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Home , 3C 66A, 3C 66B

DISCOVERY OF A VERY HIGH ENERGY GAMMA-RAY SIGNAL FROM THE /B REGION

D. MAZIN 1, M. Errando 1, E. Lindfors 2, E. Prandini 3, F. Tavecchio 4 for the MAGIC collaboration 5 1 IFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain, 2 Tuorla Observatory, Turku University, FI-21500 Piikki¨o, Finland, 3 Universit`adi Padova and INFN, I-35131 Padova, Italy, 4 INAF National Institute for Astrophysics, I-00136 Rome, Italy, 5wwwmagic.mppmu.mpg.de/

The MAGIC telescope observed the region around the distant 3C 66A for 54.2 hr in 2007 August–December. The observations resulted in the discovery of a γ-ray source centered ◦ ′ at celestial coordinates R.A. = 2 h23 m12 s and decl.= 43 0. 7 (MAGIC J0223+430), coinciding with the nearby radio . A possible association of the excess with the blazar 3C 66A is discussed. The energy spectrum of MAGIC J0223+430 extends up to ∼ 2 TeV and follows a power law with a photon index Γ = −3.10 ± 0.31 stat ± 0.2syst .

1 Introduction

As of today, there are 26 known extragalactic very high energy (VHE, defined here as E > 100 GeV) γ-ray sources. All of them are active galactic nuclei (AGNs) with relativistic jets. With the exception of the M 87 and Cen A all detected sources are , whose jets (characterized by a bulk Lorentz factor Γ ∼ 20) point, within a small angle ( θ ∼ 1/Γ), to the observer. The spectral energy distribution (SED, logarithm of the observed energy density versus logarithm of the photon energy) of AGNs shows typically a two-bump structure. For the origin of the high-frequency bump, various models have been proposed, the most popular invoking inverse Compton scattering of ambient photons. There have been several suggestions for the origin of the low-frequency seed photons that are up-scattered to γ-ray energies: they may be produced within the jet by synchrotron radiation (synchrotron self-Compton or SSC mechanism 1) or come from outside the jet (external Compton or EC mechanism 2). Relativistic effects boost the observed emission as the Doppler factor depends on the angle to the line of sight. In case the jet angle to the line of sight is large, models that depend less critically on beaming effects are needed 3. The VHE γ-ray emission of AGNs might also be of hadronic origin through the emission from secondary electrons 4. 3C 66A and 3C 66B are two AGNs separated by just 6 ′ in the sky. 3C 66B is a large Fanaroff–Riley-I-type (FRI) radio galaxy, similar to M 87, with a of 0.0215, whereas 3C 66A is a blazar with uncertain redshift. The often referred redshift of 0.444 5 for 3C 66A is based on a single measurement of one emission line only, while in later observations no lines in the spectra of 3C 66A were reported 6. Based on the marginally resolved host galaxy, a photometric redshift of ∼ 0.321 was inferred. In this paper we report the discovery of VHE γ-ray emission located 6 .′1 away from the blazar 3C 66A and coinciding with the radio galaxy 3C 66B in 2007. Detailed results and discussion can be found in 7. iue1 etpo:Sgicnempfor map Significance plot: Left 1: Figure C6Audreta pia ubrti 07Ags,a monit as August, VHE 2007 triggered in outburst The outburst optical program. an monitoring underwent 66A Analysis3C Data and Observations 2 ecie ndti in detail in described oa hrels hn10poolcrn pe nodrt a to additio order An in (phe) cleaning. photoelectrons image wh 150 after than of less rate out charge event hr, total 54.2 the on lasted 13 based and from cuts December extends and data September, the August, of distribution distance ieetsydrcin,ec t24 at each directions, sky different AI eecp olwn h agto potnt progra Opportunity VHE of new Target the following telescope MAGIC a esrdt e3 be took Gaussia the to which of sigma necessary, measured The was again. became PSF system the improved and alignment campaign mirror the (PS of function point-spread optical the worsening replaced, neeg eouinaoe10GVof GeV 150 above resolution energy an eutdi ipitn,wihwstkncr fb e poi new a by of care analysi taken the information was were starguider of which end mispointing, data the a account, at in combined into resulted were this results the take and To period December. and September ewe 5 e n e.Frtebcgon eeto loos a rejection background the For TeV. 1 and GeV 150 between iue1(etpo)sosasgicnemppoue rmth from of produced Gaussian map a significance with a smoothed shows both plot) (left 1 Figure Results 3 accu pointing the of uncertainty systematic the estimate we aaee sapidt epalrenme fgmalk eve gamma-like of number large a keep to the applied is parameter xescne fRA 2 = R.A. of center excess utbfr h tr fteosraincampaign observation the of start the before Just aawr ae nteflesuc rcig(obe oep mode (wobble) tracking false-source the in taken were Data AI a tnadtigrtrsodo 0GV naglrr angular an GeV, 60 of threshold trigger standard a has MAGIC γ

ryeiso sdrvdfo iue1 h tyed reconst yields fit The 1. Figure from derived is emission -ray DEC (deg) 42.6 42.8 43.0 43.2 43.4 43.6 02 h 26 m γ rysucsi h past. the in sources -ray 02 h 25 m 02 h 24 m 02 7 , 9 h RA (h) 8 . 23 osdrn h diinlucranycue yteofflin the by caused uncertainty additional the Considering . ′ . m n20 uut1-4 2 12-14, August 2007 in 0 ieeta nrysetu fMGCJ0223+430. MAGIC of spectrum energy Differential 02 h h 23 22 m m 02 12 h 21 3C 66B 3C 66A m s n el=43 decl.= and 02 ′ γ σ h 20 lk vnsaoe10GVi h bevdsyrgo.Rgtp Right region. sky observed the in GeV 150 above events -like itnefo h C6Actlgpsto.Tezenith The position. catalog 66A 3C the from distance m 6= ∼ 5 (see25% ′ -2 -1 0 1 2 3 4 5 crepnigt the to (corresponding Significance

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Flux -10(E>150 GeV) [cm s ] 10 42 44 46 48 40 42 54440 54420 54400 54380 54360 54340 54320 10 20 12 E 6 8 0 × d . d 10 ′ N . wyfo h aao oiino C6A hl h distance the while 66A, 3C of position catalog the from away 1 A 3 -12 d ± . t 0 1 (1= σ . 5) infiac petil ttepsto f3 6)ad5 and 66B) 3C of position the at (pre-trial, significance × . γ 7 ryflx oe ae:otcllgtcreo C6Aa measur as 66A 3C of curve light optical panel: Lower flux. -ray 10 γ ± rylgtcrei ossetwt osatflxwithin flux constant a with consistent is curve light -ray − 0 12 . 3) cm γ ryeet rmawd k ein( region sky wide a from events -ray × MAGIC J0223+430 − 10 2 3C 66A(KVA) s − − 11 1 | ( 22 fteCa euaflx n sthe is and flux) Crab the of (2.2% Alpha E/ 8 lnts h pia u f3 6 remained 66B 3C of flux optical the elengths, sw antecueta C66A 3C that exclude cannot we As . m aiblt ftesga antbe cannot signal the of variability ome ted. 0 GeV)300 nerlflxaoe10GVi iso days. 3 of bins in GeV 150 above flux integral ey sn hssuyw n that find we study this Using vely. | 6.Teoii fteemission the of origin The 66B. C tit setmtdt e3%in 35% be to estimated is rtainty e n ssoni i.1(right 1 Fig. in shown is and ded itiuinwspoue,where produced, was distribution time [MJD] ue1(ih lt ersnsa represents plot) (right 1 gure orsodn o6.% 95.4%, 68.2%, to corresponding a.Teitga u above flux integral The nal. − 3 ieeta u (TeV flux differential a s . ttsial adadding (and statistically ) 1 ± n h oreposition source the and e hteflxo C6Ain 66A 3C of flux the th 0 es. . 3 eosrce position.reconstructed et epthe keep to de ∼ 0 . 7deg07 γ dbyed -ra y . 4 (1) 2 − to ), σ 1 4 Discussion and conclusions

A new VHE γ-ray source MAGIC J0223+430 was detected in 2007 August to December. Given the position of the excess measured by MAGIC above 150 GeV, the source of the γ-rays is likely 3C 66B. The VHE γ-ray flux was found to be on the level of 2.2% Crab Nebula flux and was constant during the observations. The differential spectrum of MAGIC J0223+430 has a photon spectral index of Γ = 3 .10 ± 0.31 and extends up to ∼ 2 TeV. In view of the recent detection of 3C 66A at VHE γ-rays 10 , we note that if 3C 66A was emitting γ-rays in 2007 August to December then its flux was at a significantly lower level than in 2008. In the unlikely case, excluded with probability 85.4%, that the total signal and observed spectrum presented in this paper originates from 3C 66A, the redshift of the source is likely to be significantly lower than previously assumed due to energy-dependent absorption of VHE γ- rays with low-energy photons of the extragalactic light 7,11 . If z > 0.24 for 3C 66A, an alternative explanation for a hard intrinsic spectrum at energies above 100 GeV can be given if γ-rays are passing through a narrow band of optical-infrared photons in the vicinity of the blazar 12 . 3C 66B is a FRI radio galaxy similar to M 87, which has been detected to emit VHE γ-rays 13 . Since the distance of 3C 66B is 85.5 Mpc, its intrinsic VHE luminosity would be two to eight times higher than the one of M 87 (22.5 Mpc) given the reported variability of M 87 13 ,14 . As in the case of M 87, there would be several possibilities for the region responsible of the TeV radiation in 3C 66B: the vicinity of the , the unresolved base of the jet and the resolved jet. A possible emission scenario associated with a structured jet responsible for the observed VHE γ-ray emisson from 3C 66B is presented in 15 . Given the likely association of MAGIC J0223+430 with 3C 66B, our detection would establish radio as a new class of VHE γ-ray emitting sources (see also 16 ). Further observations of radio galaxies with the Fermi Gamma-ray Space Telescope as well as by ground-based telescopes are needed to further study the γ-ray emission properties of radio galaxies.

Acknowledgments

We thank the Instituto de Astrofisica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma.

References

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