NASA Technical Reports Server (NTRS) 20130014774: Discovery of a Giant Radio Halo in a New Planck Galaxy Cluster PLCKG171.9-40.7

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PreprinttypesetusingLATEXstyleemulateapjv.5/25/10
DISCOVERYOFAGIANTRADIOHALOINANEWPLANCKGALAXYCLUSTERPLCKG171.9-40.7
SIMONAGIACINTUCCI1,2,RUTAKALE3,4,DANIELR.WIK5,6,TIZIANAVENTURI3,MAXIMMARKEVITCH5
submittedtoTheAstrophysicalJournal
ABSTRACT
Wereportthediscoveryofagiantradiohaloinanew,hot,X-rayluminousgalaxyclusterrecentlyfoundby
Planck,PLCKG171.9-40.7.TheradiohalowasfoundusingGiantMetrewaveRadioTelescopeobservationsat
3 235MHzand610MHz,andinthe1.4GHzdatafromaNRAOVeryLargeArraySkySurveypointingthatwe
1 havereanalyzed.ThediffuseradioemissioniscoincidentwiththeclusterX-rayemission,hasanextentof∼1
0 Mpcandaradiopowerof∼5×1024WHz−1at1.4GHz. Itsintegratedradiospectrumhasaslopeofα≈1.8
2
between235MHzand1.4GHz,steeperthanthatofatypicalgianthalo. TheanalysisofthearchivalXMM-

b Newton X–ray data shows that the cluster is hot (∼10 keV) and disturbed, consistent with X-ray selected
e clustershostingradio halos. This is the first giantradio halodiscoveredin oneof the new clustersfoundby
F Planck.
1 Subjectheadings:galaxies: clusters: general— galaxies: clusters: individual(PLCKG171.9-40.7)— inter-
galacticmedium—radiocontinuum:general—X–rays:galaxies:clusters

]
O
1. INTRODUCTION 2010; Brown&Rudnick 2011; Brunettietal. 2012). In par-
C
ticular,radiohaloswith verysteepspectralindex7 (α>1.5)
h. briSgphetncteascsulaanrdd∼iffMuspecreaxdtieonts,ouarrceeso,bsweirtvhedveirnysloomwesuorffathcee have been found (the prototype case is A521 with α 2,
p Brunettietal. 2008; Dallacasaetal. 2009), consistent with
most massive galaxy clusters. Depending on their loca-
- turbulent reacceleration in weak mergers, i.e., collisions be-
tion, i.e., cluster center versus outskirts, these cluster-wide
ro sources are classified as giant radio halos or relics (see tween clusters with relatively low mass (<1015 M(cid:2)) or ac-
t Ferrarietal. 2008; Venturi 2011; Ferettietal. 2012, for re- cretionofsmallersystemsintoamassivecluster. Secondary
s
a views; see vanWeerenetal. (2012); vanWeeren (2011); models have difficulty explaining such ultra-steep spectrum
[ Kale&Dwarakanath (2010,2012);Giacintuccietal.(2011); radio halos (USSRHs), due to the large energy required in
the form of relativistic protons (e.g., Brunettietal. 2008;
Brown&Rudnick (2011); Bonafedeetal. (2012) for recent
1 results). Halos and relics are due to synchrotron emission Macarioetal.2010;vanWeerenetal. 2011). However,only
v two giantradiohalosare confirmedUSSRHs so far – A521
from ultra-relativistic electrons with energy of several GeV,
8 spinning in μG magnetic fields that permeate the hot intra- and A697 – based on a well constrained spectrum with five
1
clustermedium(ICM). data points between 150 MHz and 1.4 GHz (Macario et al.
2
Particle acceleration and magnetic field amplification 2013).OtherpossibleUSSRHsstilllackconfirmationoftheir
0
. during cluster mergers have been proposed as mecha- spectralindex(e.g.,Giacintuccietal. 2011)andarecurrently
2 nisms by which diffuse radio emission is generated in underinvestigation. Therefore,itisnecessarytoconfirmthe
0 clusters. In particular, relics should be produced by candidates and find more of these USSRHs, with an accu-
3 electron (re)acceleration at merger shocks (Ensslinetal. rate determination of their spectral index, before ruling out
1 1998; Markevitchetal. 2005; Hoeft&Brüggen 2007; thesecondarymodels.
v: Kang&Ryu 2011; Kangetal. 2012), while giant ha- A number of dedicated searches for cluster dif-
i los may be caused by reacceleration of lower energy fuse radio sources have been carried out in the past
X relativistic electrons by MHD turbulence during merg- decade (e.g., Giovanninietal. 1999; Kempner&Sarazin
r ers (Brunetti 2011, and references therein). Although 2001; Venturietal. 2007, 2008; Giovanninietal. 2009;
a alternative possibilities have been proposed for the ori- vanWeerenetal. 2009; vanWeerenetal. 2011), and the
gin of the radio-emitting electrons – the secondary mod- numberofgianthalosandrelicshasrapidlyincreased.While
els (e.g., Dennison 1980; Blasi&Colafrancesco 1999; ∼50 objects are presently known (including candidates),
because of their low surface brightness, most of them lack
Pfrommer&Enßlin2004;Keshet&Loeb2010;Enßlinetal.
detailedspectralinformation,crucialtotesttheoreticalmod-
2011) – turbulent reacceleration models seem to be fa-
els and understand the origin of the radio-emittingelectrons
voredbycurrentradioobservations(e.g.,Venturietal.2008;
(e.g.,Venturietal.2013).
Brunettietal.2008,2009;Macarioetal.2010;Donnertetal.
ThePlanck8 EarlySunyaev-Zel’dovich(ESZ)clustersam-
ple (PlanckCollaborationetal. 2011b) is an all-sky sam-
1DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD
ple of clusters, detected by their multi-frequency signa-
20742,USA;[email protected]
2JointSpace-Science Institute, UniversityofMaryland, CollegePark,
MD,20742-2421,USA 7WeadopttheconventionSν∝ν−α,whereSνistheisthefluxdensityat
3INAF-IstitutodiRadioastronomia,viaGobetti101,I-40129Bologna, thefrequencyν.
Italy 8 Planck (http://www.esa.int/Planck), a project of the European Space
4DipartimentodiFisicaeAstronomia,viaRanzani1,I-40127Bologna, Agency(ESA)withinstrumentsprovidedbytwoscientificconsortiafunded
Italy byESA member states (in particular the lead countries France and Italy),
5AstrophysicsScienceDivision,NASA/GoddardSpaceFlightCenter, withcontributions fromNASA(USA)andtelescopereflectorsprovidedby
Greenbelt,MD20771,USA acollaborationbetweenESAandascientificconsortiumledandfundedby
5NASAPostdoctoralPositionFellow Denmark.
2 S.Giacintuccietal.
235 MHz. Details on these observationsare summarized in
TABLE1
Table 2, which reports observing date, frequency and total
GENERALPROPERTIESOFPLCKESZG171.94-40.65
bandwidth (columns 1, 2 and 3), total time on source (col-
umn4),usabletimeafterdataediting(column5),full-width
half maximum (FWHM) and position angle (PA) of the full
RDAECJ2J0200000(h(◦m(cid:3)s(cid:3))(cid:3)) 003812225170.4 array(column6),rmslevel(1σ)atfullresolution(column7).
zFe 0.27 ThedatasetswerecalibratedandreducedusingtheNRAO
LX[0.1−2.4]kev(1045ergs−1) 1.13 AstronomicalImageProcessingSystem(AIPS)package.The
kT (keV) 10.65 datawereinitiallyinspectedtoidentifyandremovebadchan-
M500(M(cid:4)) 1.09×1015 nels,timeintervalsandvisibilitieswithradiofrequencyinter-
DL(Mpc) 1364.5 ference(RFI). The data were foundto be affected by severe
angularscale(kpc/(cid:3)(cid:3)) 4.135 phase instabilities caused by ionosphericscintillation during
mostoftheobservation,leavingonly∼1hourofusabletime
at both 610 MHz and 235 MHz. The data were then cali-
NotestoTable1–RAJ2000 andDECJ2000 arethecoordinates oftheX-ray
peak, zisfromtheironKline. Theluminosity LX[0.1−2.4]kev,temperature brated. Thefluxdensityscalewassetusing3C48and3C147
kT andmassM500 areestimatedwithinR500,whereR500 istheradiuscor- asamplitudecalibratorsandtheBaarsetal.(1977)scale. The
respondingtoadensitycontrastof500(PlanckCollaborationetal.2011c). source 0323+055was used as a phase calibrator. The band-
pass calibration was carried out using the flux density cali-
ture in the Planck microwave observing bands (30–857 brators. Few of the central channels free of RFI were used
GHz; PlanckCollaborationetal. 2011a). Clusters are de- tonormalizethebandpassforeachantenna. Afterthe band-
tected through the measurement of the spectral distortion passcalibration,thecentral420channelswereaveragedto35
of the cosmic microwave background (CMB) due to in- channelseach 0.78 MHz wide to reduce the size of the data
verse Compton scattering of CMB photons by the thermal setat610MHz,and,atthesametime,tominimizetheband-
electrons in the ICM – the Sunyaev Zel’dovich (SZ) effect widthsmearingeffectswithintheprimarybeamoftheGMRT
(Sunyaev&Zeldovich 1972). The Compton y-parameter, a antenna.At235MHz,72channelswereaveragedto18chan-
measure of the SZ effect, does not suffer dimming due to nelsof0.25MHzwidtheach.
distance, and its integral over the cluster is a measure of Afterfurthercarefuleditingintheaverageddata,anumber
the total thermal energy in the cluster. Therefore, SZ sur- of phase-only self-calibration cycles and imaging were car-
veysareapowerfultooltofindmassiveclustersathighred- ried out to reduce residual phase variationsand improvethe
shifts. A totalof51new clustersdiscoveredbyPlanckhave quality of the final images. Wide-field imaging was imple-
been confirmed by follow-up X-ray XMM-Newton and op- mentedateachstepoftheself-calibrationprocess,toaccount
tical observations (PlanckCollaborationetal. 2011c, 2012a, for the non-planarnature of the sky. The final images were
2012b). Most of these new clusters appear to be hot, mas- madeusingthemulti-scaleCLEANimplementedinIMAGR,
sive systems with highly irregular and disturbed X-ray mor- whichresultsinbetterimagingofextendedsourcescompared
phologies,andthusgoodtargetstosearchfornewradiohalos tothetraditionalCLEAN(e.g.,Clarke&Ensslin2006,fora
and relics. Recently, double radio relics in one of the new detaileddiscussionseeAppendixAinGreisenetal.(2009)).
Planckclusters,PLCKESZG287.0+32.9,havebeendiscov- Weusedδ-functionsasmodelcomponentsfortheunresolved
ered by Bagchietal. (2011) using the Giant Metrewave Ra- featuresandcircularGaussiansfortheresolvedones,within-
dio Telescope (GMRT). Here, we report a GMRT detection creasingwidthto progressivelyhighlightthe extendedemis-
of a giant radio halo in the newly discovered Planck cluster sionduringthe clean. Thermssensitivity levelsachievedin
PLCKESZ G171.94-40.65 (hereafter PLCK171). The clus- theimagesatfullresolutionis∼0.12mJybeam−1at610MHz
ter, whose general properties are summarized in Table 1, is and∼0.80mJybeam−1 at235MHz(Table2). We also pro-
anX-rayluminous(L ∼1045ergs−1)systemataredshiftof ducedimageswithlowerresolutions,downto40(cid:3)(cid:3),bytaper-
X
z =0.27±0.01,asdeterminedthroughtheXMMFeKline ing the uv data using UVTAPER and ROBUST in IMAGR.
Fe
spectroscopy(PlanckCollaborationetal.2011c). Anoptical The noise reached in the images at the lowest resolution is
photometricredshiftofz =0.31±0.03,basedon29cluster ∼0.3mJybeam−1 at610MHzand∼2.3mJybeam−1at235
opt
members,isreportedbyPlanckCollaborationetal.(2012a). MHz. Residual amplitude errors should be within ∼5% at
Inthispaper,weadoptz=0.27andtheΛCDMcosmology 610MHz(e.g.,Chandraetal.2004);acalibrationuncertainty
with H0=70 km s−1 Mpc−1, Ωm =0.3 and ΩΛ =0.7. Under of∼20%isestimatedat235MHz.
theseassumptions,1(cid:3)(cid:3)correspondsto4.135kpc.
2.2. VLAobservations
2. RADIOOBSERVATIONS
WealsoreprocessedandanalyzedVeryLargeArray(VLA)
2.1. GMRTobservations data at 1.4 GHz from the NVSS (NRAO VLA Sky Survey;
PLCK171 was observedwith the GMRT at 235 MHz and Condonetal. 1998) pointing containing PLCK171 (project
610MHz in October2011, as partof a projectto search for AC308). Thedata werecalibratedandreducedin AIPS.We
diffuseradioemissionin 8 Planckclusters(project21_017). usedthestandardFouriertransformdeconvolutionmethodto
The cluster was observed for ∼5 hours in dual-frequency produce the images (CLEAN and RESTORE), and applied
mode, recordingLL polarizationat 235MHz and RR polar- self-calibrationto reduce the effectsof residual phase errors
izationat610MHz. inthedata. Thermssensitivitylevelachievedinthefinalim-
The data were collected in spectral-line mode, using the ageis∼0.2mJybeam−1,witharestoringbeamof58(cid:3)(cid:3)×48(cid:3)(cid:3).
GMRT software backend(GSB; Royetal. 2010) with a to- Ourimageis∼2timesmoresensitivethanthepublicNVSS
tal observing bandwidth of 32 MHz at 610 MHz divided in image,whichhasalocalrmsnoiseof∼0.4mJybeam−1(50(cid:3)(cid:3)
512 spectral channels. A bandwidth of 6 MHz was used at restoringbeam).Residualamplitudeerrorsarewithin∼5%.
AgiantradiohaloinanewPlanckcluster 3
TABLE2
DETAILSOFTHEGMRTOBSERVATIONS.
Observation ν Δν ttot t FWHM,p.a. rms
date (MHz) (MHz) (hours) (hours) ((cid:3)(cid:3)×(cid:3)(cid:3),◦) (mJybeam−1)
Oct22,2011 235a 6 5 ∼1 15.4×9.0,64 0.80
Oct22,2011 610a 32 5 ∼1 5.5×4.5,0 0.12
NotestoTable2–a:observedinsimultaneous235MHz/610MHzmode.
TABLE3
DISCRETERADIOSOURCESINPLCK171
Source RAJ2000 DECJ2000 Stot,610MHz Stot,235MHz α263150MMHHzz morphology
(h,m,s) (◦,(cid:5),(cid:5)(cid:5)) (mJy) (mJy)
S1 031253.2 +082312 11.7±0.6 >15.5±3.3 >0.3±0.2 NAT
S2 031253.9 +082305 4.3±0.2 14.6±3.1 1.3±0.2 unresolved
S3 031256.9 +082211 2.7±0.2 † - unresolved
S4 031257.5 +082209 85.8±4.3 † - NAT
S5 031257.3 +082137 6.0±0.3 >8.1±1.9 >0.3±0.3 NAT
S6 031258.3 +082114 39.2±2.0 66.5±13.3 0.6±0.2 NAT
S7 031254.4 +082035 1.9±0.1 - - unresolved
NotestoTable3–Peakcoordinates arefromtheGMRTfull-resolution imageat610MHz(Fig.1). Fluxdensities havebeenmeasuredontheGMRTfull-
resolutionimagesat610MHzand235MHz(Fig.2). S1andS5arepartiallydetectedat235MHz,thereforelowerlimitsarereportedforStot,235MHzandα. †
SourcesS3andS4arenotresolvedinthe235MHzimage.Theircombinedfluxdensityat235MHzis248.4mJy.
3. RADIOGALAXIESINTHECLUSTERREGION
Figure 1 presents the GMRT 610 MHz contour image of
PLCK171atfullresolution(∼5(cid:3)(cid:3)),overlaidontheopticalim-
agefromtheDigitalSkySurvey(DSS).Discreteradiosources s
0 S1
are labelledfromS1 to S7. Theirflux densitiesat610MHz 0
m
and positions, both measured from the image in Fig. 1, are 3 S2
2
summarizedin Table3. All haveopticalcounterpartson the
DSS, exceptforS7. Noredshiftmeasurementsarecurrently
n
racSeol6opi)ngosscrh.tiedodewTnhftaoewrnciattlhrhuresotsthweeer-gapXanel-gaarlkxaeiyoetasfcieSlinn(4Nt.rtehAFeTo(Tu)liramtbesorloearutpru1hcr)eoesilsoa(gnSayd1p.,poTSrpoh4txei,ciStmaa5lialctsaeanoltdy-f Declinatio 8d22m00s S4 S3
S4 andS6are bothorientednorthward,while the tailsofS1 +
S5
andS5pointtowardSouth-EastandNorth-West,respectively,
almostoppositetoeachother. AssumingthattheNATsareat s
0
the redshift of the cluster, the length of their tails ranges, in 0 S6
m
projection, from ∼90 kpc (S5) to ∼180 kpc (S6), and their 1 0.5’
2 S7
radio powers at 610 MHz are of the order of ∼1024−25 W 120 kpc
Hz−1. Thesevaluesareconsistentwiththerangeofsize and
radiopowertypicallymeasuredforNATradiogalaxies(e.g., 3h13m00s 56s 12m52s
Feretti&Venturi2002). Right ascension
ThefullresolutionGMRTimageat235MHzispresented
FIG.1.—GMRTfull-resolutionimageat610MHz(contours)overlaidon
in Fig.2 (greyscale), with thepositionsof sourcesS1 toS7 theopticalDSSimage. Therestoringbeamis5.5(cid:3)(cid:3)×4.5(cid:3)(cid:3),inp.a. 0◦. The
markedby redcrosses andlabels. SourceS7 is notdetected r.m.s.levelintheimageplaneis0.12mJybeam−1.Contoursstartat0.4mJy
at 235 MHz. This is consistentwith the fact that the source beam−1 andthenscale byafactor of2. Nonegative levels corresponding
appears very compact at 610 MHz, so it is probably an ac- tothe−3σ levelarepresentinthisregion. Labelsindicate thediscretera-
diosources,whosepropertiesaresummerizedinTable3. TheclusterX-ray
tive AGN with flator invertedspectrum. The 235MHz flux
centerisatRA=3h12m57.4s,DEC=+08d22m10.3s(Tab.1).
densitiesofS1,S2, S5andS6arereportedinTable3, along
with the spectral index α computed between 235 MHz and
610 MHz. Sources S1 and S5 are only partially detected at tomeasuretheirindividualfluxdensityat235MHz. Atotal
235MHz,thusthefluxdensityandspectralindecesinTable of248.4mJyismeasuredfortheblend.
3 should be considered as lower limits. Sources S3 and S4
appearblendedtogetherinFig.2, thereforeitisnotpossible 4. THEGIANTRADIOHALO
4 S.Giacintuccietal.
the source-subtracteddata set, using only the long baselines
(>3 kλ). We indeedfoundsomeresidualemissionpossibly
0s associatedwiththeblendofS3andS4(∼7mJy)andwithS6
m0 2’ (∼6 mJy). Thiswill be takeninto accountwhen measuring
5 500 kpc thefluxdensityandspectralindexofthehaloinSect.5.
2
HintsofthehaloarealreadyvisibleonthepublicNVSSim-
ageat1.4GHz.Toobtainabetterqualityimageofthediffuse
source, we re-analyzed the NVSS pointing (see Sect. 2.2).
s Our 1.4 GHz imagesare presented in Fig. 3. Panel a shows
ation 3m00 S2 S1 tFhoelltoowtailngratdhieopermoicsesdiounre, ia.de.opthteedhaatlo23a5ndMtHhez,rawdeioreg-adlearxivieesd.
n 2 theimageusingonlythebaselineslongerthan0.8kλ(angular
ecli S4 S3 scales (cid:2)4(cid:3)) to identifythe contributionof the discrete radio
D
sourcesin the cluster area. A central regionof the resulting
s S5 image is shown in panel b, with the position of the sources
0 S6
0 detected at 610 MHz (Fig. 1) highlightedby crosses and la-
m
1 bels. Even though the resolution of the two images is very
s2 different(∼5(cid:3)(cid:3)at610MHzand∼50(cid:3)(cid:3)at1.4GHz),thereisa
8 S7
0 goodmatchbetweenthelocationofthe radiosourcesat610
+
MHzandthestructureimagedat1.4GHz.TheCLEANcom-
04s3h13m00s 56s 52s 12m48s ponentsassociatedwiththisimagewerethensubtractedfrom
Right ascension the uv-data, and the resulting data set was used to obtain an
Jy/beam
imageof just the radiohalo, shownin Figure 3c (colorsand
contours). An overlay of this 1.4 GHz diffuse emission on
theGMRT235MHzimageoftheradiohalo,convolvedwith
0.0010 0.0030 0.0050 0.0070 0.0090 0.0110 a circular beam of 40(cid:3)(cid:3), is shown in Figure 3d. The overall
shapeofthehaloissimilaratbothfrequencies,whileitsex-
FIG.2.—GMRTfull-resolutionimageat235MHz(grayscale)withover- tentisslightlylargerat1.4GHz(∼1Mpc).Thedifferenceis
laid the low-resolution image shown as contours. The restoring beam is
15.4(cid:3)(cid:3)×9.0(cid:3)(cid:3),inp.a. 64◦(showninthebottom-leftcorner)and28(cid:3)(cid:3)×25(cid:3)(cid:3), likely dueto the low sensitivity and quality of the 235MHz
inp.a. 73◦,respectively. Ther.m.s. levelis0.8mJybeam−1 atfullreso- observation,whichwasstronglyaffectedbyionosphericscin-
lutionand2.3mJybeam−1 inthelow-resolutionimage. Contourlevelsare tillationduringmostoftheobservingtime(Sect.2.1).
6.9,11.5,18.4,27.6mJybeam−1.Nonegativecontourscorrespondingtothe
−3σlevelarepresentintheportionoftheimageshown.Thediscretesources 5. INTEGRATEDSPECTRUMOFTHERADIOHALO
detectedat610MHz(Fig.1;Table3)arelabelledandmarkedbythecrosses.
Usingtheimagesofthediffusecomponentalone(Fig.2and
Fig.3c),wemeasuredatotalfluxdensityofthehaloof483±
110mJyat235MHz9and18±2mJyat1.4GHzintheregion
TABLE4
PROPERTIESOFTHERADIOHALOINPLCK171 defined by the first contour in Fig. 3d (3σ). These are also
reportedin Table 4, alongwith the other main observational
propertiesof the halo, andin Fig. 4. The derivedtotalradio
S235MHz(mJy) 483±110 powerat1.4GHzisP1.4GHz=(4.9±0.1)×1024WHz−1.
S610MHz(mJy) >50 Our flux density values at 235 MHz and 1.4 GHz corre-
S1400MHz(mJy) 18±2 spondto a verysteep spectralindex,α=1.84±0.14,which
α(235MHz÷1400MHz) 1.84±0.14 makes PLCK171 one of the steepest-spectrum giant halos
P1400MHz(WHz−1) (4.9±0.1)×1024 known so far. However, large uncertainties affect our mea-
Linearsize(Mpc) ∼1 surement. In particular, it is possible that some of the halo
flux density is missed in both the reprocessed NVSS image
andthe GMRT 235 MHz image due to the shortdurationof
theobservations,whichresultsin sparsecoveragesoftheuv
A low-resolution (28(cid:3)(cid:3) ×25(cid:3)(cid:3)) image at 235 MHz of planeandlossofsensitivitytostructure.Furthermore,thelow
PLCK171is presentedas contoursin Fig. 2, overlaidon the resolution of the 1.4 GHz image does not allow an accurate
full-resolution image in gray scale. The image shows the removalofthediscreteradiogalaxiesinthehaloregion,and,
residual emission after subtraction of the CLEAN compo- consequently,thehalofluxdensitycouldbeoverestimated.
nentsofsourcesS1toS6fromtheuvdata.TheCLEANcom- A flux density of ∼45–50 mJy is measured from low-
ponentswereobtainedfromanimageproducedusingonlythe resolution(∼30−40(cid:3)(cid:3))imagesat610MHz(notshownhere),
baselineslongerthan 3 kλ, containinginformationon struc- obtained after the subtraction of the discrete sources. How-
turesonangularscales(cid:2)1.4(cid:3). Thepresenceofalargediffuse ever,theseimagesdetectonlythebrightestpeaksofthehalo
source filling the central region of the cluster is evident. Its and,therefore,itsfluxdensityatthisfrequencyisonlyalower
largelinearsize(∼800kpcinFig.2),roundishmorphology limit. The 610 MHz measurementis ∼40% lower than the
andthecentralclusterlocationareconsistentwiththeproper- valueexpectedbasedonthespectralindexbetween235MHz
tiesofagiantradiohalo. and1.4GHz(Fig.4). OurexperiencewiththeGMRTat610
We note that the peaks of the radio halo image in Fig. 2 MHz suggests that the flux density losses at this frequency
areapproximatelycoincidentwiththepositionofsomeindi-
vidualsources. Toestimatethepossibleresidualcontribution 9Theresidual13mJy,possiblyassociatedwiththeindividualsources(see
of these sources, we madean image of the haloregionfrom Sect.4),havebeenaddedintotheerror.
AgiantradiohaloinanewPlanckcluster 5
a 2’ b
s
m00s m00 500 kpc
6 4
2 2
0s S1
0
m
Declination 2422m00sm00s Declination 22m00s SS45 S3 S2
0
2
8d 4’
+ S6
~ 1 Mpc S7
10s 3h13m00s 50s 12m40s s
0
(Jy/beam) Right ascension m0
0
2
d
8
0.00039 0.00048 0.00082 0.00219 0.00767 0.02935 + 04s 3h13m00s 56s 52s 12m48s
Right ascension
c d
m00s m00s
26 26
Declination 24m00s22m00s Declination 24m00s22m00s
m00s m00s
+8d20 4’ +8d20 4’
~ 1 Mpc ~ 1 Mpc
10s 3h13m00s 50s 12m40s 10s 3h13m00s 50s 12m40s
(Jy/beam) Right ascension (Jy/beam) Right ascension
0.00018 0.00054 0.00090 0.00126 0.00162 0.00198 0.0010 0.0046 0.0082 0.0118 0.0155 0.0191
FIG.3.—VLA–Darrayimagesat1.4GHzofPLCK171obtainedfromthere-processedNVSSdata. Contourlevelsare−1(dashed),1,2,4,8,16,...×3σ. a:
ContoursandcolorscaleimageofPLCK171.Therestoringbeamis58(cid:3)(cid:3)×48(cid:3)(cid:3),inp.a. −5◦and1σ=0.17mJybeam−1.b:Imageobtainedusingonlybaselines
longerthan0.8kλ.Therestoringbeamis50(cid:3)(cid:3)×42(cid:3)(cid:3),inp.a.−7◦and1σ=0.4mJybeam−1.Crossesandbluelabelsshowthepositionofthediscreteradiosources
detectedat610MHz(Fig.1,Table3). c: Imageofthediffuseradiohaloafterthesubtractionofthediscreteradiosourcesderivedfromtheimageinpanelb.
Theimagehasbeenrestoredwithacircularbeamof50(cid:3)(cid:3).Ther.m.s.noiseis1σ=0.12mJybeam−1.d:1.4GHzcontours(sameaspanelc)overlaidonthe235
MHzimageoftheradiohaloconvolvedwithacircularbeamof40(cid:3)(cid:3).
canindeedbeconspicuous,upto∼40%(Venturietal.2008; tively),excludinganomalousMOSCCDs(inthiscaseCCDs
Dallacasaetal.2009;Macarioetal.2010). 4, 5, and 6 for MOS1), and identifying and excluding point
sources. Spectraandimagesarethenextracted,includingfor
6. X-RAYANALYSIS the quiescent particle background, which is derived from a
PLCK171 was observed with XMM-Newton for 14 ks in databaseoffilterwheelclosedobservationsthatarematched
2010 August under ObsID 0656201101. The EPIC data todatafromtheunexposedcornersofthechips. Theremain-
were reduced using the Extended Source Analysis Software ingbackgroundcomponents(residualsoftprotoncontamina-
(XMM-ESAS)10packagepartofSASversion11.0.0.Forthe tion,instrumentallines,solarwindchargeexchange(SWCX)
EPICMOSdetectors,theanalysisofgalaxyclusterswiththis emission,andthecosmicfore-andbackgroundsfromthelo-
software was introduced by Snowdenetal. (2008), but the calhotbubble,theGalaxy,andextragalacticunresolvedpoint
methodology has since been expanded to include the EPIC sources) are all explicitly modeled and determined empiri-
pndata(e.g.,Bulbuletal.2012, foramoredetaileddescrip- cally when fitting spectra in XSPEC. While robust best fits
tion). Insummary,afilteredeventlistiscreatedbyremoving ofthesecomponentscanbetrickiertoobtaininshorterexpo-
periods of excessive count rate in the light curve caused by suressuchasthisone,thesmallerangularextentofthiscluster
proton flaring events (retaining 13.7 ks, 13.4 ks, and 8.2 ks providesforalargelocalbackgroundregionfromwhichtheir
oftheexposuresfortheMOS1, MOS2, andpndata,respec- parameterscanbesufficientlyconstrained.
In this case, nearly all of the cluster emission is taken to
10http://heasarc.gsfc.nasa.gov/docs/xmm/xmmhp_xmmesas.html fallwithinacircleof320(cid:3)(cid:3)radius;EPICMOSandpnspectra
6 S.Giacintuccietal.
purelycosmeticpurposes).Then,anabsorbedMeKaLmodel
with fixed absorption (2.8×1021 cm−2), abundance (0.3 so-
lar), and redshift (0.27) is fit to the coarse spectrum in the
sevenenergybinscorrespondingtoeachpixel. Theresulting
best-fit temperaturesare shown in Figure 5b, which have 1-
sigmaerrorsontheorderof1keVinthecenterand∼2keV
neartheedge.
The general picture that emerges is a roughly isothermal
centralregion, withouta central coolcore and with a slight,
butonlymarginallysignificant,temperaturepeak(∼11keV)
coincidentwiththeX-raypeak.Thetemperaturepeakextends
in the same direction, but more narrowly, as does the broad
bandsurfacebrightness. Thiselongationfollowsthatevident
in the core of the radio halo, suggesting a merger along the
NW-SEaxis. ThetemperatureofthecoolerregiontotheSW
of the X-ray peak (∼7−8 keV) deviates from the tempera-
turesalongtheNW-SEaxisatthe2.3σlevelandisconfirmed
in direct spectral fits. Deeper X-ray observationsare neces-
sarytodiscernexactlywhatthisfeatureisanditsrelationship
tothemerger.
7. DISCUSSION
ThePlanckmissionhasdiscoveredseveralnewclustersus-
FIG.4.—Spectrumoftheradiohalobetween235MHzand1.4GHz. ingtheSZsignal. Thesesystemswerenotdetectedfromthe
allskyX-raysurveys,suchastheROSATAllSkySurvey,be-
causeoftheirlow surfacebrightnessandbeingnearthe flux
fromthisregionandtheremainingfieldofviewoutto800(cid:3)(cid:3) limitofthesurveys.ThenewlydiscoveredPlanckclustersare
(containingonlybackground)aresimultaneouslyfit. Wefind mostlymassivesystemswithhighlydisturbedICM.Withthe
aglobalclustertemperatureof10.1±0.7keV,ingoodagree- current knowledge of the connection between cluster merg-
mentwiththatreportedinPlanckCollaborationetal. (2011c) ersandthepresenceofgiantradiohalos(Cassanoetal.2010,
of 10.65±0.42 keV. Our slightly lower temperature likely and referencestherein; see also Rossettietal. (2011)), these
reflects a different source region and background treatment, newclustersareexcellenttargetsforsearchingfornewhalos.
andourgreateruncertaintyisaconsequenceoftreatingback- In this paper, we reported the discoveryof the first giant ra-
groundcomponentsasfreeparameters. Fortheimageanaly- diohalointhesenewPlanckclusters–inPLCK171–based
sis, the best-fit valuesfor the non-cosmicbackgrounds(qui- on GMRT observations at 235 MHz and 610 MHz and re-
escent particle, soft proton, and SWCX) are translated into analysisoftheVLA1.4GHzNVSSdata.
imagesandsubtractedfromthedata. Thediffusehalois∼1Mpcinextentandiscospatialwith
Theresultingadaptivelysmoothed,fullband(0.4-7.2keV) the brightestX-ray emission, as typically observedin radio-
imageispresentedin Figure5a with the1.4GHz radiohalo halo clusters (e.g., Govonietal. 2004; Venturietal. 2013).
contoursoverlaid(same as Fig. 3c). Structurally,the central The spectral index of the halo between 235 MHz and 1400
region is elongated in the North–West/South–East direction MHzisquitesteep,α=1.84±0.14.Thisisoneofthesteep-
with the X-ray peak slightly towards the SE side from the est slopes measured for a giant radio halo so far – in fact,
large-scaleemissioncentroid. TheradiohaloandX-raysur- it is similar to α ∼ 1.9 of the prototype USSRH in A521
facebrightnessgenerallyfolloweachother,asoftenobserved (Dallacasaetal. 2009). However, the spectral index of the
in clusters hosting radio halos. The asymmetric ICM mor- PLCK171haloisaffectedbylargeuncertainties. In particu-
phologyisconsistentwitharecentmergerevent. lar,duetotheshortusabletimeofour235MHzobservation
To investigate the merger state in more detail, we con- andshortintegrationtimeoftheNVSSpointing,itispossible
structedatemperaturemapusingsmoothednarrow-bandim- that the halo extends more than imaged here, and a fraction
agesfittedwiththeMeKaLplasmamodel,usingamethodde- of its flux density is missed at both frequencies. Moreover,
scribedin Markevitchetal. (2000). Imagesare extractedin the low resolution of the 1.4 GHz image does not allow an
sevenbands(0.4-0.8keV, 0.8-1.1keV, 1.1-1.35keV, 1.9-2.1 accuratesubtractionofthediscreteradiogalaxiesenclosedin
keV,2.35-3.2keV,3.2-4.8keV,and4.8-7.2keV)thatavoidin- thehaloemission.Therefore,deeperobservationsat1.4GHz
strumentallines. A localbackground,accountingforGalac- withhigherangularresolution,anddeeperexposuresatlower
tic and extragalactic non-cluster sources, is estimated from frequenciesareessentialbeforeanyconclusiveinterpretation
the region outside the cluster in each image and subtracted, of the spectral indexof this halo can be made. If future ob-
after which the image is corrected by vignetting and expo- servationsconfirmitssteepradiospectrum,PLCK171would
sure maps. Using the ESAS task comb, which adjusts each be another important case of USSRH (of which only a few
exposuremaptotheresponseoftheMOS2,mediumfilterre- other examples are known so far; e.g., Brunetti et al. 2008,
sponse,thethreeEPICinstrumentsarecombinedtomaximize Macarioetal. 2010,vanWeerenetal. 2011,andacandidate
thesignal-to-noisebeforesmoothing. Theimages,with2.5(cid:3)(cid:3) in Giacintucciet al. 2011), whose study is crucialto under-
pixels, are variably smoothed by a 10 pixel Gaussian kernel standthephysicsbehindtheoriginofradiohalosingeneral.
attheimageedges,whichtransitionstoa∼5pixelkernelin- While such objects are expected in turbulent reacceleration
verselyfollowingthewidebandsurfacebrightnesstothe0.2 models, the existence of halos with α > 1.5 poses serious
power. Chipandpointsourcegapsareinterpolatedover(for problemsforsecondarymodelsdueto therequiredlargeen-
AgiantradiohaloinanewPlanckcluster 7
2’
2’
500 kpc
500 kpc
6 7 8 9 10 11 12 13 14 15 17
FIG.5.—a:AdaptivelysmoothedXMM-Newtonimageinthe0.4-7.2keVband. b:Temperaturemap.ThecolorbardisplaysthetemperatureinkeV.Inboth
panelsthe1.4GHzimageofthehaloisoverlaidascontours(sameasFig.3c).
26
Hz] 25
W/
P [1.4
g
lo 24
23
-4.2 -4.0 -3.8 -3.6 -3.4 -3.2 -3.0
log Y (<5R ) [sr Mpc2]
SZ 500
FIG.7.— Distribution of the radio-halo clusters in the P1.4GHz−YSZ(<
5R500)srMpc2 plane (Basu2012). Thegreen symbolmarks the position
ofPLCK171.Theredsymbolsaretheknownradio-haloclusters,theyellow
symbolisthemini-haloinA2390andbluesymbolsaretheupperlimitsfrom
theGMRTradio-halosurvey(Brunettietal.2007).
FIG.6.—Distributionoftheradio-haloclustersintheP1.4GHz−LX plane tures of a recent merger in both the surface brightness and
fromBrunettietal. (2009). FilledblacksymbolsaretheGMRTradio-halo gas temperature distributions. A NW-SE elongation of the
clustersandopenblacksymbolsareotherradio-haloclustersfromtheliter-
central X-ray emission, coupled with a similar structure in
ature. ThemagentapointmarksthepositionofPLCK171,usingthecluster
X-rayluminosityestimatedwithinR500(PlanckCollaborationetal. 2011c) thetemperaturemap,suggeststhatthemergeroccurredalong
andthe1.4GHzradiopowerinTable4. Thesolidlineisthebestfittothe thisaxis. Theradiohalo displaysa similar asymmetryin its
distributionofgiantradiohalosfromBrunettietal.(2009). central region, suggesting a correlation between the thermal
and non-thermal emissions, as typically observed in radio-
ergyin cosmicray protons(Brunettietal. 2008, Macarioet haloclusters.
al. 2010, vanWeeren et al. 2011). Such energiesare above Figure6showsthedistributionoftheclustersoftheGMRT
thecurrentupperlimitsfromtheγ-rayobservationsofnearby radio-halosurvey(Venturietal. 2007,2008),alongwithother
clusters(Aharonianetal. 2009,Ackermannetal. 2010,Jel- radio-haloclustersfromtheliterature,intheP1.4GHz–LX plane
tema & Profumo 2011), assuming the μG cluster magnetic (from Brunetti et al. 2009). The bimodality evident in this
fieldvaluesindicatedbyFaradyrotationmeasurestudies(e.g., diagram (with some clusters lacking diffuse radio emission,
Bonafedeetal. 2010andreferencestherein). while those that exhibit radio halos follow a correlation) is
The analysis of the X-ray XMM-Newton observation of oneofthemainoutcomesoftheGMRTradio-halosurveyand
PLCK171 reveals high temperature (∼10 keV) and signa- providesquantitativesupporttoreaccelerationmodelsforthe
8 S.Giacintuccietal.
radiohalo(α∼1.8,withthecaveatsgivenabove).Ifthesteep
spectrumis confirmed,it would be the hottestcluster with a
USSRH–allotherUSSRHshavekT ∼5−9keV,asvisiblein
Fig.8,thatreportsthedistributionofradio-haloclustersinthe
α235MHz −kT planeadaptedfromVenturietal. (2013). The
1400MHz
magenta point marks the position of PLCK171 using kT =
10.1±0.7keV,asmeasuredinSect.6. Thus,thisobjectmay
provideanimportantdatapointforourunderstandingofgiant
radiohalosinclusters.
8. CONCLUSIONS
We have discovered a ∼ 1 Mpc-size radio halo in
PLCK171, using GMRT observations at 235 MHz and 610
MHz and re-analysis of NVSS 1.4 GHz data. This is the
firstgiantradiohalofoundinthenewclustersdiscoveredby
Planck.
Withaspectralindexα≈1.8,thissourcemightbeanother
systemwithaverysteepradiospectrum,sofarfoundinonly
afewotherclusters. However,deepermulti-frequencyobser-
vationsarerequiredbeforeanydefinitiveinterpretationofthe
spectralindexofthisradiohalo.
TheX-rayluminosityoftheclusterandradiopowerofthe
haloat1.4GHzareconsistentwiththecorrelationknownto
befollowedbyradio-haloclusters.ItslocationintheP1.4GHz-
FIG.8.—Spectralindexofradiohalosinthe235MHz-1.4GHzintervalas Y plane also follows the scaling relation found for clusters
functionoftheclusterX-raytemperature(adaptedfromVenturietal.2013).
ThemagentapointsmarksthepositionofPLCK171. withradiohalos. TheasymmetriesobservedintheX-raysur-
facebrightnessandtemperauredistributions,derivedfromthe
originofradiohalos(Brunettietal. 2007,2009).Thesepara- XMMdata,indicateadisturbedICMresultingfromarecent
tion betweenclusters hostinga radio haloand those without merger. This is in line with the expectation that giant radio
(upperlimits)ismostlikelycausedbythedifferentdynamical halosareexclusivelyassociatedwithclustermergers.
propertiesoftheclusters:clusterswithoutaradiohalotendto With an X-rayluminosityof 1.13×1045 ergs−1 and a red-
be more relaxed systems, while radio halos are exclusively shiftofz=0.27,PLCK171representsthekindofclustersthat
foundindisturbedclusters(Cassanoetal. 2010). have been missed in X-ray flux limited surveys, such as the
PLCK171(magentasymbol)fallson the radiohalo corre- Rosat All Sky Survey, and, consequently, in radio surveys
lation.Furthermore,ourX-rayanalysisshowsthatthecluster based on X-ray selection criteria, such as the GMRT radio-
is unrelaxed, as expected for a system hosting a radio halo. halo survey (Venturi et al. 2007, 2008). It is therefore im-
Ongoing dynamical activity in PLCK171 is also suggested portanttoconsiderbiasesduetoclusterssuchasPLCK171in
bythepresenceoffourNATsinitscentralMpcregion,with X-rayfluxlimitedsamplesselectedforradiosurveys.
tails pointingaway from the cluster center, suggesting infall
ofmultiplesubctructuresontothemainclusterandbulkmo-
tionsoftheICMdrivenbytheongoingmerger(e.g.,Blitonet WearedeeplygratefultoRossellaCassanoforusefulcom-
al. 1998). Thus,theradiohaloinPLCK171providesfurther ments and suggestions, and for providing Fig. 6. We thank
supporttotheexistenceofadirectlinkbetweentheradiohalo KaustuvBasuforkindlyprovidingFig.7. Wethankthestaff
phenomenonandclustermergers. of the GMRT for their help duringthe observations. GMRT
Ascalingrelationbetweentheradiopowerat1.4GHzand is run by the National Centre for Radio Astrophysics of the
the integrated SZ effect measurement (Y) has been recently TataInstituteofFundamentalResearch. TheNationalRadio
presentedbyBasu(2012).Sucharelationisadirectprobeof AstronomyObservatoryis a facility of the National Science
theconnectionbetweenthemassofthe clusterandpresence Foundation operated under cooperative agreement by Asso-
of a radio halo. In Fig. 7 we report the position PLCK171 ciated Universities, Inc. SG acknowledges the support of
in the P1.4GHz-Y plane using the integrated Y published in NASAthroughEinsteinPostdoctoralFellowshipPF0-110071
PlanckCollaborationetal.(2011c).PLCK171(greensquare) awardedbytheChandraX-rayCenter(CXC),whichisoper-
followsthe scaling relationobtainedforthe otherradio-halo atedbySAO.Thisresearchwassupportedbyanappointment
clusters. to the NASA Postdoctoral Program at the Goddard Space
WhatisunusualaboutPLCK171isitscombinationofhigh FlightCenter,administeredbyOakRidgeAssociatedUniver-
gas temperature (∼10 keV) and ultra-steep spectrum of its sitiesthroughacontractwithNASA.
REFERENCES
Ackermann,M.,Ajello,M.,Allafort,A.,etal.2010,ApJ,717,L71 Bonafede,A.,Brüggen,M.,vanWeeren,R.,etal.2012,MNRAS,426,40
Aharonian,F.,Akhperjanian,A.G.,Anton,G.,etal.2009,A&A,502,437 Brown,S.,&Rudnick,L. 2011,MNRAS,412,2
Baars,J.W.M.,Genzel,R.,Pauliny-Toth,I.I.K.,&Witzel,A. 1977,A&A, Brunetti,G.,Venturi,T.,Dallacasa,D.,etal.2007,ApJ,670,L5
61,99 Brunetti,G.,etal. 2008,Nature,455,944
Bagchi,J.,Sirothia,S.K.,Werner,N.,etal.2011,ApJ,736,L8 Brunetti,G.,Cassano,R.,Dolag,K.,&Setti,G. 2009,A&A,507,661
Basu,K.2012,MNRAS,421,L112 Brunetti,G.2011,JournalofAstrophysicsandAstronomy,32,437
Blasi,P.,&Colafrancesco,S.,1999,APh,12,169 Brunetti,G.,Blasi,P.,Reimer,O.,etal.2012,MNRAS,426,956
Bliton,M.,Rizza,E.,Burns,J.O.,Owen,F.N.,&Ledlow,M.J.1998, Bulbul,G.E.,Smith,R.K.,Foster,A.,etal.2012,ApJ,747,32
MNRAS,301,609
AgiantradiohaloinanewPlanckcluster 9
Cassano,R.,Ettori,S.,Giacintucci,S.,Brunetti,G.,Markevitch,M., Markevitch,M.,Govoni,F.,Brunetti,G.,Jerius,D.,2005,ApJ,627,733
Venturi,T.,&Gitti,M. 2010,ApJ,721,L82 Pfrommer,C.,&Enßlin,T.A. 2004,JournalofKoreanAstronomical
Chandra,P.,Ray,A.,&Bhatnagar,S. 2004,ApJ,612,974 Society,37,455
Clarke,T.E.,&Ensslin,T.A.2006,AJ,131,2900 PlanckCollaboration,Ade,P.A.R.,Aghanim,N.,etal.2011,A&A,536,
Condon,J.J.,Cotton,W.D.,Greisen,E.W.,etal.1998,AJ,115,1693 A1(a)
Dallacasa,D.,Brunetti,G.,Giacintucci,S.,etal.2009,ApJ,699,1288 PlanckCollaboration,Ade,P.A.R.,Aghanim,N.,etal.2011,A&A,536,
Dennison,B.1980,ApJ,239,L93 A8(b)
Donnert,J.,Dolag,K.,Cassano,R.,&Brunetti,G. 2010,MNRAS,407, PlanckCollaboration,Aghanim,N.,Arnaud,M.,etal.2011,A&A,536,A9
1565 (c)
Ensslin,T.,A.,Biermann,P.L.,Klein,U.,Kohle,S.,1998,A&A,332,395 PlanckCollaboration,Aghanim,N.,Arnaud,M.,etal.2012,A&A,543,
Enßlin,T.,Pfrommer,C.,Miniati,F.,&Subramanian,K.2011,A&A,527, A102(d)
A99 PlanckCollaboration,Ade,P.A.R.,Aghanim,N.,etal.2012,
Ferrari,C.,Govoni,F.,Schindler,S.,Bykov,A.M.,&Rephaeli,Y. 2008, arXiv:1205.3376(e)
SSRv134,93 Rossetti,M.,Eckert,D.,Cavalleri,B.M.,etal.2011,A&A,532,A123
Feretti,L.,&Venturi,T.2002,MergingProcessesinGalaxyClusters,272, Roy,J.,Gupta,Y.,Pen,U.,Peterson,J.B.,Kudale,S.,Kodilkar,J.,2010,
163 ExA,28,25
Feretti,L.,Giovannini,G.,Govoni,F.,&Murgia,M.2012,A&ARev.,20, Snowden,S.L.,Mushotzky,R.F.,Kuntz,K.D.,&Davis,D.S.2008,A&A,
54 478,615
Giacintucci,S.,Dallacasa,D.,Venturi,T.,etal.2011,A&A,534,A57 Sunyaev,R.A.,&Zeldovich,Y.B.1972,CommentsonAstrophysicsand
Giovannini,G.,Tordi,M.,&Feretti,L. 1999,NewA,4,141 SpacePhysics,4,173
Giovannini,G.,Bonafede,A.,Feretti,L.,etal.2009,A&A,507,1257 vanWeeren,R.J.,Röttgering,H.J.A.,Brüggen,M.,Cohen,A.,2009,
Govoni,F.,Markevitch,M.,Vikhlinin,A.,etal.2004,ApJ,605,695 A&A,508,75
Greisen,E.W.,Spekkens,K.,&vanMoorsel,G.A.2009,AJ,137,4718 vanWeeren,R.J.,Brüggen,M.,Röttgering,H.J.A.,etal.2011,A&A,533,
Hoeft,M.,&Brüggen,M.,2007,MNRAS,375,77 A35
Jeltema,T.E.,&Profumo,S. 2011,ApJ,728,53 vanWeeren,R.J.,2011,PhDT,14
Kale,R.,&Dwarakanath,K.S.,2010,ApJ,718,939 vanWeeren,R.J.,Bonafede,A.,Ebeling,H.,etal.2012,MNRAS,425,L36
Kale,R.,&Dwarakanath,K.S.,2012,ApJ,744,46 Venturi,T.,Giacintucci,S.,Brunetti,G.,etal.2007,A&A,463,937
Kang,H.,&Ryu,D.,2011,ApJ,734,18 Venturi,T.,Giacintucci,S.,Dallacasa,D.,etal.2008,A&A,484,327
Kang,H.,Ryu,D.,&Jones,T.W.2012,ApJ,756,97 Venturi,T.2011,Mem.Soc.Astron.Italiana,82,499
Kempner,J.C.,&Sarazin,C.L. 2001,ApJ,548,639 Venturi,T.,Giacintucci,S.,Dallacasa,D.,etal.2012,arXiv:1210.7617
Keshet,U.,&Loeb,A. 2010,ApJ,722,737 A&A,inpress,
Macario,G.,Venturi,T.,Brunetti,G.,etal.2010,A&A,517,A43
Macario,G.,Venturi,T.,Intema,H.T.,etal.2013,A&A,submitted
Markevitch,M.,Ponman,T.J.,Nulsen,P.E.J.,etal.2000,ApJ,541,542