Discovery of a very high energy gamma-ray signal from the 3C 66A/B region

0.21 MB·English

by  MAGIC Collaboration

Draftversion January22,2009
PreprinttypesetusingLATEXstyleemulateapjv.10/09/06
DISCOVERY OF A VERY HIGH ENERGY GAMMA-RAY SIGNAL FROM THE 3C 66A/B REGION
E. Aliua, H. Anderhubb, L. A. Antonellic, P. Antoranzd, M. Backese, C. Baixerasf, S. Balestrad, J. A. Barriod,
H. Bartkog, D. Bastierih, J. Becerra Gonza´lezi, J. K. Beckere, W. Bednarekj, K. Bergerj, E. Bernardinik,
A. Bilandb, R. K. Bockg,h, G. Bonnolil, P. Bordasm, D. Borla Tridong, V. Bosch-Ramonm, T. Bretzn,
I. Britvitchb, M. Camarad, E. Carmonag, A. Chilingariano, S. Commichaub, J. L. Contrerasd, J. Cortinaa,
M. T. Costadoi,p, S. Covinoc, V. Curtefe, F. Dazzih, A. De Angelisq, E. De Cea del Pozor, R. de los Reyesd,
B. De Lottoq, M. De Mariaq, F. De Sabataq, C. Delgado Mendezi, A. Dominguezs, D. Dornerb, M. Doroh,
D. Elsaessern, M. Errandoa,**, D. Ferenct, E. Ferna´ndeza, R. Firpoa, M. V. Fonsecad, L. Fontf, N. Galanteg,
R. J. Garc´ıa Lo´pezi,p, M. Garczarczykg, M. Gaugi, F. Goebelg,*, D. Hadasche, M. Hayashidag, A. Herreroi,p,
9 D. Ho¨hne-Mo¨nchn, J. Hoseg, C. C. Hsug, S. Hubern, T. Joglerg, D. Kranichb, A. La Barberac, A. Laillet,
0 E. Leonardol, E. Lindforsu,**, S. Lombardih, F. Longoq, M. Lo´pezh, E. Lorenzb,g, P. Majumdark, G. Manevav,
0 N. Mankuzhiyilq, K. Mannheimn, L. Maraschic, M. Mariottih, M. Mart´ıneza, D. Mazina,**, M. Meuccil,
2 M. Meyern, J. M. Mirandad, R. Mirzoyang, J. Moldo´nm, M. Moless, A. Moralejoa, D. Nietod, K. Nilssonu,
n J. Ninkovicg, N. Otteg,w,aa, I. Oyad, R. Paolettil, J. M. Paredesm, M. Pasanenu, D. Pascolih, F. Paussb,
a R. G. Pegnal, M. A. Perez-Torress, M. Persicq,x, L. Peruzzoh, F. Pradas, E. Prandinih, N. Puchadesa,
J A. Raymerso, W. Rhodee, M. Ribo´m, J. Ricoy,a, M. Rissib, A. Robertf, S. Ru¨gamern, A. Saggionh, T. Y. Saitog,
M. Salvatic, M. Sanchez-Condes, P. Sartorih, K. Sataleckak, V. Scalzottoh, V. Scapinq, T. Schweizerg,
2
M. Shaydukg, K. Shinozakig, S. N. Shorez, N. Sidroa, A. Sierpowska-Bartosikr, A. Sillanpa¨a¨u, J. Sitarekg,j,
2
D. Sobczynskaj, F. Spaniern, A. Stamerral, L. S. Starkb, L. Takalou, F. Tavecchioc, P. Temnikovv, D. Tescaroa,

M. Teshimag, M. Tluczykontk, D. F. Torresy,r, N. Turinil, H. Vankovv, A. Venturinih, V. Vitaleq,
]
h R. M. Wagnerg, W. Wittekg, V. Zabalzam, F. Zandanels, R. Zanina, J. Zapaterof
p Draft version January 22, 2009
-
o
ABSTRACT
r
t The MAGIC telescope observed the region around the distant blazar 3C 66A for 54.2hr in 2007
s
a August–December. The observations resulted in the discovery of a γ-ray source centered at ce-
[ lestial coordinates R.A. = 2h23m12s and decl.= 43◦0.′7 (MAGIC J0223+430), coinciding with

the nearby radio galaxy 3C 66B. A possible association of the excess with the blazar 3C 66A
3
is discussed. The energy spectrum of MAGIC J0223+430 follows a power law with a normal-
v ization of (1.7±0.3 ±0.6 ) × 10−11 TeV−1 cm−2 s−1 at 300GeV and a photon index Γ =
2 stat syst
−3.10±0.31 ±0.2 .
1 stat syst
7 Subject headings: gammarays: observations—BLLacertaeobjects: individual(3C66A)—galaxies:
4 individual (3C 66B) — ISM: individual (MAGIC J0223+430)
.
0
1
8 aIFAE,EdificiCn.,CampusUAB,E-08193Bellaterra,Spain 1. INTRODUCTION
0 bETHZurich,CH-8093Switzerland
: cINAFNationalInstituteforAstrophysics,I-00136Rome,Italy As of today, there are 23 known extragalactic very
v
dUniversidadComplutense,E-28040Madrid,Spain high energy (VHE, defined here as E > 100GeV) γ-ray
i
X eTechnische Universita¨t Dortmund, D-44221 Dortmund, Ger- sources. All of them are active galactic nuclei (AGNs)
many with relativistic jets. With the exception of the ra-
r fUniversitatAuto`nomadeBarcelona,E-08193Bellaterra,Spain
a dio galaxy M 87 all detected sources are blazars, whose
gMax-Planck-Institutfu¨rPhysik,D-80805Mu¨nchen, Germany
hUniversita`diPadovaandINFN,I-35131Padova, Italy jets (characterized by a bulk Lorentz factor Γ ∼ 20)
iInst. deAstrof´ısicadeCanarias,E-38200LaLaguna,Tenerife, point, within a small angle (θ ∼ 1/Γ), to the ob-
Spain server. The spectral energy distribution (SED, loga-
jUniversityofL o´d´z,PL-90236Lodz,Poland
rithm of the observed energy density versus logarithm
kDeutschesElektronen-Synchrotron(DESY),D-15738Zeuthen,
of the photon energy) of AGNs shows typically a two-
Germany
lUniversita`diSiena,andINFNPisa,I-53100Siena,Italy bump structure. The lower-energy bump originates
mUniversitat de Barcelona (ICC/IEEC), E-08028 Barcelona, from synchrotron radiation of relativistic electrons spi-
Spain raling in the magnetic field of the jet. For the origin
nUniversita¨tWu¨rzburg,D-97074Wu¨rzburg,Germany
of the high-frequency bump, various models have been
oYerevanPhysicsInstitute, AM-375036Yerevan,Armenia
pDepto. deAstrofisica,Universidad,E-38206LaLaguna,Tener- proposed, the most popular invoking inverse Compton
ife,Spain
qUniversita`diUdine,andINFNTrieste,I-33100Udine,Italy
rInstitut de Cienci`es de l’Espai (IEEC-CSIC), E-08193 Bel- wHumboldt-Universita¨tzuBerlin,D-12489Berlin,Germany
xINAF/Osservatorio Astronomico and INFN, I-34143 Trieste,
laterra,Spain
sInst. de Astrof´ısica de Andalucia (CSIC), E-18080 Granada, Italy
yICREA,E-08010Barcelona,Spain
Spain
tUniversityofCalifornia,Davis,CA-95616-8677, USA zUniversita`diPisa,andINFNPisa,I-56126Pisa,Italy
uTuorla Observatory, Turku University, FI-21500 Piikkio¨, Fin- aanowat: UniversityofCalifornia,SantaCruz,CA95064,USA
land *deceased
vInst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, **Send offprint requests to M.Errando [email protected],
Bulgaria E.Lindforselilin@utu.fi,[email protected]
2
scattering of ambient photons. There have been sev- 3C66Aunderwentanopticaloutburstin2007August,
eral suggestions for the origin of the low-frequency seed as monitored by the Tuorla blazar monitoring program.
photons that are up-scattered to γ-ray energies: they The outburst triggered VHE γ-ray observations of the
may be produced within the jet by synchrotron radia- source with the MAGIC telescope following the Target
tion (synchrotron self-Compton or SSC mechanism, e.g. ofOpportunityprogram,whichresultedindiscoveriesof
Maraschi et al. 1992; Bloom & Marscher 1996) or come new VHE γ-ray sources in the past (Albert et al. 2006,
from outside the jet (external Compton or EC mecha- 2007a; Teshima 2008).
nism, e.g. Dermer & Schlickeiser 1993). Relativistic ef- MAGIC has a standard trigger threshold of 60GeV,
fects boost the observed emission as the Doppler factor anangularresolutionof ∼0.◦1 and anenergy resolution
depends on the angle to the line of sight. For sources above 150GeV of ∼ 25% (see Albert et al. (2008a) for
with a large angle between the jet and the line of sight details).
(e.g.,theradiogalaxyM87),theseclassicinverseComp- Data were taken in the false-source tracking (wobble)
tonscenarioscannotaccountfortheVHEγ-rayemission. mode (Fomin et al. 1994) pointing alternatively to two
In this case, models that depend less critically on beam- different sky directions, each at 24′ distance from the
ing effects are needed (e.g. Neronov & Aharonian 2007; 3C 66A catalog position. The zenith distance distribu-
Tavecchio & Ghisellini 2008a). The VHE γ-ray emis- tion of the data extends from 13◦ to 35◦. Observations
sion of AGNs might also be of hadronic origin through were made in 2007 August, September, and December
the emission from secondary electrons (e.g. Mannheim and lasted 54.2hr, out of which 45.3hr passed the qual-
1993; Mu¨cke et al. 2003). itycutsbasedontheeventrateafterimagecleaning. An
3C 66A and 3C 66B are two AGNs separated by just additional cut removed the events with total charge less
6′ in the sky. 3C 66B is a large Fanaroff–Riley-I-type than150photoelectrons(phe)inordertoassureabetter
(FRI) radio galaxy, similar to M 87, with a redshift of backgroundrejection.
0.0215 (Stull et al. 1975), whereas 3C 66A is a blazar Justbeforethestartoftheobservationcampaign∼5%
with uncertain redshift. The often referred redshift of of the mirrors on the telescope were replaced, worsen-
0.444(Miller et al.1978) for 3C66Ais basedona single ing the optical point-spreadfunction (PSF). As a conse-
measurement of one emission line only (and the authors quence,anewcalibrationofthemirroralignmentsystem
were not certain on the realness of the feature), while in became necessary, which took place within the observa-
laterobservationsnolines inthe spectraof3C 66Awere tion campaign and improved the PSF again. The sigma
reported (Finke et al. 2008). Based on the marginally of the Gaussian PSF (40% light containment) was mea-
resolved host galaxy (Wurtz et al. 1996), a photometric suredtobe3.′0in2007August12-14,2.′6in2007August
redshift of ∼0.321 was inferred. 15-26and2.′1in2007SeptemberandDecember. Totake
3C 66A, a promising candidate for VHE γ-ray thisintoaccount,datawereanalyzedseparatelyforeach
emission, was observed several times with satellite- period and the results were combined at the end of the
borne and ground-based γ-ray detectors. The EGRET analysis chain. However, the realignment resulted in a
source 3EG J0222+4253 was associated with 3C 66A mispointing, which was taken care of by a new pointing
(Hartman et al. 1999), but the association was am- model(Bretz et al.2009)appliedofflineusingstarguider
biguous because the error box is large enough to information (Riegel et al. 2005). Considering the addi-
cover 3C 66B and the nearby pulsar PSR J0218+4232 tional uncertainty caused by the offline corrections, we
(Verbunt et al. 1996; Kuiper et al. 2000). In the estimatethesystematicuncertaintyofthepointingaccu-
TeV regime the Crimean Astrophysical Observatory’s racytobe2′ onaverage. Notethatinthecaseofanopti-
GT-48 imaging atmospheric Cerenkov telescope has mal pointing model the systematic uncertainty is below
claimedrepeateddetectionsofthissourceabove900GeV 2′, being 1′ on average (Bretz et al. 2009; Albert et al.
(Neshpor et al. 1998; Stepanyan et al. 2002) with a flux 2008a).
as high as (3±1) × 10−11cm−2s−1. HEGRA and The data analysis consists of several steps. Initially,
WHIPPLE reported upper limits, F (>630GeV) < a standard calibration of the data (Albert et al. 2008d)
1.42 × 10−11cm−2s−1 (Aharonian et al. 2000) and is performed. In the next step, an image cleaning pro-
F (>350GeV) < 0.59 × 10−11cm−2s−1 (Horan et al. cedure is applied using the amplitude and timing infor-
2004), from non-simultaneous observations. The mation of the calibrated signals. In particular, the ar-
STACEE solar array also provided an upper limit of rival times of the photons in core pixels (> 6phe) are
F (>184GeV) < 1.2 × 10−10cm−2s−1 (Bramel et al. required to be within a time window of 4.5ns and for
2005). In 2008 September, the Veritas collaboration re- boundarypixels(>3phe)withinatimewindowof1.5ns
portedacleardetectionof3C66A(Swordy 2008)above from a neighboring core pixel. For the surviving pix-
100GeV with an integral flux on the level of 10% of the elsofeacheventimageparametersarecalculated(Hillas
Crab Nebula flux. Shortly after, a high state of 3C 66A 1985). Using the good time resolution of the recorded
was also reported by the Fermi Gamma-ray Space Tele- signals (∼400ps), unique to MAGIC, the time gradient
scope at energies above 20MeV (Tosti 2008). along the main shower axis and the time spread of the
In this paper we report the discovery of VHE γ-ray showerpixels are computed (Aliu et al. 2009). Hadronic
emission located 6.′1 away from the blazar 3C 66A and background suppression is achieved using the Random
coincidingwiththeradiogalaxy3C66Bin2007. InSec- Forest(RF)method(Albert et al.2008c),whereforeach
tion2,wedescribetheobservationsandthedataanalysis eventtheso-calledHadronnessparameteriscomputed,
chain. TheresultsoftheanalysisarepresentedinSection basedon the image and the time parameters. Moreover,
3 and discussed in Section 4. the RFmethodis usedforthe energyestimationtrained
ona Monte Carlosimulatedγ-raysample with the same
2. OBSERVATIONS AND DATA ANALYSIS
3
43.6 5
33CC 6666AB (cid:176)nts / 212800000
4 ve
43.4 e1600
3 1400
43.2 1200
e
DEC (deg)43.0 12 Significanc 1680000000 NNexc == 566573.89.1
0 400 Obnk/dOff normalization = 0.33
42.8 200 Significance = 6.0 s
-1 0
0 10 20 30 40 50 60 70 80 90
42.6 |Alpha| (deg)
-2
Fig. 2.— |Alpha| distribution after all cuts evaluated with re-
spect to the position of MAGIC J0223+430. A γ-ray excess with
02h26m 02h25m 02h24m 02h23m 02h22m 02h21m 02h20m a significance of 6.0σ is found, which corresponds to a post-trial
RA (h) significanceof5.4σ.
Fig. 1.— Significance map for γ-like events above 150GeV in
theobservedskyregion. Thegreencrosscorrespondstothefitted catalog position of 3C 66A, the exclusion probability is
maximumexcess positionofMAGICJ0223+403. Theprobability
85.4%.
of the true source to be inside the green circles is 68.2%, 95.4%,
and 99.7% for the inner, middle, and outer contour, respectively. To calculate the significance of the detection, an
The catalog positions of 3C 66A and 3C 66B are indicated by a |Alpha|distributionwasproduced,whereAlpha is the
whitesquareandablackdot,respectively. angle between the major axis of the shower image el-
lipse and the source position in the camera. For the cal-
zenith angle distribution as the data sample.
culation of the source-dependent image parameters we
3. RESULTS considered the fitted position of the excess. Background
rejection was achieved by a cut in Hadronness, which
Figure 1 shows a significance map produced from the
was optimized using Crab Nebula data taken in similar
signal and background maps, both smoothed with a
conditions and diluted to 5% of its real flux. The cut
Gaussian of σ = 6′ (corresponding to the γ-PSF), for
in |Alpha| that defines the signal region was also opti-
photon energies between 150GeV and 1TeV. For the
mized in the same way. The |Alpha| and Hadronness
background rejection a loose cut in the Hadronness
cutstogetherhaveanefficiencyof40%inkeepingMonte
parameter is applied to keep a large number of gamma-
Carlosimulatedγ events,andresultinanenergythresh-
like events. The center of gravity of the γ-ray emission oldofapproximately230GeV. 1 A signalof 6.0σ signifi-
isderivedfromFigure1byfitting abell-shapedfunction
cance(pre-trial)wasfound(seeFigure2). We estimated
of the form
the number of trials of the signal search by projecting
(x−x¯)2+(y−y¯)2 the γ-rayacceptance of the camera into the field of view
F(x,y)=A·exp − (1)
(cid:20) 2σ2 (cid:21) of the observations,and defined the searchregion where
the γ-ray acceptance after cuts is larger than 50%. In
forwhichthedistributionoftheexcesseventsisassumed this way, we obtained an area of 2.18deg2. Given that
to be rotationally symmetric, i.e., σ = σ = σ. The
x y the 68% containment radius for γ-rays from a point-like
fit yields reconstructed coordinates of the excess center source is 0.◦152, we calculated the number of indepen-
of R.A. = 2h23m12s and decl.= 43◦0.′7. The detected
dent trials to be 30.
excess, which we name MAGIC J0223+430,is 6.′1 away
Figure 3 shows the light curve of MAGIC J0223+430
from the catalog position of 3C 66A, while the distance
together with the flux of 3C 66A in optical wavelengths.
to 3C 66B is 1.′1.
As we integrate over γ-ray events from a wide sky re-
In order to estimate the statistical uncertainty of the gion (∼0.07deg2), we cannot exclude that 3C 66A con-
reconstructed position, we simulated 104 sky maps with
tributes to the measured signal. The integral flux above
the same number of background and excess events as in 150GeV corresponds to (7.3±1.5) × 10−12cm−2s−1
the data. The excesspositionin the sky maps wasfitted
(2.2% of the Crab Nebula flux) and is the lowest ever
and the distance to the simulated source position calcu-
detected by MAGIC. The γ-ray light curve is consistent
lated. From the histogram of the distances we obtained
with a constant flux within statistical errors. These er-
probabilities for an offset between the true source and
rors,however,arelarge,andsomevariabilityofthesignal
thefittotheexcess. TheprobabilitiesshowninFigure1
cannot be excluded.
by the green contours correspond to 68.2%, 95.4%, and
For the energy spectrum of MAGIC J0223+430,loose
99.7% for the inner, middle, and outer contour, respec-
cuts are made to keep the γ-ray acceptance high.
tively. Usingthisstudywefindthatthemeasuredexcess
The differential energy spectrum was unfolded using
coincideswiththecatalogpositionof3C66B.Theorigin
theTikhonovunfoldingtechnique(Tikhonov & Arsenin
oftheemissionfrom3C66Acanbestatisticallyexcluded
1979; Albert et al. 2007b) and is shown in Fig. 4. The
withaprobabilityof95.6%. Addinglinearlythe system-
atic uncertainty of the pointing of the data set (2′, see 1 Definedas thepeakofthedistributionofMonteCarlogener-
above),i.e., shifting the excess position by 2′ towardthe atedgamma-rayevents afterallcuts.
4
· 10-12
-2-1]seV) [cm1200 MAGIC J0223+430 -1-1] TeVs 1100--98 dfGc02N =F=// id n-t( 3E1dF..u f17= n0=4 cf –10–ti .o· 010n .0.(3:2E 1/8 /2) 0· . 31T0e-1V1 [)c Gm-2 s-1 TeV-1]
0 G 0 -2 m
15 c10-10
Flux (E>-10 dA dT [10-11
E
mJy]12 3C 66A (KVA) / d10-12
nd flux [108 dNg10-13
a
R-b 6 10-14
54320 54340 54360 54380 54400 54420 54440 70 100 200 300 1000 2000
Energy [GeV]
time [MJD]
Fig. 3.— Light curve of MAGIC J0223+430. Upper panel: Fig. 4.— Differential energy spectrum of MAGIC J0223+430.
MAGIC integral flux above 150GeV inbins of 3 days (except for The fit to the data is shown by the solid gray line and the fit
periods where the sampling was coarser). The gray dashed line parameters are listed in the inset. No correction for the γ −γ
indicates the average γ-ray flux. Lower panel: optical light curve attenuation due to the EBL has been made. The Crab Nebula
of3C66AasmeasuredbytheKVAtelescope. DuringtheMAGIC spectrum(Albertetal.2008a)isalsoshownasareference(dashed
observations 3C66A was verybrightatoptical wavelengths vary- grayline).
ing from 6mJy to 12mJy in the R-band (the baseline flux in the
historicaldatabeing∼6mJy). Inthesameperiodtheopticalflux
of3C66Bremainedconstant,whichisatypicalbehaviorforlarge
radiogalaxies.
background(EBL,Gould & Schr´eder 1967),theVHEγ-
rayfluxofdistantsourcesissignificantlysuppressed. We
investigated the measured spectrum by MAGIC follow-
spectrum can be well fitted by a power law which gives
a differential flux (TeV−1 cm−2 s−1) of: ing the prescription of Mazin & Raue (2007), and de-
rived a redshift upper limit of the source to be z <0.17
dN =(1.7±0.3)×10−11(E/300GeV)−3.1±0.3 (2) (z <0.24)undertheassumptionthattheintrinsicenergy
dEdAdt spectrum cannot be harder than Γ = 1.5 ( Γ = 0.666).
This assumption of Γ >1.5 is based on particle acceler-
The quoted errors are statistical only. The systematic
ation arguments (Aharonian et al. 2006a), and the fact
uncertainty is estimated to be 35% in the flux level and
thatnoneofthesourcesintheEGRETenergyband(not
0.2 in the power law photon index (Albert et al. 2008a).
affected by the EBL) have shown a harder spectrum.
As in the case of the lightcurve,we cannotexclude that
The latter assumption of Γ > 0.666 can be considered
3C 66A contributes to the measured signal. Thus, the
as anextreme case of the spectrum hardness,suggesting
spectrumshowninFigure4representsacombinedγ-ray
a monochromatic spectrum of electrons when interact-
spectrum from the observed region.
ing with a soft photon target field (Katarzynskiet al.
2006).2 If z > 0.24 for 3C 66A, an alternative expla-
4. DISCUSSION AND CONCLUSIONS
nation for a hard intrinsic spectrum at energies above
A new VHE γ-ray source MAGIC J0223+430was de- 100GeV can be given if γ-rays are passing through a
tected in 2007 August to December. Given the position narrow band of optical-infrared photons in the vicinity
of the excess measured by MAGIC above 150GeV, the of the blazar. Such narrow radiation fields can produce
source of the γ-rays is most likely 3C 66B. The VHE arbitrarily hard intrinsic spectra by absorbing specific
γ-ray flux was found to be on the level of 2.2% Crab energies of γ-rays (Aharonian et al. 2008). We also note
Nebula flux and was constant during the observations. that,inthiscase,theintrinsicVHEluminosityof3C66A
The differential spectrum of MAGIC J0223+430 has a should exceed 1047ergs−1, which is an unusually large
photon spectralindex of Γ=3.10±0.31and extends up valueforaBLLacobject(Wagner 2008),alsoinviewof
to ∼2TeV. In view ofthe recent detectionof 3C 66Aat its spectral characteristics (Persic & De Angelis 2008).
VHE γ-rays(Swordy 2008), wenote that if3C 66Awas 3C 66B is a FRI radio galaxy similar to M 87, which
emittingγ-raysin2007AugusttoDecemberthenitsflux hasbeen detected to emit VHE γ-rays(Aharonian et al.
was at a significantly lower level than in 2008. We also 2003; Aharonian et al. 2006b). Since the distance
note that we cannot exclude the scenario suggested in a of 3C 66B is 85.5Mpc, its intrinsic VHE luminosity
recent work by Tavecchio & Ghisellini (2008b) that the would be two to eight times higher than the one of
observed spectrum would be a combination of emission M 87 (22.5Mpc) given the reported variability of M 87
from 3C 66B (dominating at energies above 150GeV) (Aharonian et al. 2006b; Albert et al. 2008b).
and blazar 3C 66A (at lower energies). As in the case of M 87, there would be several pos-
In the unlikely case, excluded with probability 85.4%, sibilities for the region responsible of the TeV radia-
thatthe totalsignalandobservedspectrumpresentedin tion in 3C 66B: the vicinity of the supermassive black
this paper originates from 3C 66A, the redshift of the hole (Neronov & Aharonian 2007), the unresolved base
source is likely to be significantly lower than previously of the jet (in analogy with blazar emission models;
assumed. Due to the energy-dependent absorption of
VHEγ-rayswithlow-energyphotonsoftheextragalactic 2 SeealsoStecker etal.(2007)formoredetailedcalculations.
5
Tavecchio & Ghisellini 2008a)andthe resolvedjet. Un- anewclassofVHEγ-rayemittingsources. Accordingto
like for M 87,we do not observesignificantvariability in Ghisellini et al. (2005), there are eightFR I radio galax-
the VHE γ-ray flux and therefore we have no constrains ies in the 3CR catalog that should have a higher γ-ray
onthe size ofthe emissionregion. However,as the angle fluxat100MeVthan3C66B,butpossiblymanyofthese
to line of sight is even larger than in M 87 (M 87: 19◦, sourcesareratherweakinthe VHEγ-rayband. Further
Perlman et al. (2003); 3C 66B: 45◦, Giovannini et al. observationsofradiogalaxieswiththeFermiGamma-ray
(2001))theresolvedjetseemsanunlikelysiteoftheemis- SpaceTelescopeaswellasbyground-basedtelescopesare
sion. On the other hand, the unresolved base of the jet needed to further study the γ-ray emission properties of
seems a likely candidate for the emission site as it could radio galaxies.
point with a smaller angle to the line of sight. If the We thank the Instituto de Astrofisica de Canarias for
viewinganglewassmall,blazar-likeemissionmechanisms the excellent working conditions at the Observatorio del
cannotbeexcluded. Theorbitalmotionof3C66Bshows Roque de los Muchachos in La Palma. The support of
evidence for a supermassiveblack hole binary(SMBHB) the German BMBF and MPG, the Italian INFN, and
with a period of 1.05±0.03 years (Sudou et al. 2003). Spanish MCINN is gratefully acknowledged. This work
TheSMBHBwouldlikelycausethejettobehelical,and wasalsosupported by ETHResearchGrantTH 34/043,
the pointing direction of the unresolved jet could differ by the Polish MNiSzW Grant N N203 390834, and by
significantly from the direction of the resolved jet. the YIP of the Helmholtz Gemeinschaft.
GiventhelikelyassociationofMAGICJ0223+430with
3C 66B, our detection would establish radio galaxies as
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