3C66B as a TeV radiogalaxy

0.13 MB·

by  F. Tavecchio

Mon.Not.R.Astron.Soc.000,1–??(2008) Printed22January2009 (MNLATEXstylefilev2.2)
3C66B as a TeV radio-galaxy
⋆
Fabrizio Tavecchio and Gabriele Ghisellini
INAF/OsservatorioAstronomicodiBrera,viaE.Bianchi46,I–23807Merate,Italy
9
0
22January2009
0
2

n
ABSTRACT
a
TheMAGICcollaborationreportedthedetectionofanewVHEsource,MAGICJ0223+430,
J
located close to the position of the blazar 3C66A, considereda candidate TeV blazar since
2
a long time. A careful analysis showed that the events with energies above 150 GeV are
2
centered on the position of the FRI radiogalaxy 3C66B (at 6 arcmin from 3C66A), with a

probabilityof95.4%(85.4%includingsystematicuncertainties)thatthesourceisnotrelated
]
h to3C66A.Wepresentamodelforthepossibleemissionof3C66Bbasedonthestructuredjet
p modelalreadyusedtointerprettheTeVemissionoftheradiogalaxyM87.Themodelrequires
- parameterssimilartothoseusedforM87butalargerluminosityforthelayer,toaccountfor
o
themoreluminousTeVemission.WealsoshowthatthespectrumobtainedbyMAGICcanbe
r
t interpretedasthecombinedemissionof3C66B,dominatingabove∼200GeV,andof3C66A.
s
a Thehigh–energyemissionfromthelattersource,beingstronglyattenuatedbytheinteraction
[ withtheextragalacticbackgroundlight,canonlycontributeatlowenergies.Ifweweretosee

thejetemissionof3C66Batsmallviewingangleswewouldseeaspectralenergydistribution
2
closelyresemblingtheoneofS50716+714,atypicalblazar.
v
3
Keywords: galaxies:active–galaxies:individual:3C66B–BLLacertaeobjects:individual:
8
3C66A–BLLacertaeobjects:individual:0716+714–radiationmechanisms:non–thermal.
8
1
.
1
1
8 1 INTRODUCTION 2008).However,acarefulanalysisoftheMAGICdatashowsthat
0 theevents withenergies above ∼150 GeV arerather centered on
Blazars represent the great majority of extragalactic objects de-
: thepositionofthenear(6arcmin)FRIradiogalaxy3C66B(located
v tectedinthe veryhigh energy (VHE,E > 30 GeV) γ–rayband
at85.5Mpc),withaprobabilityof95.4%(decreasingto85.4%if
i (Aharonianetal.2008,DeAngelis,Mansutti&Persic2008forre-
X systematicuncertaintiesonthepositioningaretakenintoaccount)
centreviews)1.Untilnowonlyonenon–blazarsource,thenearby
r thatMAGICJ0223+430 isnotrelatedto3C66A.Theassociation
(16Mpc)FRIradiogalaxyM87isanestablishedVHEsource(Aha-
a with3C66Bisalsoindirectlysupportedbytherelativelyhardmea-
ronianetal.2003,2006a,Acciarietal.2008,Albertetal.2008a).
suredspectrum(photonindexΓ=3.1±0.3),difficulttoreconcile
DuetothelimitedspatialresolutionofCherenkovtelescopesitis
withtheexpectedstrongabsorptionofVHEphotons, throughin-
not possible to identify the TeV emission region of M87. How-
teraction with the extragalactic background light, for a source at
ever,thedetectionofvariabilityonshorttimescales(∼days,Aha-
theredshift af3C66A. However, contamination from3C66A (es-
ronianetal.2006a,Albertetal.2008a)suggestsacompactemis-
peciallyatlowγ–rayenergies)cannotbecompletelyexcluded.If
sionregion,possiblyrelatedtoknotHST-1(e.g.Cheung,Harris&
the association of the excess with 3C66B will be confirmed, this
Stawarz 2007), to the black hole horizon (Neronov & Aharonian
wouldbethesecond known TeVemittingradiogalaxy, indicating
2007)ortotheinnermostregionsoftherelativisticjetconsidered
thatM87isnotanexceptionalcase.
forblazars(Georganopoulos,Perlman&Kazanas2005,Lenainet
In this letter we assume that the VHE emission detected by
al.2008,Tavecchio&Ghisellini2008,hereafterTG08).
MAGICcomesfrom3C66Bandpropose(Section2)thatitispro-
Recently,theMAGICcollaboration(Aliuetal.2008)reported
duced, as in the case of M87, in the misaligned inner structured
the detection, during observations taking place between August
jet (Ghisellini, Tavecchio & Chiaberge 2005, hereafter GTC05,
andDecember2007,ofanewsource,MAGICJ0223+430,located
TG08).For“structuredjet”wemeanajetcomposedbyafastin-
closetothepositionoftheBLLacobject3C66A.Thisblazar(with
ternalspinesurrounded byaslower layer.Boththespineandthe
probable redshift z = 0.444, Miller et al. 1978, but seeFinke et
layeremitandthereisastrongradiativeinterplaybetweenthetwo,
al. 2008) has been considered a TeV candidate since a long time
sinceonecomponent seestheradiationemittedbytheotherrela-
(Neshpor et al. 1998, Stepanyan et al. 2002) and it has been re-
tivisticallyboostedbytherelativevelocity.Inalignedsources(i.e.
centlydetectedbyVERITAS(withasoftspectrum,Swordyetal.
blazars)wepreferentiallyseetheradiationproducedbythespine.
Instead,formisalignedsources,asradiogalaxies,wetendtoseethe
⋆ E–mail:[email protected] radiationproducedbythelayer,sinceitslowerbulkLorentzfactor
1 seealsohttp://www.mppmu.mpg.de/∼rwagner/sources impliesthatitsradiationiscollimatedwithinalargersolidangle.
2 F.Tavecchio & G.Ghisellini
As in the case of M87, we show that, if observed under a small
viewingangle,theSEDof3C66Bwouldresemblethatofatypical
BLLacobject,S50716+714,alsorecentlydetectedinTeVbandby
MAGIC(Teshimaetal.2008).InSection3wediscussthepossible
contributionofthe(stronglyattenuated)TeVemissionof3C66Ato
theobservedMAGICspectrum.Hereafterweassumethefollowing
cosmologicalparameters:H0 = 70 km s−1 Mpc−1,ΩΛ = 0.7,
ΩM =0.3.
2 VHEEMISSIONOF3C66BFROMAMISALIGNED
STRUCTUREDJET
2.1 Themodel
The spectral energy distribution (SED) of the core of 3C66B
is shown in Fig.1. We use the equivalent isotropic luminosities
Lν, calculated from the corresponding fluxes Fν with νrLνr =
4πd2LνoFνo (where dL is the luminosity distance) and the fre-
quencyν isintherestframeofthesource,relatedtotheobserved
r
one by νr = (1 +z)νo. The TeV spectrum (red open squares)
is taken from Aliu et al. (2008) and it has been corrected (blue
opensquares)forthe(small)attenuationusingtheLowSFRmodel
of Kneiske et al. (2004) which predicts a low level of the extra-
galacticbackground lightclosetowhatispresentlyinferredfrom
observations(e.g.Franceschinietal.2008)andindirectarguments
(Aharonianetal.2006b,Mazin&Raue2007,Albertetal.2008b). Figure1. SEDofthe coreof3C66B (lower part), represented withthe
In the X–rays this radiogalaxy shows a rather peculiar be- equivalent isotropic luminosityasafunctionoftherestframefrequency.
haviour,withthespectrumapparently changingfromsofttovery Radio through UV data (green filled triangles) are from Trussoni et al.
hard (Trussoni et al. 2003, Balmaverde, Capetti & Grandi 2006, (2003)andreferencestherein.X–raydata(bow-ties)arefromTrussoniet
Evansetal.2006). Inparticular, comparingChandra andROSAT al.(2003)(cyanandorange),Balmaverdeetal.(2006)(blue)andEvanset
al.(2006)(green).MAGICdata(redopensquares)havebeencorrectedfor
observations, Trussonietal.(2003) concludedthattherearehints
absorption(blueopensquares)withtheLowSFRmodelofKneiekeetal.
thattheX–rayfluxfromthecoreofthesourcedecreasedbyafac-
(2004).Thecurvescorrespondtothestructuredjetmodel,consideringthe
tor ∼ 4in10 yearswithanimportant softeningof thespectrum.
spine(red)andthelayer(blue)emission.Wealsoreport(upperpart)the
Thisbehaviour,unusualforaradiogalaxy,isreminiscentofthatof emissionseenbyanobserveralmostalignedwiththejetaxis(θv = 2◦)
blazars and could support the idea that the emission comes from with(solid)andwithout(dashed)thecontribution oftheseedphotonsof
themisalignedjet.However,notethat,duetothecomplexityofthe thelayer forthescattering byelectrons inthespine. Forcomparison we
observed X-ray spectrum, requiring several spectral components, alsoreporthistorical data (taken from NED,Tagliaferri etal. 2003,Fos-
a strong conclusion is not possible. Optical-UV data for the core chinietal.2006,Hartmanetal.1999)describingtheSEDoftheBLLac
havebeenobtainedwiththeHubbleSpaceTelescope(Chiaberge, S50716+714.Theblueopendiamondindicates theTeVfluxasdetected
Celotti&Capetti1999). byMAGIC (Teshima et al. 2008). Theblack dashed line in the TeV re-
gionshowstheγ–rayfluxreachingtheobserver,consideringtheabsorption
There is large consensus that the radio, the optical-UV and,
through theinteraction withthe extragalactic background light estimated
possibly, the X-ray emission from the core of FRI radiogalaxies
withtheLowSFRmodelofKneiekeetal.(2004)assumingz = 0.31for
isduetosynchrotronemissionfromtheinnerjet(Chiabergeetal.
theredshift ofS50716+714. The(negligible) emissionfromthelayerat
1999,2002,Hardcastle&Worrall2000,VerdoesKleijnetal.2002,
smallanglesisshownbythedottedline.
Balmaverdeetal.2006).For3C66Badirectpossibilitycouldthere-
fore be that the VHE emission is part of the high-energy inverse
Comptoncomponentfromthesameregionsofthejet.However,as Comptonemissionofbothcomponents.Eachcomponentseesthe
detailedinTG08,asingle–zoneemissionmodelfailsinreproduc- emissionoftheotheramplifiedbecauseoftherelativespeed:this
ingaSEDwithtwoemissionpeakshighlyseparatedinfrequency: externalradiationcontributestothetotalenergydensity,enhancing
3C66B, in fact, has the first peak in the IR and the second peak theemittedinverseCompton radiation. Depending on theparam-
probablyclosetoTeVenergies.Thestructuredjetmodel,instead, eters, this “external Compton” (EC) emission can dominate over
allowstocorrectlyreproducetheemissionwithparameterssimilar theinternalsynchrotronself–Compton(SSC)component that,es-
tothosecommonlyinferredforblazarjets. pecially in TeV blazars, is depressed because scatterings mainly
WemodeltheSEDusingthestructuredjetmodelofGTC05 occur in the Klein–Nishina (KN) regime. We refer the reader to
and applied to M87 in TG08. Briefly, we assume that the jet has GTC05foramoredetaileddiscussionofthetreatmentusedtocal-
ainnerfastcore(spine),withbulkLorentzfactorΓS,surrounded culatetheECterm.
by a slower layer, with bulk Lorentz factor ΓL. In both regions, The model is specified by the following parameters: i) the
relativisticelectronsemitthroughsynchrotronandinverseComp- spineisassumedtobeacylinderofradiusR,heightH (asmea-
S
tonmechanisms. Theexistenceofavelocitystructureimpactson sured inthespine frame) and inmotion withbulkLorentz factor
theobservedemissionpropertiesofthejet.Specifically,theradia- ΓS;ii)thelayerismodeledasanhollowcylinderwithinternalra-
tiveinterplaybetweenthelayerandthespineamplifiestheinverse diusR,external radiusR ,height H (asmeasured intheframe
2 L
3C66B asa TeV radiogalaxy 3
R H Lsyn nae B γmin γb γmax n1 n2 Γ θv
cm cm ergs−1 cm−3 G deg.
3C66Bspine 5×1015 5×1015 1041 1.3×103 1.8 1 5×103 6×104 2 4 10 20
3C66Blayer 5×1015 2×1016 1041 1.4 0.8 1 3×106 2×107 1.7 3.2 3 20
M87spine 7.5×1015 3×1015 5.2×1041 140.8 1 600 5×103 1×108 2 3.7 12 18
M87layer 7.5×1015 6×1016 4.0×1038 2.04 0.2 1 6×106 1×109 2 3.7 4 18
Table1.InputparametersofthemodelsforthelayerandthespineshowninFig.1.Allquantities(exceptthebulkLorentzfactorsΓandtheviewingangle
θv)aremeasuredintherestframeoftheemittingplasma.TheexternalradiusofthelayerisfixedtothevalueR2=1.2×R.Fordirectcomparison,wealso
reporttheparametersusedforM87(highstate,fromTG08).a:densityoftherelativisticelectrons.Notethatthisisnotadirectinputparameter,sinceitis
determinedbytheassumedLsyn.
ofthelayer)andbulkLorentzfactorΓL.Eachregioncontainstan- electronsandmagneticfieldareclosetoequipartition,whileinthe
gled magnetic field with intensity B , B and it is filled by rel- layerthemagneticenergydensityisabouttwoordersofmagnitude
S L
ativisticelectronsassumedtofollowa(purelyphenomenological) largerthantheelectronenergydensity.
smoothed broken power–law distribution extending from γ to
min
γ andwithindicesn ,n belowandabovethebreakatγ .The
max 1 2 b
normalisation of this distribution is calculated assuming that the 2.2 3C66BandM87
systemproducesanassumed(bolometric)synchrotronluminosity
It is interesting to compare the SED and the parameters of the
L (asmeasuredinthelocalframe),whichisaninputparameter
syn model obtained for for 3C66B with those of M87 (also reported
ofthemodel.AsinGTC05andTG08weassumethatH > H .
L S inTable1).
Assaidabove,theseedphotonsfortheICscatteringarenotonly
TheshapeoftheSEDofthecoreofbothradiogalaxiesinthe
thoseproducedlocallyinthespine(layer),butwealsoconsiderthe
radioandtheoptical-UVbandsappearsrathersimilar,withapeak
photonsproducedinthelayer(spine). intheIRregion.Alsotheluminosities(between1040 ergs−1and
In Fig. 1 (lower part) we report the model reproducing the 1041 ergs−1)arecomparable.IntheX-rayband,M87displaysa
dataassumingtheemissionfromthespine(red),accountingforthe
soft spectrum, close to that of 3C66B as measured by Chandra.
datafromtheradiototheoptical–UVbands,andthelayer(blue),
However,forM87therearenohintsofvariability,asinthecaseof
assumed toemit the VHE radiation. In view of thepossible con-
3C66B.
tributionof3C66AtotheobservedMAGICspectrum(seeSect.3),
Aremarkabledifference,instead,concernstheimportanceof
weassumethattheemissionfromthelayerof3C66Blies(slightly)
theVHEemission.WhiletheluminosityofM87and3C66Binra-
belowthepointatthelowestenergy.Parametersforthespineand
dio,opticalandX–rays(assumedtobeproduced bythespine)is
thelayerarereportedinTable1.ToproduceTeVphotons,electrons
comparable,theVHEluminosity(fromthelayer)isverydifferent:
ofthelayermust haveahighvalueof γ .Consequently, fortyp-
b evenifthefluxemittedat150GeViscontaminatedby3C66A(see
icalvaluesofthemagneticfieldintensity,thecorresponding syn-
alsobelow)theoutputatVHEof3C66Bismorethananorderof
chrotronemissionpeaksintheX-rayband,determiningahardX-
magnitudelargerthanforM87.Thedifferenceintheluminosityat
rayspectrum.TheSSCemissionfromthelayerisnegligible,since
TeVenergiestranslatesinthedifferent(comoving)luminositiesfor
mthoeshtiogfh–theenesrcgaytteerminigsssiohnapopfetnhedelaeypelyr,ianccthoeunKtiNngrefgoirmthee.Tmheearseufroerde ethrgelsa−y1erfoLrsyMn8u7s.eIdnin3Co6u6rBmothdeell:a1y0e4r1reeqrguisr−es1tfhoers3aCm6e6Bpoawnedr1o0u3t8-
MAGICspectrum,isdominatedbytheECcomponent.Onthecon-
putofthespine.Alltheotherparametersareinsteadverysimilar.
trary,theECemissionfromthespinedoes not contributesignifi-
Thevalueofγ ,differentbetweenthetwosources,isphysically
cantlytotheemission(attheviewinganglesconsideredhere:see max
unimportant, as long as γ ≫ γ . Of course, these values re-
thediscussion inSection2.3)andthehigh–energy bumpisdom- max b
fertothespecific(probablyhigh)leveloftheemissionof3C66B
inated by the SSC process. Since the spine has to reproduce the
attheepochoftheobservationsofMAGIC.Lowerluminositiesin
optical-UVpeakthecorrespondingvalueofγ islowerthanwhat
b theTeVbandarepossible.Indeed,inourscenarioimportantvari-
assumedforthelayer.
ationsoftheemissionfromthelayerarerequiredtoreproducethe
Withtheadoptedparametersandassuming1protonperrela-
variationsobservedintheX-rayband.
tivisticelectrons(seee.g.Celotti&Ghisellini2008)theestimated
energyfluxis4.5×1044ergs−1forthespine(dominatedbypro-
tons), and 2.5×1041 erg s−1 for the layer (magnetically domi-
2.3 Pairing3C66BandS50716+714
nated). A value around few 1044 erg s−1 is within the range of
energyfluxesestimatedforFRIs(e.g.Bicknell&Begelman1996, OnecanwonderwhatwouldbetheSEDcalculatedbyourmodel
Laing&Bridle2002).Thevalueoftheenergyfluxforthespine, foran observer located atasmall viewingangle. Sinceweought
largelydominatedbyprotons,dependsontheirnumberdensity,in toobtainaSEDsimilartotheSEDofknownblazars,thisshould
turn related to γmin. In our fit we used γmin = 50. Smaller val- be considered as a consistency check for our model and for the
ues of γ do not affect theshape of theSEDsand in principle derived parameters. In Fig. 1 we report the predicted SED for a
min
couldbeacceptable, thoughthederivedenergyfluxwouldlarger. viewingangleof2degreesandcompareittothemulti–frequency
InthethespinetheratioofPoyntingfluxtoparticleenergyfluxis observationsoftheBLLacS50716+714attheredshiftz = 0.31
dominated(byafactor∼10)byparticles(coldprotons).Thelayer, (Witzeletal.1988;butpossiblylarger,Sbarufatti,Treves&Falomo
instead,appearstobemagneticallydominated, withthePoynting 2005).ThisblazarhasbeenrecentlydetectedintheTeVbandby
fluxabouttentimestheparticleenergyflux.Inthespine,relativistic MAGIC (Teshima et al. 2008). Although we do not pretend that
4 F.Tavecchio & G.Ghisellini
themodelexactlyreproducesthedata,onecanseethatthetwoare
rathersimilar.AsdiscussedinTG08,theconditionthattheSEDat
smallanglesresemblesthoseofknownblazarsdictatesthechoice
thattheVHEradiationcomesfromthelayer.
At small angles, the emission is entirely dominated by the
spine,whichischaracterisedbyalargerLorentzfactor.Thedirect
emissionfromthelayer(dottedline)providesanegligiblecontri-
butiontothetotalobservedemission.Animportantpointtonoteis
theratherhardspectrumpredictedintheGeVband.Theemission
inthisspectralbandismainlyduetotheECcomponentofthespine
Comptonizingtheseedphotonsprovidedbythelayer.Forcompar-
ison, the SSC component of the spine is reported by the dashed
orangeline.Asalreadynoted,theECcomponentfromthespineis
notprominentatlargerangles(notethedifferentshapeofthehigh
energycomponentofthespineforθv =20deg).Thereasonisthat,
contrarytothecaseofthelayer,theECbeamingconeofthespine
isnarrowerthanthecorrespondingsynchrotronandSSCcones,an
effect originally pointed out by Dermer (1995) in discussing the
external inverse Compton emission in blazars. This effect is due
tothefactthattheseedphotonsproducedinthelayerareseen,in
the spine frame, as coming from the forward direction. The pat- Figure 2. SED of 3C66B as in Fig. 1 and of 3C66A. Data for 3C66A
ternofthescatteredradiation,eveninthespineframe,istherefore (bluesquaresandbow-ties)arefromNED(radio,optical),Impey&Neuge-
anisotropic: morepower isproduced indirectionsclosetothejet bauer(1988)(IRAS),Donatoetal.(2001)(ROSAT),Donatoetal.(2005)
axis.Whentransformingintheobserverframe,thistranslatesinto (BeppoSAX),Foschinietal.(2006)(XMM–Newton),Hartmanetal.(1999)
aradiationpatternthatismoreenhancedalongthevelocity(andjet (EGRET).Thesquareincyanreportstheaverageoptical(R–band)fluxdur-
ingtheobservationsofMAGIC.TheSEDof3C66Ahasbeenreproduced
axis)directionthantheoneusuallyderivedwhendealingwithra-
withthe model forthe blazar emission described inCelotti &Ghisellini
diationthatisisotropicinthecomovingframe(seeFig.2ofTG08
(2008).Thesolidlineisthespectrumemittedatthesource,whilethedashed
thatillustratesthispoint).
line reports the γ–ray emission reaching the Earth after being absorbed
The presence of the layer is thus important even when the
bytheinteractionwiththeextragalacticbackgroundlight(weassumedthe
source is observed at small viewing angles and the entire radia- LowSFRmodelofKneiskeetal.2004andaredshiftofz = 0.444).The
tion is provided by the spine. It enhances the observed radiation measuredVHEspectrumofMAGIC(cyancircles)isreproducedasthesum
producedbythespineathighenergies(GeVband),justwherethe oftheemissionfrombothsources,3C66Abeingdominantbelow200GeV,
suppressionoftheSSCfluxduetoKNeffectsstartstobeimpor- and3C66Baccountingfortheemissionabove200GeV.Thegreylinere-
tant.Thecontributionoftheseedphotonsofthelayerisimportant portstheFermisensitivity(for1yearand5σsignificance).
whenthelayerluminosityislarge,asinthecaseof3C66B.Thisis
thereasonwhy,inthecaseofM87pairedwithBLLac,itwasnot
datawiththemodel,thecurveoftheintrinsicemission(solidline)
noticeable:thelayerluminosity,inthatcase,wasmuchsmaller.
hasbeenabsorbedwiththeLowSFRmodelofKneiskeetal.(2004)
(dashedline).Duetothesevereabsorption, thefluxfrom3C66A
above100GeVisstronglydepressedandonly3C66Bcontributes
3 THEVHESPECTRAOF3C66A/B totheobservedspectrum.However,3C66Acouldentirelyaccount
forthefluxmeasuredatthelowestenergies.Inthiscase,thecon-
Wecannotexcludethatthenearby3C66Acontributestothetotal
tributionof3C66Aalleviatestherequirementontheluminosityof
emissionmeasuredbyMAGIC,alsoinviewoftherecentdetection
thelayerof3C66Bbyafactorof3(asalreadynotedthelineforour
byVERITAS(Swordyetal.2008).However,duetothe(uncertain)
modelof3C66BstaysbelowthefirstbinoftheMAGICspectrum).
highredhiftofthisblazar,asizeablecontributiontothemeasured
Ofcourse,sincenosimultaneousdata(especiallyathighen-
spectrumisexpectedonlyatthelowestenergies,∼100GeV.
ergy)areavailablefor3C66A,ourmodeloftheSEDissomewhat
TheSEDsof3C66AandBarecomparedinFig.2.For3C66A
arbitrary.Inparticular, despitethestrong absorption, adetectable
weusedhistoricaldata(seefigurecaptionforreferences),together
emissionintheTeVbandisexpectedinstatesofhighactivityof
with the R-band flux averaged over the period of observation of
thesource. Infact,therecentdetectionbyVERITAS,withaflux
MAGIC(Aliuetal.2008).Unfortunatelytherearenoobservations
above10−11ergcm−2s−1(Swordyetal.2008),hasbeenobtained
inthe GeV band of 3C66A around theepoch of the MAGIC ob-
duringaperiodofratherintenseγ–rayactivity,detectedbyFermi
servations. Therefore, inour model weassume the average spec-
(Tostietal.2008).
trum(bow-tie)reportedinthe3rdEGRETcatalogue(Hartmanet
al.1999).
Wequantifythepossiblecontributionof3C66Atothespec-
trummeasuredbyMAGICmodelingtheSED,withthemodelfor 4 DISCUSSION
blazarsdescribedinCelotti&Ghisellini(2008)2.Tocomparethe
ThepossibledetectionofVHEphotonsfromtheFRIradiogalaxy
3C66B (at a distance of 85.5 Mpc) suggests that M87 is not an
2 Weuse the following parameters: R = 1.5×1016 cm, ∆R = R, isolatedcasebut,instead,radiogalaxiescouldrepresentanewclass
L′ = 3.8×1042 ergs−1,B = 1.4G,γmin = 1,γb = 4.5×103, of extragalactic TeV sources. The fact that radiogalaxies can be
γmax =105,n2 =3.6,Γ=18,θv =3deg.SeeCelottiandGhisellini relativelyluminousγ–raysourceswasproposedbyGTC05based
(2008)forafulldescriptionoftheseparameters. on the idea of the existence of structured jets. According to this
3C66B asa TeV radiogalaxy 5
scenario, in sources with misaligned jets, the emission from the ACKNOWLEDGEMENTS
fastregionsofthejetisdeboostedwhiletheslowerlayercanstill
WethankL.Maraschi,D.Mazin,M.PersicandR.Wagnerforuse-
provideanimportantcontributionbecauseofthebroaderemission
ful comments and M. Chiaberge and E. Trussoni for discussions.
cone.
Wearegratefultothereferee,G.Bicknell,fortheconstructiveand
The best VHEcandidates aretheradiogalaxies in whichthe usefulreport.ThisresearchhasmadeuseoftheNASA/IPACEx-
jet axis liesat a relatively small angle with respect to the line of tragalacticDatabase(NED)whichisoperatedbytheJetPropulsion
sight.M87,withananglebetween30and20degreesisprobably Laboratory,CaliforniaInstituteofTechnology,undercontractwith
oneofthebestcases.For3C66Bweassumeinourmodelasimilar theNationalAeronauticsandSpaceAdministration.Thisworkwas
angle,20degrees.Forlargeranglesthedeboostingbecomesmore partlyfinanciallysupportedbya2007COFIN-MiURgrant.
importantandthusthepowerrequirementgrowssubstantially.The
consequent increaseoftheintrinsicluminosityofthespineinthe
optical-IRbandresultsinalargeopticaldepthforthepairproduc-
tion process for VHE photons within the layer. Radio VLBI ob-
REFERENCES
servations (Giovannini et al. 2001) show a complex structure for
3C66B, with the possible presence of a counter-jet at small dis- AcciariV.A.,etal.,2008,ApJ,679,397
tances, disappearing at larger distances. Giovannini et al. (2001) AharonianF.,BuckleyJ.,KifuneT.,SinnisG.,2008,RPPh,71,096901
AharonianF.,etal.,2006a,Sci,314,1424
interpreted this behavior as the evidence that the jet accelerates,
AharonianF.,etal.,2006b,Natur,440,1018
fromβ = 0.6 at 1.5mas (linear deprojected distance of about 1
AharonianF.,etal.,2003,A&A,403,L1
parsec)toβ = 0.99at4.5masfromthecoreandajetinclination
AlbertJ.,etal.,2008a,ApJ,685,L23
ofabout45degrees.However,thisinterpretationisnotunique,the
AlbertJ.,etal.,2008b,Sci,320,1752
sameeffectbeingproducedbyajetwithconstantspeed,β=0.83, AliuE.,etal.,2008,ApJ,submitted(arXiv:0810.4712)
varyingtheangle(ofboththejetandthecounter-jet)fromθ =60 BalmaverdeB.,CapettiA.,GrandiP.,2006,A&A,451,35
degtoθ = 30deg.Moreover,thisconclusionisbasedontheim- BicknellG.V.,BegelmanM.C.,1996,ApJ,467,597
plicitassumptionthatthejetsareexactlysymmetric.Localvaria- CelottiA.,GhiselliniG.,2008,MNRAS,385,283
tions of the emissivity in one of the two jets could easily mimic CheungC.C.,HarrisD.E.,StawarzŁ.,2007,ApJ,663,L65
suchaphenomenology.Giventheseuncertaintiesweconsiderthat Chiaberge M., Macchetto F. D., Sparks W. B., Capetti A., Allen M. G.,
θ=20degreesfortheinnerjet(atdistancesof0.1pc)ispossible. MartelA.R.,2002,ApJ,571,247
ChiabergeM.,CapettiA.,CelottiA.,1999,A&A,349,77
AsextensivelydiscussedinTG08,themodel parametersare deAngelisA.,MansuttiO.,PersicM.,2008,NCimR,31,187
notcompletelyconstrainedbythedata.Awholefamilyofsolutions DermerC.D.,1995,ApJ,446,L63
is allowed. However, the requirement that the parameters of the DonatoD.,SambrunaR.M.,GliozziM.,2005,A&A,433,1163
spine,anditsSEDatsmallangles,aresimilartothoseofblazars DonatoD.,GhiselliniG.,TagliaferriG.,FossatiG.,2001,A&A,375,739
EvansD.A.,WorrallD.M.,HardcastleM.J.,KraftR.P.,BirkinshawM.,
allowsustopartlyconstraintheparameters.
2006,ApJ,642,96
In our model we implicitly assume that the region emitting FinkeJ.D.,ShieldsJ.C.,Bo¨ttcherM.,BasuS.,2008,A&A,477,513
γ-rays from 3C66B is, as for M87, the inner jet. However, since FoschiniL.,etal.,2006,A&A,453,829
FranceschiniA.,RodighieroG.,VaccariM.,2008,A&A,487,837
novariations havebeen detected, thealternativemodelsinvoking
GeorganopoulosM.,PerlmanE.S.,KazanasD.,2005,ApJ,634,L33
emission from more distant regions along the jet (e.g. Stawarz,
GhiselliniG.,TavecchioF.,ChiabergeM.,2005,A&A,432,401(GTC05)
Sikora & Ostrowski 2003) cannot be excluded. Further observa-
GiovanniniG.,CottonW.D.,FerettiL.,LaraL.,VenturiT.,2001,ApJ,552,
tionsareneededtoclarifythispoint.
508
HardcastleM.J.,WorrallD.M.,2000,MNRAS,314,359
Animportantpointtonoteistheimportantcontributionofthe
HartmanR.C.,etal.,1999,ApJS,123,79
ECcomponenttothespineemissionatsmallangles.Thecontribu-
ImpeyC.D.,NeugebauerG.,1988,AJ,95,30
tionisparticularrelevantabovetheGeVband,determiningahard
KneiskeT.M.,BretzT.,MannheimK.,HartmannD.H.,2004,A&A,413,
spectrumuptotheVHEband.InthepreviousmodelforM87this
807
component wasnot soprominent due tothelowerintrinsiclumi- LaingR.A.,BridleA.H.,2002,MNRAS,336,1161
nosityrequiredforthelayer.InBLLacsthiscomponentcouldplay LenainJ.-P.,BoissonC.,SolH.,Katarzyn´skiK.,2008,A&A,478,111
animportantroleindeterminingthehigh-energyγ-rayspectrum. NeronovA.,AharonianF.A.,2007,ApJ,671,85
In particular, since SSC dominates below and the EC above the NeshporY.I.,StepanyanA.A.,KalekinO.P.,FominV.P.,ChalenkoN.N.,
GeV band, variability of the layer emission could determine dif- ShitovV.G.,1998,AstL,24,134
ferent variability of the emission in the two bands. Simultaneous MazinD.,RaueM.,2007,A&A,471,439
MillerJ.S.,FrenchH.B.,HawleyS.A.,1978,inPittsburghConferenceon
observationswithFermiandCherenkovtelescopescouldtestthis
BLLacObjects,ed.A.M.Wolfe,176
possibility.
SbarufattiB.,TrevesA.,FalomoR.,2005,ApJ,635,173
Another important point to mention is that, due to its lim- StawarzŁ.,SikoraM.,OstrowskiM.,2003,ApJ,597,186
itedangularresolution,Fermicannotdistinguishbetweenthetwo StepanyanA.A.,NeshporY.I.,AndreevaN.A.,KalekinO.R.,Zhogolev
N.A.,FominV.P.,ShitovV.G.,2002,ARep,46,634
sources, 3C66A and B. However, the MeV-GeV band should be
SwordyS.etal.,2008,TheAstronomer’sTelegram,1753
largelydominatedby3C66A,witharathersmallcontributionfrom
TagliaferriG.,etal.,2003,A&A,400,477
the radiogalaxy only at the largest energies covered by Fermi.
TavecchioF.,GhiselliniG.,2008,MNRAS,385,L98(TG08)
Above100GeV,however,thesituationreverses,andthustheemis-
TeshimaM.etal.2008,TheAstronomer’sTelegram,1500
sioncanbedominatedby3C66Bwhen3C66Aisatlowlevel.In TostiG.etal.2008,TheAstronomer’sTelegram,1754
theseoccasions,Cherenkovtelescopes,characterizedbyagoodan- TrussoniE.,CapettiA.,CelottiA.,ChiabergeM.,FerettiL.,2003,A&A,
gularresolution,couldpossiblysimultaneouslydetectbothsources. 403,889
6 F.Tavecchio & G.Ghisellini
VerdoesKleijnG.A.,BaumS.A.,deZeeuwP.T.,O’DeaC.P.,2002,AJ,
123,1334
WitzelA.,SchalinskiC.J.,JohnstonK.J.,BiermannP.L.,KrichbaumT.P.,
HummelC.A.,EckartA.,1988,A&A,206,245