Deep Observation of the Giant Radio Lobes of Centaurus A with the Fermi Large Area Telescope

0.96 MB·English

by  Rui-zhi Yang

Astronomy&Astrophysicsmanuscriptno.cena˙v6 (cid:13)c ESO2012
January6,2012
Deep Observation of the Giant Radio Lobes of Centaurus A with the
Fermi Large Area Telescope
Rui-zhiYang1,2,NarekSahakyan3,4,EmmadeOnaWilhelmi1,FelixAharonian1,5,andFrankRieger1
1 Max-Planck-Institutfu¨rKernphysik,P.O.Box103980,69029Heidelberg,Germany
2 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing,
210008,China
3 UniversityofRomeSapienzaandICRANet,Dip.Fisica,p.leA.Moro2,00185-Rome,Italy
2 4 InstituteforPhysicalResearch,NASofArmenia,Ashtarak-2,0203,Armenia
1 5 DublinInstituteforAdvancedStudies,31FitzwilliamPlace,Dublin2,Ireland
0
2 Preprintonlineversion:January6,2012

n
a ABSTRACT
J
Thedetection of high energy (HE)g -ray emissionup to ∼3GeV from thegiant lobes of the radiogalaxy Centaurus A hasbeen
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recentlyreportedbytheFermi-LATCollaborationbasedontenmonthsof all-skysurveyobservations. Adatasetmorethanthree

timeslargerisusedheretostudythemorphologyandphotonspectrumofthelobeswithhigherstatistics.Thelargerdatasetresults
]
E inthedetectionofHEg -rayemission(upto∼6GeV)fromthelobeswithasignificanceofmorethan10and20s fortheNorthand
H theSouthlobe,respectively.Basedonadetailedspatialanalysisandcomparisonwiththeassociatedradiolobes,wereportevidence
forasubstantialextensionoftheHEg -rayemissionbeyondtheWMAPradioimageinthecaseoftheNorthernlobeofCenA.We
h. reconstructthespectralenergydistribution(SED)ofthelobesusingradio(WMAP)andFermi-LATdatafromthesameintegration
p region.Theimplicationsarediscussedinthecontextofhadronicandleptonicscenarios.
-
o Keywords.Gammarays:galaxies
r
t
s
a 1. Introduction
[
The bright, nearby radio galaxy Centaurus A (Cen A; NGC 5128) has been extensively studied from radio to very-high-energy

1 (VHE)g -rays(e.g.,seeIsrael1998,Steinle2010forreviews).Itsuniqueproximity(d∼3.7Mpc;Ferrareseetal.2007)andpeculiar
v
morphologyallowforadetailedinvestigationofthe non-thermalaccelerationandradiationprocessesoccurringbothinitsactive
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nucleusanditsrelativisticoutflows.AtradiofrequenciesCenArevealsgiantstructures,theso-called”lobes”,withatotalangular
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sizeof∼10◦(Shain1958,Burnsetal.1983),correspondingtoaphysicalextensionof∼600kpc(d/3.7Mpc).
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1 At high-energy(HE;200 MeV<E<100 GeV) Fermi-LAT hasrecently detected g -rayemission fromboth the core (i.e., within
. ∼0.1◦)andthegiantradiolobesofCenA(Abdoetal.2010a,Abdoetal.2010b):Ananalysisoftheavailable10-monthsdataset
1
revealsapoint-likeemissionregioncoincidentwiththepositionoftheradiocoreofCenA,andtwolargeextendedemissionregions
0 detectedwithasignificanceof5and8s fortheNorthernandtheSouthernlobe,respectively.TheHEemissionfromthecoreextends
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upto∼10GeVanditwelldescribedbyapower-lawfunctionwithphotonindex∼2.7.Itcanbesuccessfullyinterpretedasorigi-
1
natingfromsynchrotronself-Compton(SSC)processesintheinnermostpartoftherelativisticjet.However,asimpleextrapolation
:
v oftheHEcorespectrumtotheTeVregimetendstounder-predicttheTeVfluxobservedbyH.E.S.S.(Aharonianetal.2009),afact
Xi thatmayindicateanadditionalcontributionrelatedtoe.g.non-thermalmagnetosphericprocessesemergingatthehighestenergies
(seeRieger2011forreview).TheextendedHEemissionregions,ontheotherhand,seemmorphologicallycorrelatedwiththegiant
r
a radiolobesandcontributemorethanone-halftothetotalHEsourceemission.Itisagainspectrallywelldescribedbyapower-law
functionextendingupto2or3GeVwithphotonindicesofG ∼2.6.
If the extended HE emission indeed arises due to inverse-Compton up-scattering of CMB and EBL (Extragalactic Background
Light)photons,thenthiscouldoffersauniquepossibilitytospatiallymaptheunderlyingrelativisticelectrondistribution.Thede-
tectionofGeVg -raysfromtheradiolobesimpliesmagneticfieldstrengths ∼< 1µG(e.g.,Abdoetal.2010a).Thisestimatecanbe
obtainedquitestraightforwardlyfromthecomparisonofradioandg -rays,assumingthattheseradiationcomponentsareproduced
inthesameregionbythesamepopulationofelectronsthroughsynchrotronandinverse-Comptonprocesses.Ingeneral,however,
theradioandtheg -rayregiondonotneedtocoincide.Whiletheradioluminositydependsontheproductoftherelativisticelectrons
densityN andthe magnetic-fieldsquareB2, the inverse-Comptong -ray luminosityonly dependson N . Thisimpliesthatg -rays
e e
cangiveusmodel-independentinformationaboutboththeenergyandthespatialdistributionofelectrons,whiletheradioimageof
synchrotronradiationstronglydependsonthemagneticfield.Asaconsequence,theg -rayimagecanbelargerthantheradioimage
if the magneticfield dropsatthe peripheryofthe regionoccupiedbyelectrons.Thisprovidesoneofthe motivationfora deeper
studyoftheextendedHE(lobe)emissionregioninCenA.Tothisendweanalyze3yrofdata,increasingtheavailableobservation
timebymorethanafactorthreewithrespecttothepreviouslyreportedresults.Thelargerdatasetallowsforadetailedinvestigation
of the spectrum andmorphologyof the lobeswith better statistics, especially above1 GeV, where the spectralshape may reflect
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Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
cooling effects and/or maximum energy constraints on the parent population of particles generating the HE g -ray emission. We
alsore-analyzeradiodatafromWMAP(Pageetal.2003)forthesameregionfromwheretheHEemissionisevaluatedfrom,and
discusstheimplicationsoftheresultantspectralenergydistribution(SED)fordifferentemissionscenarios.
Thepaperisstructuredasfollows.InSec.2thespectralandspatialHEanalysisresultsaredescribed,whereastheanalysisofthe
WMAPresultsforthelobesarepresentedinSec.3.ImplicationsforleptonicandhadronicemissionmodelsarediscussedinSec.4
andconclusionsarepresentedinSec.5.
2. Fermi-LATDataAnalysis
TheLargeAreaTelescope(LAT)onboardoftheFermig -RaySpaceTelescope,operatingsinceAugust4,2008,candetectg −ray
photonswithenergiesintherangebetween100MeVandafew100GeV.DetailsabouttheLATinstrumentcanbefoundinAtwood
et al. (2009). Here we analyze the field of view (FoV) of Cen A, which includes the bright core and the giant radio lobes. We
selecteddataobtainedfromthebeginningoftheoperationuntilNovember14,2011,amountingto∼3yrofdata(MET239557417–
342956687).WeusedthestandardLATanalysissoftware(v9r23p1)1.Inordertoavoidsystematicerrorsduetopoordetermination
of the effective area at low energies, we selected only events with energies above 200 MeV. The region-of-interest (ROI) was
selectedtobearectangularregionofsize14◦×14◦ centeredonthepositionofCenA(RA=201◦21′54′′,DEC=−43◦1′9′′).To
reducethe effectof Earth albedobackgrounds,time intervalswhen the Earth was appreciablyin the FoV (specifically, when the
centeroftheFoVwasmorethan52◦ fromzenith)aswellastimeintervalswhenpartsoftheROIwereobservedatzenithangles
>105◦ were also excludedfromthe analysis.The spectralanalysiswas performedbased onthe P7v6versionofthe post-launch
instrumentresponsefunctions(IRFs).WemodeledtheGalacticbackgroundcomponentusingtheLATstandarddiffusebackground
modelgal 2yearp7v6 v0 and we left the overallnormalizationand index as free parameters.We also used iso p7v6sourceas the
isotropicg -raybackground.
TheresultantFermi-LATcountsmapforthe3yrdatasetisshowninFig.1(a).The(green)crossesshowthepositionofthepoint-
like sources from the 2FGL catalog (Abdoetal.2011) within the ROI. The core of Cen A is clearly seen with a test statistic of
TS>800,correspondingtoadetectionsignificanceof28s .ExtendedemissiontotheNorthandSouthofCenAisdetectedwith
significancesofTS>100(10s )andTS>400(20s ),respectively.
2.1. SpatialAnalysis
Eventswith energiesbetween 200 MeV and 30 GeV were selected. The residualimage after subtractingthe diffuse background
and point-like sources including the core of Cen A is shown in Fig. 1(b). The fluxes and spectral indices of 11 other point-like
sourcesgeneratedfromthe2FGLcatalogwithintheROIarealsoleftasfreeparametersintheanalysis.The2FGLcatalogsource
positionsareshowninFig.1(a),where2FGLJ1324.0-4330eaccountsforthelobes(bothNorthandSouth).Anewpoint-likesource
(2FGLJ1335.3-4058),locatedatRA=203◦49′30′′,DEC=−40◦34′48′′accountsforsomeresidualemissionfromtheNorthlobe,
althoughno knownsource at other wavelengthsis foundto be associated. We treat it as partof the North lobe here.The core of
CenAismodeledasapoint-likesource.Thenthefollowingstepswereperformed:
(1)Toevaluatethetotal(extended)HEg -rayemissionwefirstusedatemplatebasedontheresidualmap(T1;correspondingtothe
bluecontoursinFig.2).TheTSvaluesfortheSouthandtheNorthlobeinthistemplateare411and155,respectively.Theresidual
mapwasalsocomparedwithradio(WMAP,22GHz)lobecontours(greencontoursoverlaidonFig.1(b)).WefindthattheSouth
lobeoftheHEg -rayimageissimilartotheSouthlobeoftheradioone,whereastheHEemissionintheNorthextendsbeyondthe
radiolobeemissionregion.
(2)Inordertounderstandthisfeaturebetter,were-fittheexcessusinganadditionaltemplate(T2;redcontoursinFig.3)generated
fromtheradio(WMAP)image.ThetwotemplatesareshowninFig.2,andthecorrespondingresidualmapsareshowninFig.1.
WhilethereissomeresidualemissiontotheNorthofCenAfortemplateT2,thisresidualemissionisobviouslyabsentfortemplate
T1.ThequalitativefeaturesofthedifferentresidualmapsareconfirmedbythecorrespondingTSvalues,whicharelistedinTable1.
Accordingly,theHESouthlobeseemreasonablywellinagreementwiththeradioSouthlobe,whereasfortheNorth,thetemplate
generatedfromtheradiolobe(T2)fitstheHEexcesssubstantiallyworsethanT1(110vs155).
Table1:TSvalueforthetwotemplatesused.
Model NorthLobe SouthLobe
T1 155 411
T2 110 406
(3) To further investigate a possible extension (or contribution of a backgroundsource) of the North lobe, we evaluated the
projectionofarectangularregionontheexcessimage(inwhiteinFig.1(b)).Fig.3showstheprojectionfortheNorthandSouth
regionsforboth,theFermi-LATimage(inblack)andtheradioone(inred).TheSouthprojectionfortheradiomapiswell-fitted
bya singleGaussiancenteredat∼0.05(0 isdefinedasthe centeroftherectangleonRA=201◦21′54′′,DEC=−43◦1′9′′) with
an extension of s =0.99◦. For the Fermi-LAT map the Gaussian is centered at ∼0.5 and has s =1.01◦, compatible with the
radiomapprojection.Onthecontrary,theNorthprojectionfortheFermi-LATmaphasaGaussianprofilewiths =1.68◦, while
forthe radiomaps is0.97◦. Theextensionin thenorthprojectionsfortheFermi-LATmapindicatesthattheg -rayNorthlobeis
1 http://fermi.gsfc.nasa.gov/ssc
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Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
moreextendedthantheradiooneorthatan(otherwiseunknown)sourceinthebackgroundmaybecontributingtothetotalemission.
2.2. SpectralAnalysis
Our morphological analysis indicates some incongruity between the morphology of the radio lobe and g -ray lobe in the North.
Hence, in order to model the g -ray lobe as self-consistently as possible, we used the template generated with the residual map
(T1).Integratingthe whole g -rayemission observed,we then derivedthe totalflux and index in the 100MeV to 30 GeV energy
range. For the North lobe the integral HE flux is (0.93±0.09)×10−7ph cm−2s−1 and the photon index is 2.24±0.08, while
for the South lobe we find (1.4±0.2)×10−7ph cm−2s−1 and 2.57±0.07, respectively. The core region of Cen A has a flux
(1.4±0.2)×10−7ph cm−2s−1 while the photon index is 2.7±0.1. The results are summarized in Table 2, where the subscripts
3aand10mrefertothe3-yeardata(analyzedhere)andthe10-monthdata(reportedinAbdoetal.2010a),respectively.We find
thatthefluxandphotonindicesintheT2templatesaresimilartothe10-monthdata.Ontheotherhand,theanalysisusingtheT1
templateresultsinaharderspectrumfortheNorthlobe.
Toderivethespectralenergydistribution(SED)wedividedtheenergyrangeintologarithmicallyspacedbandsandapplygtlikein
eachofthesebands.Onlytheenergybinsforwhichasignalisdetectedwithasignificanceofatleast2s areconsidered,whilean
upperlimitiscalculatedforthosebelow.Asaresult,thereare7binsintheSEDfortheSouthlobe.TheSEDisshowninFig.4.
Toclarifytheoriginoftheg -rayemission,weevaluatedthespectrumindifferentpartsofeachlobe.Tothisend,wedividedeach
lobeintotwoparts(seeFig.5)andusedgtliketoevaluatethespectrum.IntheSouthlobethephotonindexis2.8±0.2neartheCen
Acoreand2.3±0.1farawayfromthecore.FortheNorthlobethevaluesare2.2±0.2forbothparts.Theenergyspectrumofthe
Southlobeseemstobehardening(DG =-0.5±0.3)farfromthecoreofCenAwhereasinNorthlobethereisnoobviouschange
S
inthespectrum.Unfortunately,thestatisticsisstillnotenoughtoclaimahardeinngonthespectrumoftheSouthlobe.Thespectra
forthefourregionsareshowninFig.6andFig.7.
Table2:Fluxesandspectraofthelobes
SourceName F 3a(T1) G 3a(T1) F 10m G 10m F 3a(T2) G 3a(T2)
SouthLobe 1.43±0.15 2.57±0.07 1.09±0.24 2.60±0.15 1.40±0.15 2.56±0.08
NorthLobe 0.93±0.09 2.24±0.08 0.77±0.20 2.52±0.16 0.64±0.15 2.56±0.08
F istheintegralflux(100MeVto30GeV)inunitsof10−7ph·cm−2s−1andG isthephotonindex.Thesubscripts”3a”and”10m”
referto the3-yeardataanalyzedhereandto the 10-monthresults(basedona WMAP template)reportedinAbdoetal. (2010a),
respectively.
3. WMAPdata
InSec.2wefoundevidencethattheHEg -rayemissionregionsdonotcoincidewellwiththeradiolobes,especiallyfortheNorth
lobe.Tocorrectlycomparetheg -rayemissionwiththeradioone,ananalysisoftheradiodataforthesameregionisrequired,rather
thansimplyfortheNorthradiolobeitself.WeperformedthisanalysisfollowingthemethoddescribedinHardcastleetal.(2009).
SevenyearsofWMAPdatawere analyzed(Komatsuetal.2011). TheWMAPmapsin allfivebandsareconvolvedwitha 0.83◦
Gaussiantogetsimilarresolutionforallbands.Theinternallinearcombination(ILC)CosmicMicrowaveBackground(CMB)map
was treated as backgroundand subtractedfrom all maps. The intensity maps(in mK) were convertedto flux maps (in Jy/beam)
(Pageetal.2003)andintegratedintheregiondefinedbytheNorthg -raylobetoobtainthetotalflux.Fluxvaluesalmosttwiceas
largeareobtainediftheg -rayregionisused.Inordertocross-checkouranalysismethod,wealsoderivedthefluxoftheNorthradio
lobeforthe sameregionasusedin Hardcastleetal. (2009).AllresultsaresummarizedinTab.4.OurresultsfortheNorthradio
lobeiscompatiblewithinerrorswiththeoneobtainedbyHardcastleetal.(2009)(showninbracketsinTab.5).NotethatintheV
Table3:RadiofluxfortheNorthlobe(in1011Jy·Hz)fromWMAPdata.
band Northg -rayLobe NorthradioLobe
K(22.5GHz) 5.34±0.37 2.71±0.19(2.61±0.25)
Ka(33GHz) 5.24±0.41 2.55±0.22(2.50±0.24)
Q(41GHz) 5.53±0.56 2.94±0.29(2.58±0.25)
V(61GHz) 7.65±1.19 <4.13
W(94GHz) <25.6 <9.43
andtheWbandthesignalislessthanthe5s ,soweonlygiveupperlimitshere.
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Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
13
2
0 12
2FGL J-36.00123F5G9L. 9J-13374476.7-3752 2FGL J1259.8-3749 11 36.000 11..68
-
2FGDeclinationL J1420-40.000-44.000F7G.L5 -J41235257F2G.L6 -2J42F14FG31GL33L 5 J.J1313-324225042F.522.FG68FF0GL-GG-L 4LL4 J3 3J10JJ31301103033e07006.34.5..9-53-4--434400632025823 5789...823 Declination -40.00044.000 0011....792488
22FFGGLL JJ11332286..54--44772289 -
2FGL J1446.8-4701 -48.000 2FGL J1305.8-4925 34..57 -48.000 00..3599
212.000 206.000 200.000 194.000 2.3 212.000 206.000 200.000 194.000 0.19
Right ascension Right ascension
1.2 0
(a)LATcountsmapofthe14◦×14◦ROI. (b)Excessmapafterbackgroundsubtraction.
2 2
0 1.8 0 1.8
0 0
0 0
36. 1.6 36. 1.6
- -
0 1.4 0 1.4
0 0
0 0
on -40. 1.2 on -40. 1.2
Declinati 44.000 01.8 Declinati 44.000 01.8
- -
0 0.6 0 0.6
0 0
0 0
48. 0.4 48. 0.4
- -
212.000 206.000 200.000 194.000 0.2 212.000 206.000 200.000 194.000 0.2
Right ascension Right ascension
0 0
(c)ResidualmapfortemplateT1. (d)ResidualmapfortemplateT2.
Fig.1:ThedifferentmapsfortheCenAregion:(a)LATcountsmapofthe14◦×14◦ regionofinterest(ROI)aroundtheposition
ofCenA.ThecountsmapissmoothedwithaGaussianofkernel0.8◦.Thegreencrossesmarkthepositionofthe2FGLpoint-like
sources.(b)Excessmapaftersubtractionofdiffusebackground,point-likesourcesandCenAcore.ThecontoursareWMAPradio
lobecontours,whilethewhiteboxesrepresenttheprojectionregionsdiscussedinSec.2.1.(c)ResidualmapusingtemplateT1for
thelobes.(d)ResidualmapusingtheradiotemplateT2forthelobes.
4. OntheOriginoftheNon-ThermalLobeEmission
UsingtheWMAPandFermi-LATresultsreportedhere,wecancharacterizethespectralenergydistributions(SEDs)fortheNorth
and the South lobe, respectively. While the radio emission is usually taken to be caused by electron synchrotron emission, the
originof the HE g -ray emissioncouldin principlebe related to bothleptonic (inverse-Comptonscattering)or hadronic(e.g.,pp-
interaction)processes.Inthefollowingwediscusspossibleconstraintsfortheunderlyingradiationmechanismasimposedbythe
observedSEDs.
4.1. Inverse-Comptonoriginofg -rays
Both theHE g -rayandthe radioemissioncouldbe accountedforina leptonicscenario.Inthe simplestversion,a single popula-
tion ofelectronsN(g ,t) is used to modelthe SED throughsynchrotronandinverse-Comptonemission, with particleacceleration
beingimplicitlytreatedbyaneffectiveinjectiontermQ=Q(g ,t).Thelaterallowsustodisentangleaccelerationby,e.g.,multiple
shocksorstochasticprocesses(e.g.,O’Sullivanetal.2009)fromemission,andenablesastraightforwardinterpretation.Thekinetic
equationdescribingtheenergeticandtemporalevolutionoftheradiatingelectronscanthenbewrittenas
¶ N ¶ N
= (PN)− +Q, (1)
¶ t g¶ t
esc
whereP=P(g )=−dg isthe(time-independent)energylossrateandt isthecharacteristicescapetime.Fornegligibleescape(as
dt esc
appropriateheregiventhelargesizeoftheg -rayemittingregion)andquasi-stationaryinjectionQ(g ,t)=Q(g ),thesolutionofthe
kineticequationbecomes
N(g ,t)= 1 g0Q(g )dg , (2)
P(g )Zg
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Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
where g 0 is found by solving the characteristic equation for a given epoch t, t = gg0 Pd(gg) (e.g., Atoyan&Aharonian1999). If
synchrotronorinverse-Compton(Thomson)losses(P(g )=ag 2)providethedominanRtlosschannel,theng =g /(1−ag t),sothat
0
attheenergyg = 1 thestationarypower-lawelectroninjectionspectrumsteepensbyafactorof1(coolingbreak)duetoradiative
br at
losses,naturallygeneratingabrokenpower-law.
We use the above particle distribution described in eq. (2), for a representation of the observed lobe SEDs. The magnetic field
strength B, the maximum electron energyg and the epoch time t are left as free parametersto modelthe data. Klein-Nishina
max
(KN)effectsontheinverse-Compton-scatteredHEspectrumaretakenintoaccount.
Fig.8showstheSEDresultsobtainedfortheNorthandSouthlobes.TheHEpartofbothspectracanbedescribedbyapower-law
withphotonindexG g ≃2.2and2.6fortheNorthandtheSouthlobe,respectively.Atlowenergies,theSouthlobespectrumshows
asynchrotronpeakatabout5GHz,whiletheNorthoneiswelldescribedbyapower-lawwithanindex>2.Notethatifonewould
use a simple power-lawelectron injection spectrumQ(g )(cid:181)g −a , evolvingin time with a cooling break,to describe the HE g -ray
spectrum,apowerindexa =3.2wouldberequiredfortheSouthlobe.Yet,assumingthatthesameelectronpopulationisresponsible
forboththeradio-synchrotronandHEinverse-Comptonemission,suchavaluewouldbeinconflictwiththeresultsobtainedfrom
theWMAPdataanalysis,indicatinganelectronpopulationwithpower-lawindexa ≃2basedonthedetectedsynchrotronemission.
As it turnsout, however,this issue could be accommodatedby consideringa more naturalspectral inputshape, e.g., an electron
injectionspectrumwithanexponentialcut-off
g
Q(g )=Q g −a exp − , (3)
0 (cid:18) g (cid:19)
max
wheretheconstantQ canbeobtainedfromthenormalizationtotheinjectionpowerL=m c2 Q(g )g dg .
0 e
The lifetime of the giant radio lobes is somewhat uncertain. Dynamical arguments suggestRa minimum age of several 106 yr,
whilesynchrotronspectralagingargumentsindicateanageofsome107yr(e.g.,Alvarezetal.2000,Hardcastleetal.2009).Inthe
followingwethusdiscusstheSEDimplicationsforaminimumandamaximumepochtimetoft =106yrandt =8×107yr.
min max
For the South lobe, the radio data suggest a break frequency n =5 GHz above which the spectrum drops down. The break in
br
the synchrotron spectrum is related to the break in the electron spectrum via n =1.3g 2B Hz. In principle, a change in the
br 1µG
spectralshapeoftheelectronpopulationcouldbeduetocoolingeffectsor/andtheexistenceofamaximumenergyfortheelectron
population.Foraminimumepochtimet =106 yr,coolingwouldaffectthesynchrotronspectrumatfrequency≈7.5B THz,
min 1µG
much higher than inferred from the radio data. Therefore, to obtain a break at 5 GHz in the South lobe, a maximum energy in
theelectronpopulation(g ),lowerthang definedbyt =t wouldbeneeded.Ontheotherhand,foramaximumepochtime
max br min
t =8×107 yr, the power-lawspectralindex changesat frequency≃1B GHz, providinga satisfactory agreementwith the
max 1µG
radiodata.Inthiscasethemaximumelectronenergyisobtainedfromtheradiodataabovethebreakfrequency5GHz.Resultsfor
theconsideredminimumandmaximumepochtime,andforafixedpower-lawelectronindexa =2areillustratedinFigs.8and9.
NotethatforB≤3µG,theenergylossrateP(g )isdominatedbytheICchannelonly,sothattheresultsofthecalculationsarequite
robust.
Figure 8 shows a representation of the SED for the North and the South lobe, respectively, using the parameters t =106 yr
min
andg =9.5×104.ThedashedlineshowstheHEcontributionproducedbyinverse-ComptonscatteringofCosmicMicrowave
max
Background (CMB) photons by relativistic electrons within the lobes. In this case the resulting g -ray flux can only describe the
first two data points and then drops rapidly. Thus, in order to be able to account for the observed HE spectrum, Extragalactic
BackgroundLight(EBL)photonsneedtobeincludedinadditiontoCMBphotons(seedot-dashedlineinFigure8).Upscattering
of infrared-to-optical EBL photons was already required in the stationary leptonic model reported in the original Fermi paper
(Abdoetal.2010a).InourapproachweadoptthemodelbyFranceschinietal.(2008)toevaluatethisEBLcontribution.Thesolid
lineinFig.8representsthetotal(CMB+EBL)inverse-Comptoncontribution.Themaximumtotalenergyofelectronsinbothlobes
isfoundto be≃2×1058 erg.Dividingthisbythe epochtime 106 yr,wouldimplya meankineticpowerofthe jetsinflatingthe
lobesof≃6×1044erg/s,roughlyanorderofmagnitudesmallerthantheEddingtonpowerinferredfortheblackholemassinCen
A,yetwellabovetheestimatedpowerofthekpc-scalejetinthecurrentepochofjetactivity(Crostonetal.2009).Thiswouldimply
thatthejetwasmorepowerfulinthepast.However,therequirementonthejetpowercanbesignificantlyreducedifoneassumes
anageofthelobes≫106yr.
Figure 9 shows a representation of the SED for the maximum epoch time t =8×107 yr, with a maximum electron Lorentz
max
factor g =2.5×106 and 1.5×106 for the North lobe and the South lobe, respectively.Note that in this case the contribution
max
byinverse-ComptonscatteringofCMBphotonsaloneissufficienttoaccountfortheobservedHEspectrum(seethesolidlinein
Figure9).Theinverse-ComptoncontributionofEBLphotonsonlybecomesimportantathigherenergies(seethedot-dashedline
inFig.9).Ontheotherhand,foranepochtimet exceedingt =8×107yr,thehighenergypartoftheSEDwouldnolongerbe
max
consistentwiththedata(seethedashedlineinFig.9fort=108yr).Thiscouldbeinterpretedasadditionalevidenceforafiniteage
<108yrofthelobes.Themaximumtotalenergyofelectronsinbothlobesisfoundtobe≈6×1057erg,requiringonlyarelatively
modestmeankineticjetpowerof2×1042erg/s.
4.2. Hadronicg -rays?
Once protons get efficiently injected, they are likely to remain energetic since the cooling time for pp-interactions is t ≈
pp
1015(n/1cm−3)−1 s. High-energy protons interacting with the ambient low density plasma, can then produce daughter mesons
andthosep 0 componentdecaysintotwog -rays.Thedatareportedhereallowustoderiveanupperlimitontheenergeticprotons
5
Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
containedinthelobesofCenA.Asbefore,weuseapower-lawprotondistributionwithanexponentialcut-off,i.e.,
g
N(g )=N g −a exp − p
p 0 p (cid:18) g (cid:19)
max
where the constant N can be expressed in terms of the total proton energyW =m c2 g N(g )dg . Current estimates for the
0 p p p p p
thermalplasmadensityinthegiantradiolobesofCenAsuggestavalueintherangen≃(1R0−5−10−4)cm−3(e.g.,Isobeetal.2001,
Feainetal.2009). We use n=10−4 cm−3 for the modelrepresentationshown in dotted line in Fig. 9. In both lobes, the power-
law index of the proton populationis a =2.1, and the high-energycut-off is E ≃55 GeV. The maximum total energyW is
max p
proportionaltothegasnumberdensityn,sothatW ≃1061(n/10−4cm−3)−1 erg,obtainedhere,shouldbeconsideredasanupper
p
limit.Inprinciple,protonscouldbeaccumulatedoverthewholeevolutionarytimescaleofthelobes.Forlongtimescale≥109yr,an
averageinjectionpower≤3×1044 erg/sandameancosmic-raydiffusioncoefficientofD∼R2/t∼< 3×1030(R/100kpc)2 cm2/s
wouldbeneeded.
5. DiscussionandConclusion
Results based on an detailed analysis of 3 yr of Fermi-LAT data on the giant radio lobes of Cen A are described in this paper.
We haveshownthatthedetectionoftheHElobeswithasignificancemorethantwice ashighasreportedbefore(i.e.,withmore
than 10 and 20s forthe Northernand the Southernlobe, respectively)allows for a better determinationof their spectralfeatures
andmorphology.AcomparisonoftheFermi-LATdatawithWMAPdataindicatesthattheHEg -rayemissionregionsdonotfully
coincidewiththeradiolobes.Thereisofcoursenoapriorireasonforthemtocoincide.Theresultsreportedhereparticularlysupport
thecase of asubstantialHEg -rayextensionbeyondthe WMAPradioimageinthe case oftheNorthernlobeofCen A. We have
reconstructedtheSEDbasedondatafromthesameemissionregion.Asatisfactoryrepresentationispossibleinaleptonicscenario
withradiativecoolingtakenintoaccountself-consistentlyandinjectiondescribedbyasinglepower-lawwithexponentialcut-off.
Theresultsimplyafiniteage<108yrofthelobesandameanmagneticfieldstrengthB∼< 1µG.Whileforlobelifetimesoftheorder
of8×107yr,inverse-Comptonup-scatteringofCMBphotonsalonewouldbesufficienttoaccountfortheobservedHEspectrum,
up-scattering of EBL photons is needed in the case of shorter lobe lifetimes. In a leptonic framework,the HE emission directly
traces (via inverse-Comptonscattering) the underlying relativistic electron distribution and thereby provides a spatial diagnostic
tool.Theradioemission,arisingfromsynchrotronradiation,ontheotherhandalsotracesthemagneticfielddistribution.Together,
theHE g -rayandtheradioemissionthusofferimportantinsightsintothe physicalconditionsofthe source.Thefactthatthe HE
emissionseemsextendedbeyondtheradioimagecouldthenbeinterpretedasduetoachangeinthemagneticfieldcharacterizing
theregion.Thiswouldimplythataquasi-homogeneousSEDmodelforthelobescanonlyserveasafirst-orderapproximationand
thatmoredetailedscenariosneedtobeconstructedtofullydescribethedata.Thisalsoappliestotheneedofincorporatingelecton
re-accelerationself-consistently.ExtendedHEemissioncouldinprinciplealsoberelatedtoacontributionfromhadronicprocesses.
Thecoolingtimescalesforprotonsappearmuchmorefavourable.Ontheotherhand,boththespectralshapeofthelobesandthe
required energetics seem to disfavor pp-interaction processes as sole contributor. One of the insights emerging from the present
paperistheneedforamoredetailedtheoreticalSEDapproachinordertobeabletotakefulladvantageofthecurrentobservational
capabilities.
References
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FranceschiniA.,RodighieroG.,VaccariM.2008,Astron.Astrophys.487,837
Hardcastle,M.J.,Cheung,C.C.,Feain,I.J.,Stawarz,L.2009,MNRAS,393,104
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O’Sullivan,S.,Reville,B.,Taylor,A.2009,MNRAS,400,248
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Steinle,H.2010,PASA,27,431
http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone/
http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Pass7 usage.html
6
Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
0
0
0
6.
3
-
0
0
0
0.
on -4
ati
n
cli 00
e 0
D 4.
4
-
0
0
0
8.
4
-
212.000 206.000 200.000 194.000
Right ascension
Fig.2:Thetwotemplatesusedintheanalysis.ThebluecontourscorrespondtoT1andtheredtoT2.
be) 500be) be)350 350be)
o o o o
mi LAT - South L450000 340000WMAP - South L mi LAT - North L223050000 223050000WMAP - North L
mber of Events (Fer123000000 0120000Number of Events ( mber of Events (Fer11055000 051100500Number of Events (
Nu Nu 0 -50
-4 -2 0 2 4 -6 -4 -2 0 2 4 6
q q
(a)ProjectionoftheSouthlobe (b)ProjectionoftheNorthlobe
Fig.3:TheprojectionoftherectangularregionshowninFig.1(b)forbothlobes.ThecurvesareGaussianfits,withWMAPdatain
redandFermi-LATdatainblack.
7
Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
1e-10
)
2
-
m
c
1
-
s 1e-11
g
r
e
(
x
u
l
F
)
h
ut 1e-12
o
S 2
/
h
t
r 1.5
o
N
( 1
o
i
t
a 0.5
R
x
u 0
L
100 1000 10000
F
Energy(MeV)
Fig.4: SED for template T1. Squares and crosses are for the North lobe and the South lobe, respectively.The ratio of the fluxes
(North/South)areshowninthebottompanel.
0
0
0
6.
3
-
0
0
0
0.
on -4
ati
n
cli 00
e 0
D 4.
4
-
0
0
0
8.
4
-
212.000 206.000 200.000 194.000
Right ascension
Fig.5: The two lobes are both divided into two parts (near core of Cen A core and further away) for spectral comparison. The
contoursindifferentcolorsrepresentthedifferentparts.
8
Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
1e-10 1e-10
-2m) 1e-11 -2m) 1e-11
-1Flux(erg s c 1e-12 -1Flux(erg s c 1e-12
1e-13 1e-13
100 1000 100 1000
Energy(MeV) Energy(MeV)
Fig.6:TheSEDsforthecorrespondingsubregionsintheNorth.TheleftpanelistheSEDofthesubregionfarfromthecore,while
therightpanelreferstothesubregionnearthecore.ThecolorsarethesameasusedinthedefinitioninFig.5.
1e-10 1e-10
-2m) 1e-11 -2m) 1e-11
-1Flux(erg s c 1e-12 -1Flux(erg s c 1e-12
1e-13 1e-13
100 1000 100 1000
Energy(MeV) Energy(MeV)
Fig.7:SameasFig.6butfortheSouthernsubregions.TheleftpanelistheSEDofthesubregionfarfromthecore,whiletheright
panelreferstothesubregionnearthecore.
13.5 13.5
13.0 13.0
-2LL -2LL
m m
-1gsc 12.5 -1gsc 12.5
-2310er 12.0 -2310er 12.0
´ ´
ΥHFΥ 11.5 ΥHFΥ 11.5
Hg10 Hg10
lo 11.0 lo 11.0
10.5 10.5
10 15 20 25 10 15 20 25
log HΥHHzLL log HΥHHzLL
10 10
(a)Southlobe (b)g -rayexcess(lobe)regionintheNorth
Fig.8:Synchrotronandinverse-Comptonfluxesfort=106 yr.TheradiodatafortheSouthlobearefromHardcastleetal.(2009)
(sum of Region 4 and Region 5 in their Table 1), while the radio data for the North region are from the WMAP analysis in this
paper.ThemeanmagneticfieldvalueBusedfortheNorthandtheSouthlobeis0.38µGand0.45µG,respectively.Thedot-dashed
linereferstotheICcontributionduetoEBLupscattering.
9
Rui-zhiYangetal.:DeepObservationoftheGiantRadioLobesofCentaurusAwiththeFermiLargeAreaTelescope
13.0 13.0
-2LLm 12.5 12.5
-1gsc 12.0
-´2310er 12.0
HFΥ 11.5
ΥHg10 11.5
lo 11.0
11.0
10.5
10 15 20 25
log10HΥHHzLL 10.5 10 15 20 25
(a)Southlobe (b)g -rayexcess(lobe)regionintheNorth
Fig.9: Synchrotronand inverse-Comptonfluxes fort =8×107 yr. The mean magnetic field value B for the South lobe and the
g -ray excess region in North is 1.17µG and 0.91µG respectively. The dot-dashed line refers to the IC contribution due to EBL
upscattering.Thedashedline(a)showstheresultfort=108yr.Thepossibleg -rayfluxexpectedfrompp-interactionsforathermal
gasdensityn=10−4cm−3arealsoshown(dottedline).
10