisassumedtobeapowerlawinthisfrequency low-frequencyradiointerferometerssuchasSKAandLWAwill
range(N(E) ∝ E−p),with p = 2α+1.Weusedthemeanvalue likely shed lightonthis point.On the otherhand,it is possible
ofα =1.5and1.3forA2345-1andA2345-2,respectively,and toderiveanindependentestimateofthemagneticfieldfromX-
found B ∼1.0μGinA2345-1and0.8μGinA2345-2.These rayfluxduetoinverseComptonscatteringofCMBphotonsby
eq
valuesareconsistentwithequipartitionmagneticfield foundin relativistic electrons in the relic source. These studies have so
otherrelics. far been performedon a smallnumberof radiorelicsand have
It has been pointed out by Brunetti et al. (1997) that syn- led to lower limits on the magnetic field strength: B > 0.8 μG
chrotronluminosityshouldbecalculatedinafixedrangeofelec- in the relic 1140+203of Abell 1367(Henriksen & Mushotzky
tron energies rather than in a fixed range of radio frequencies 2001); B > 1.05 μG in 1253+275 of the Coma cluster (Feretti
(see also Beck & Krause 2005). In fact, electron energy cor- & Neumann 2006); B > 0.8 μG in 0917+75in Rood27 cluster
responding to a fixed frequency dependson the magnetic field (Chen et al. 2008); and B > 2.2 μG in the relic 1401-33in the
value,andthustheintegrationlimitsarevariableintermsofthe AbellS753cluster(Chenetal.2008).Inthesecases,thelower
energyoftheradiatingparticles.Giventhepowerlawofthera- limitsderivedfromICargumentsareconsistentwithequiparti-
diating particles and the high value of the radiospectral index, tionestimates,thusindicatingthattheequipartitionvaluecould
the lower limit is particularly relevant here. We adopted a low be used as a reasonable approximations of the magnetic field
energycutoffofγ =100andassumedγ (cid:10)γ ,obtaining strengthinrelics.
min max min
B(cid:5) ∼2.9μGinA2345-1and2.2μGinA2345-2.
eq
We derived the minimum nonthermal energy density
in the relic sources from B(cid:5) obtaining U ∼ 8.1 and 3.4.Polarizationanalysis
eq min
4.3 × 10−13 erg/cm−3 for A2345-1 and A2345-2. The cor- Another important set of information about the magnetic field
responding minimum non-thermal pressure is then ∼5.0 and intherelicscanbederivedthroughthestudyofpolarizedemis-
2.7×10−13erg/cm−3. sion. As previouslymentioned,we could calibrate polarization
We are aware that the extrapolation to low energies or fre- onlyforobservationsat1.4GHz withthe Carray.The Pradio
quencies could overestimate the number of low-energy elec- imageoftheclusterisshowninFig.5.Thenoiseachievedinthe
trons,leadingtooverestimatingtheequipartitionmagneticfield P,Q,andUimages(Table4)arelowerthanthoseobtainedinthe
436 A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240
totalintensityimage.Infact,totalintensityimagesareaffected projectioneffects couldfurthercomplicatethe observedra-
by dynamicalrange limitation due to the presence of powerful dioemission.Takingalloftheseintoaccount,theobserved
radiosourcesnearourtarget.Thesesourcesarenotstronglypo- spectralindextrendofA2345-1cannotbe usedasanargu-
larized,sothatPimagesarenotaffectedbysuchlimitation,and menttoexcludeanoutgoingshockwave.
weakerpolarizedemissioncanberevealed.Wenoteinfactthat Wedo,however,notethatthepositionofA2345-1isinbe-
polarizedradioemissionoftherelicA2345-2revealsanarc-like tweenthemainclusterandthepossiblymerginggroupX1.
structurethatismoreextendedthanin totalintensityemission. Thuswesuggestthepossibilitythatitsradiopropertiescould
Thearc-likestructureofthisrelicindicatesthattheshockwave, beaffectedbythisongoingmerger.Inparticular,iftherelic
possiblyresponsiblefortheradioemission,hasbeenoriginated isseenedgeon,andifthemagneticfieldstrengthisalmost
∼2.6(cid:5)southernthepresentX-raycenter. uniformintherelicregion,theobservedspectralindextrend
Themeanfractionalpolarizationis∼22%inA2345-2,reach- couldbethesignofashockwavemovinginward,towardthe
ing values up to 50% in the eastern region. The relic A2345-1 cluster center. It could result from the interaction with X1.
shows a mean fractionalpolarization of ∼14% with higher po- DetailedopticalandX-rayobservationswouldbeneededto
larizedregion(∼60%)inthenorthwesternpartoftherelic.The shedlightonthispoint.
amountoffractionalpolarizationallowsustoestimatethelevel – TherelicA2345-2showstheclassicalfeatureof“elongated
of order of the magnetic field in the source. Following Burn relicsources”alsofoundindoublerelicsofAbell3667and
(1966),if weassumethatthemagneticfieldiscomposedofan Abell 3376, as well as in single relic sources as 1253+275
orderedcomponentB plusarandomisotropiccomponentrep- (Andernach et al. 1984; Giovannini et al. 1991) and A521
o
resentedbyaGaussianwithvarianceequalto2/3B2,itresultsin (Ferrari2003;Giacintuccietal.2008).Itislocatedfarfrom
r
theclustercenter,itsspectralindexissteepwithmeanvalue
1
P = P , (1) ∼1.3andsteepenstowardstheclustercenter,asexpectedby
oss int1+(B2r/B2o) relicformationtheoriesiftherelicisobservededgeon.The
valueoftheequipartitionmagneticfield,thedirectionofthe
where P is the observed fractional polarization, while P
oss intr
is given by P = 3δ+3. For the relic A2345-1, we obtain Evectors,andthedetectedlevelofpolarizationareconsis-
B2/B2 ∼ 4, minetarning3tδh+a7t the magnetic energy density in the tentwithpreviousobservationsofelongatedrelicsandagree
r o with expectationsfrom theoreticalmodelsas well. The po-
random component is four times higher than the one in the
larized emission image reveals the arc-like structure of the
ordered component. For the relic A2345-2, instead, we obtain
B2/B2 ∼2.Thisindicatesthatthemagneticfieldintheregionof relic A2345-2. If we assume that the relic originated in a
r o sphericalshockwave,wecaninferthepropagationcenterof
therelicA2345-2hasahigherdegreeoforder.Wealsohaveto
the shockby extrapolatingthe curvatureradiusof the relic.
consider possible beam depolarization, internal depolarization,
Itturnsoutthatthepropagationcenteris∼2.6(cid:5),southofthe
and ICM depolarization, so that what we can conclude from
this analysis is B2/B2 < 4 and <2 in A2345-1 and A2345-2, presentX-raycenteroftheclusterAbell2345(seeFig.3).
r o This corresponds to a physical distance of 450 kpc at this
respectively.
redshift.Fromweaklensinganalysisthegalaxyvelocitydis-
In A2345-1 the magnetic field is mainly aligned with the
persioninthisclusterresults∼900kms−1(Dahleetal.2002;
sharp edge of the radio emission, i.e. in the SW-NE direction.
Cypriano et al. 2004). As we will see in Sect. 5, the ex-
In the northernpartofthe relic, the E vectorsrotateand inthe
pectedMachnumberisofabout2.2forthisrelic.Sincethe
NWparttheyarealmostalignedtowardtheSW-NEdirection.In
galaxyvelocitydispersioniscomparabletothesoundspeed
A2345-2theEvectorsareperpendiculartotherelicmajoraxis,
intheICM(seee.g.Sarazin1988),aMachnumber2.2cor-
followingthe arc-like structurethat is marginallyvisible in the
respondstoavelocityof∼2000kms−1.TherelicA2345-2is
totalintensityimage.
∼800kpcfarfromthespherical-shockcenter.Ashockwave
with M ∼2.2travelsthisdistancein∼0.4Gyr(iftheshock
3.5.ResultsforAbell2345 speedremainsconstant).Thusthemergingbetweenthetwo
substructures should have occurred at ∼1200 kms−1 to ex-
Thepresentedanalysisconfirmsthatnonthermalemissionisas-
plaintheshiftoftheX-raycenterinthisscenario.Thisisa
sociatedwiththeICMofAbell2345.
reasonablevaluefortheclustermergervelocity.
– The properties of the western relic, A2345-1 are quite pe- Althougha precise estimate should consider the amountof
culiar.Indeed,itsmorphologyisrathercircularandfilamen- energyinjectedintheICMastheshockwavepassesthrough
tary, its brightnessdistributionis higher in the inner region it, and despite the number of assumptions and approxima-
oftherelic,anditsspectralindexsteepenstowardthecluster tions, we suggestthat the relic indicates the position of the
periphery.Althoughthestatisticisreallypoor,thesefeatures mergercenterasitwas∼0.4Gyago.Thetimethattheshock
havenotbeenfoundin otherdoublerelicsso far. The level wave has taken to get the present relic position is the time
of polarization,the magnetic field direction mainly aligned thatthesub-clusterhastakentogetthecurrentX-raycenter
with the sharp edge of the radio emission, and the value of position.
theequipartitionmagneticfieldinsteadagreeswithotherob-
servedrelics.
Diffusiveshockaccelerationmodelspredictasteepeningof
4. TheclusterAbell1240
the radio spectrum towards the cluster center (e.g. Ensslin
etal.1998;Hoeft&Brüggen2007)asaconsequenceofthe Little is known in the literature about this cluster. It is a rich
electron energy losses after shock acceleration. It is worth cluster classified as Bautz-Morgantype III. In Table 5, general
mentioningherethattheoreticalpredictionsrelyonsomeas- dataaboutthisclusterarereported.
sumptionsabouttheshocksymmetryandthemagneticfield Kempner & Sarazin (2001) have revealed the presence of
structurethatcouldbenotrepresentativeofthisspecificclus- tworoughlysymmetricrelicsfromtheWesterborkNorthernSky
ter environment.Moreover,if the relic is notseen edge on, Survey(WENSS).FromWENSSimages,relicsarevisibleat2
A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240 437
areanoiseartifactinthiscase.IntherelicA1240-2agradientis
visible alongthe mainaxisofthe relic,asfoundinAbell2256
byClarke&Ensslin(2006).
InFig.8theradialspectralindextrendisshownforA1240-1
and A1240-2. They were obtained as described in Sect. 3.1.
Spherical shells are centered close to the X-ray cluster center
andareparalleltothemainaxisofbothrelics.
Despite the small extension of the relics at 325 MHZ, it is
stillpossibletoderivesomeimportantresultsonthespectralin-
dex radial trendsin these relics: in the relic A1240-1the spec-
tral index is steeper in the inner part of the relic and flatter in
theouterpart,asfoundinA2345-2andpredictedby“outgoing
mergershock”modelsifrelicsareseenedgeon(Roettigeretal.
1999;Bagchietal.2006).Thesametrendisconsistentwiththe
spectralindexprofilederivedinA1240-2,althougha firmcon-
clusioncannotbederivedfromthesedata.Wenote,infact,that
errorsandupperlimitintheinnershellcannotexcludeaconstant
spectralindexorevenanoppositetrend.
Fig.6.Abell1240.Colors:opticalemissionfromDPOSSII(redband);
Contours:radioemissionat1.4GHz(HRimage).Contoursstartat±3σ
andarethenspacedby2.RedcrosssignstheX-raycenter,labelsrefer 4.2.Radio-X-raycomparison
tothediscretesourcesembeddedinA1240-1.
We retrievedX-rayobservationsinthe energyband0.5−2keV
from the ROSAT data archive. The cluster is ∼28(cid:5) offset from
and 2.5σ level. Our VLA observation confirm the presence of
thecenteroftheROSATpointing.Observationswereperformed
two weak radio-emitting regions in the cluster’s outskirts. The
with the ROSAT PSPC detector for a total exposure time of
radio image of the cluster is shown in Fig. 6 (contours) over-
∼12ks.Afterbackgroundsubtractiontheeventfilewasdivided
laid on optical emission (from the DPOSSII, red band). The
northern relic (A1240-1) is located at ∼270(cid:5)(cid:5) from the cluster bGyauthsseiaenxpoofsσur=em60a(cid:5)p(cid:5).. We smoothedthe resulting image with a
X-raycenter.Thisdistancecorrespondsto∼700kpcattheclus-
ter’sredshift.ThisrelicismainlyelongatedintheEWdirection, The resulting image is shown in Fig. 9, where the X-ray
anditsradiobrightnessdecreasesgoingfromthewesterntothe emissionoftheclusterissuperimposedontoradiocontours.The
easternpartsoftherelic(seeFig.7).At325MHz,onlytheeast- X-ray emission of this cluster is elongated in the SN direction
ernbrightestpartisvisible.Thisislikelyduetothehighernoise and shows a double X-ray morphology. As already stated by
inthe325MHzimage.Infactfromthemeanbrightnessofthe Kempner & Sarazin (2001) this morphologyis consistent with
weakerpartoftherelic,weestimatedthatitshouldhaveaspec- aslightlyasymmetricmerger.
tral index >3 to be detected at 325 MHz. Three radio sources Relics are located at the edge of the X-ray emission. Their
are embedded in the relic emission, and are labeled with A−C emission shows the characteristic elongated shape, and their
inFig.6.ThesourcesAandBarenotdetectedinthe325MHz main axis is perpendicularto the main axis of the X-ray emis-
observations.Thisisconsistentwithspectralindexvalues<1,as sion,asfoundindoublerelicsofAbell3367andAbell3376.
commonlyfoundinradiogalaxies.Aweakemissionat1.4GHz
linkstheAradiosourceattherelic(seeFig.7).
Thesouthernrelic(A1240-2)islocatedat∼400(cid:5)(cid:5)(1.1Mpc) 4.3.Equipartitionmagneticfield
from the cluster X ray center. At 1.4 GHz, it is elongated in
the EW direction extending ∼480(cid:5)(cid:5). No discrete sources have Under the same assumptions explained in Sect. 3.4, we cal-
been found embedded in the relic emission. Also in this case culated the equipartition magnetic field for the relics A1240-1
at325MHz,therelic’sextensionisreducedto∼350(cid:5)(cid:5)alongthe andA1240-2.ValuesobtainedarereportedinTable7.We note
mainaxis,andonlythebrightestregionsarevisibleat325MHz. that these values were computed considering the brightness of
The relic’s physical parameters are reported in Table 7. The those pixels for which we have well-constrained information
quantity are computed excluding the region where discrete aboutthespectralindexvalue,i.e.thoseregionswhoseemission
sources(A−C)arepresent. isdetectedatbothfrequencies.Sincetheemissionat325MHz
is only detected in a small regionof the relics, while relics are
more extended at 1.4 GHz, the equipartition estimates refer to
4.1.Spectralindexanalysis the same small regions, and differentestimates could be repre-
sentativeofthewiderrelicemissiondetectedat1.4GHz.
WereportinFig.7thespectralindexmapandthespectralindex
maperrorfortherelicsofAbell1240.Theywereobtainedcon- We derivedthe minimum nonthermalenergydensity in the
sideringonlythosepixelsthathaveabrightness>2σatbothfre- relicsourcesfromB(cid:5) obtainingU ∼5.5×10−13erg/cm−3for
eq min
quencies.Figure7showsthatthespectralindeximageispatchy. A1240-1 and A1240-2. The corresponding minimum nonther-
Thespectralindeximagerms,σ ,is∼0.3and0.4forA1240-1 malpressureisthen∼3.4and∼3.5×10−13 erg/cm−3.Thecon-
spix
and A1240-2, respectively, while the mean of the spectral in- sistency between magnetic field equipartition values and mag-
dex error image, (cid:7)Spix Noise(cid:8) is ∼0.2 for both of the relics. netic field lower limits derivedby X-ray emission in otherfew
We canthen concludethatfeaturesin A1240-2are statistically clusters(seediscussioninSect.3.3)indicatesthatequipartition
significant, while given the small difference between σ and magneticfieldcanbeusedasareasonableapproximationofthe
spix
(cid:7)SpixNoise(cid:8)inA1240-1,wecannotexcludethatlocalfeatures magneticfieldinrelics.
438 A.Bonafedeetal.:DoublerelicsinAbell2345andAbell1240
A1240-1 spectral index A1240-1 spectral index error
DEC DEC
43:12 43:12
A1240
43:11 12:00.0 43:11
43:10
43:10
10:00.0
43:09
43:09
11h23m40s 11h23RmA30s 11h23m20s 11h23m10s 08:00.0 11h23m40s 11h23mR3A0s 11h23m20s 11h23m10s
n
atio 06:00.0
n
0 1 (SP INDEX) 2 Decli 04:00.0 0 0.1 (SP 0IN.2DEX) 0.3
A1240-2 spectral index A1240-1 spectral index error
DEC 02:00.0 DEC
43:02
43:02
43:00:00.0
43:01 500 kpc 43:01
42:58:00.0 43:00
43:00 11:24:00.0 40.0 23:20.0
42:59 Right ascension 42:59
11h24m00s 11h23m45s 11h23m30s
11h24m00s 11h23m45s 11h23m30s RA
RA
Fig.7. Center: thecluster Abell 1240 radioemission at 1.4GHz.Thebeam is42(cid:5)(cid:5) ×33(cid:5)(cid:5). Contours startat 3σ (0.13mJy/beam) andarethen
spacedbyafactor2.ThecrossmarkstheclusterX-raycenter.Left:colorsrepresentthespectralindexoftherelicA1240-1(top)andA1240-2
(bottom)superimposedovertheradioemissionat325MHz(contours)Thebeamis42(cid:5)(cid:5) ×33(cid:5)(cid:5),firstcontoursare2σ(2mJy/beam),3σandare
thenspacedbyafactor2.Right:spectralindexerrorimage(colors)superimposedontotheemissionat325MHz(contoursareasabove).
Fig.8.SpectralindexradialtrendofA1240-1 (left)andA1240-2 (right),computedinshellsof∼50(cid:5)(cid:5) inwidth.Itwascomputed excluding the
contributionofthediscretesources.Crossesrefertospectralindexvaluescomputedinshellswherethemeanbrightnessis>3σatboth325MHz
and1.4GHz.Arrowsare3σupperlimitsonthespectralindexmeanvalue(seetext).Intheinset:displacementoftheshellsoverwhichthemean
spectralindexhasbeencomputed.Circlesrefertothediscretesourcesembeddedintherelicemission.TheredcrossreferstotheclusterX-ray
center,thebluecrossisthecenterofthesphericalshells.
4.4.Polarizationanalysis polarization of A1240-1 is 26%, reaching values up to 70%.
In the relic A1240-2 the mean fractional polarization is 29%,
We obtainedthepolarizedintensityimagesfortherelicsasde- reachingvaluesupto70%.FromEq.(1)wederivethatB2/B2∼
r o
scribed in Sect. 3.4. In Table 4 the parameters relative to the 1.4 and 1.2 respectively.Because of possible beam depolariza-
polarizationimages of the relics A1240-1and A1240-2are re- tion, internal depolarization and ICM depolarization, we con-
ported. In Fig. 10 the polarized emission of the two relics is cludethat B2/B2 <1.4and<1.2.Thismeansthatthemagnetic
r o
shown. Observationsperformedwith C array cannotrevealthe energydensityintherandomandorderedcomponentissimilar.
weak extended emission, and thus only the most compact and
bright regions are visible in this image. In these regions, the
magnetic field is mainly aligned along the relic main axis in 4.5.ResultsforAbell1240
both of the relics. This is consistent with what has been ob-
served in the relics of Abell 2345 and with what is expected Our observations confirm the presence of two relics in
from the models that explain the origin of these sources (e.g. Abell 1240 with a spectral index values as steep as ∼1.2 and
Ensslin etal. 1998; Roettiger etal. 1999). The mean fractional ∼1.3.Thespectralindextrendsderivedfortheserelicsindicate