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2Astronomy & Astrophysicsmanuscript no. on325 c© ESO 2012March 5, 2012

Discovery of VHE γ-rays from the blazar 1ES 1215+303 with theMAGIC Telescopes and simultaneous multi-wavelength

observationsJ. Aleksic1, E. A. Alvarez2, L. A. Antonelli3, P. Antoranz4, S. Ansoldi15, M. Asensio2, M. Backes5, U. Barres de

Almeida6, J. A. Barrio2, D. Bastieri7, J. Becerra Gonzalez8,31,∗, W. Bednarek9, K. Berger8,10, E. Bernardini11,A. Biland12, O. Blanch1, R. K. Bock6, A. Boller12, G. Bonnoli3, D. Borla Tridon6, T. Bretz13,27, A. Canellas14,

E. Carmona6,29, A. Carosi3, P. Colin6,∗, E. Colombo8, J. L. Contreras2, J. Cortina1, L. Cossio15, S. Covino3, P. DaVela4, F. Dazzi15,28, A. De Angelis15, G. De Caneva11, E. De Cea del Pozo16, B. De Lotto15, C. Delgado Mendez8,29,A. Diago Ortega8,10, M. Doert5, A. Domınguez17, D. Dominis Prester18, D. Dorner12, M. Doro19, D. Eisenacher13,

D. Elsaesser13, D. Ferenc18, M. V. Fonseca2, L. Font19, C. Fruck6, R. J. Garcıa Lopez8,10, M. Garczarczyk8, D. GarridoTerrats19, M. Gaug19, G. Giavitto1, N. Godinovic18, A. Gonzalez Munoz1, S. R. Gozzini11, D. Hadasch16, D. Hafner6,

A. Herrero8,10, D. Hildebrand12, J. Hose6, D. Hrupec18, B. Huber12, F. Jankowski11, T. Jogler6,30, V. Kadenius20,H. Kellermann6, S. Klepser1, T. Krahenbuhl12, J. Krause6, A. La Barbera3, D. Lelas18, E. Leonardo4,

N. Lewandowska13, E. Lindfors20,∗, S. Lombardi7,∗, M. Lopez2, R. Lopez-Coto1, A. Lopez-Oramas1, E. Lorenz6,12,M. Makariev21, G. Maneva21, N. Mankuzhiyil15, K. Mannheim13, L. Maraschi3, M. Mariotti7, M. Martınez1,D. Mazin1,6, M. Meucci4, J. M. Miranda4, R. Mirzoyan6, J. Moldon14, A. Moralejo1, P. Munar-Adrover14,

A. Niedzwiecki9, D. Nieto2, K. Nilsson20,32, N. Nowak6, R. Orito22, S. Paiano7, D. Paneque6, R. Paoletti4, S. Pardo2,J. M. Paredes14, S. Partini4, M. A. Perez-Torres1, M. Persic15,23, M. Pilia24, J. Pochon8, F. Prada17, P. G. Prada

Moroni25, E. Prandini7, I. Puerto Gimenez8, I. Puljak18, I. Reichardt1, R. Reinthal20, W. Rhode5, M. Ribo14, J. Rico26,1,S. Rugamer13, A. Saggion7, K. Saito6, T. Y. Saito6, M. Salvati3, K. Satalecka2, V. Scalzotto7, V. Scapin2, C. Schultz7,

T. Schweizer6, S. N. Shore25, A. Sillanpaa20, J. Sitarek1,9,∗, I. Snidaric18, D. Sobczynska9, F. Spanier13, S. Spiro3,V. Stamatescu1, A. Stamerra4, B. Steinke6, J. Storz13, N. Strah5, S. Sun6, T. Suric18, L. Takalo20, H. Takami6,

F. Tavecchio3, P. Temnikov21, T. Terzic18, D. Tescaro8, M. Teshima6, O. Tibolla13, D. F. Torres26,16, A. Treves24,M. Uellenbeck5, P. Vogler12, R. M. Wagner6, Q. Weitzel12, V. Zabalza14, F. Zandanel17, R. Zanin14 (the MAGICCollaboration), A. Berdyugin20,32, S. Buson7, E. Jarvela33, S. Larsson34,35,36, A. Lahteenmaki33, and J. Tammi33

(Affiliations can be found after the references)

ABSTRACT

Context. We present the discovery of very high energy (VHE, E> 100 GeV)γ-ray emission from the BL Lac object 1ES 1215+303 by the MAGICtelescopes and simultaneous multi-wavelength data in a broad energy range from radio toγ-rays.Aims. We study the VHEγ-ray emission from 1ES 1215+303 and its relation to the emissions in other wavelengths.Methods. Triggered by an optical outburst, MAGIC observed the sourcein January-February 2011 for 20.3 hrs. The target was monitored in theoptical R-band by the KVA telescope that also performed optical polarization measurements. We triggered target of opportunity observations withthe Swift satellite and obtained simultaneous and quasi-simultaneous data from theFermi Large Area Telescope and from the Metsahovi radiotelescope. We also present the analysis of older MAGIC data taken in 2010.Results. The MAGIC observations of 1ES 1215+303 carried out in January-February 2011 resulted in the first detection of the source at VHEwith a statistical significance of 9.4σ. Simultaneously, the source was observed in a high optical and X-ray state. In 2010 the source was observedin a lower state in optical, X-ray, and VHE, while the GeVγ-ray flux and the radio flux were comparable in 2010 and 2011. The spectral energydistribution obtained with the 2011 data can be modeled witha simple one zone SSC model, but it requires extreme values for the Doppler factoror the electron energy distribution.

Key words. Gamma rays:galaxies–BL Lacertae objects:individual: 1ES1215+303

1. Introduction

Most of the extragalactic sources from which very high en-ergy (VHE,>100 GeV)γ-ray emissions have been detected areblazars. These objects are commonly believed to be a subtypeof active galactic nuclei (AGN) whose relativistic jet points veryclose to the line of sight of the observer. Blazars are character-ized by a large variability in all wavebands from radio toγ-rays.

The correlations between the different energy bands are com-plicated, but in general it seems that high states in lower en-ergy bands (e.g. optical) are accompanied with high states in thehigher energies (i.e.γ-rays) at least in some sources (see e.g.Reinthal et al., 2011).

The spectral energy distribution (SED) of blazars exhibitsageneric two-bump structure: one peak with a maximum in radioto X-ray band and a second peak located in X-ray toγ-ray bands.

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J. Aleksic et al.: 1ES 1215+303

The radiation is produced in a highly beamed plasma jet and thedouble peaked SED is can often be explained by a single popula-tion of relativistic electrons. The first peak is due to synchrotronemission in the magnetic field of the jet and the second peak iscaused by inverse Compton (IC) scattering of low-energy pho-tons (Rees, 1967). The low-energy photons can be external tothejet (external Compton scattering, Dermer & Schlickeiser, 1993)or are produced within the jet via synchrotron radiation (syn-chrotron self-Compton scattering, SSC, Maraschi et al., 1992).

Blazar is a common term used for Flat Spectrum RadioQuasars (FSRQs) and BL Lac objects (BL Lacs), which arethought to be intrinsically different. The FSRQs show broademission lines in their optical spectra while the BL Lacs havefeatureless spectra with weak or no emission lines possiblymasked by a strong emission from the jet. This indicatesthat in BL Lac objects the main population of seed photonsfor Compton scattering should originate from the synchrotronemission. Indeed most of the SEDs of BL Lacs are well de-scribed with simple SSC model (e.g. Bloom & Marscher, 1996;Tavecchio et al., 1998).

MAGIC has been successfully performing optically trig-gered VHEγ-ray observations of AGN since the beginning of itsoperations. The triggers have been provided by the Tuorla blazarmonitoring program1 and the target of opportunity (ToO) obser-vations with MAGIC have up to now resulted in the discoveryof five new VHEγ-ray emitting sources (Mrk 180, Albert et al.(2006b); 1ES 1011+496, Albert et al. (2007); S5 0716+714,Anderhub et al. (2009); B3 2247+381, Aleksic et al. (2012a);and 1ES 1215+303, this paper). However, in many cases it hasnot been possible to confirm with high statistical significance ifthe sources were in higher VHEγ-ray state than usually dur-ing the observations. Also, the long-term studies of individ-ual VHE γ-ray blazars like Mrk 421 (Acciari et al., 2011) andPG 1553+113 (Aleksic et al., 2012b) have yielded controversialresults about the correlation between the two energy regimes.Therefore the connection between the optical and VHEγ-raystates has remained an open question.

1ES 1215+303 (also known as ON 325) is a high syn-chrotron peaking BL Lac object (Abdo et al., 2010b) with red-shift z = 0.130 (Akiyama et al., 2003, however, alsoz = 0.237 isreported in the literature, e.g. NED2). The source was classifiedas a promising candidate TeV blazar (Costamante & Ghisellini,2002; Tavecchio et al., 2010) and has been observed severaltimes in VHEγ-rays prior to the observations presented here.The previous observations have yielded only upper limits,Whipple: F (> 430GeV)< 1.89× 10−11 cm−2 s−1, (Horan et al.,2004); MAGIC: F (> 120GeV) < 3.5 × 10−11 cm−2 s−1,(Aleksic et al., 2011a). The source was also in theFermi LargeArea Telescope (LAT) bright AGN catalog (Abdo et al., 2009)showing a hard spectrum (Γ = 1.89 ± 0.06). The sourceunderwent a large outburst in late 2008, and in this catalog1ES 1215+303 was the only high energy peaking source thatshows significant variability. In the secondFermi-LAT AGN cat-alog (Ackermann et al., 2011) other high synchrotron peakingsources have also been flagged as variable.

In the first days of January 2011 1ES 1215+303 was ob-served to be in a high optical state. This triggered MAGIC ob-servations of the source, which were carried out until February2011. These observations resulted in the discovery of VHEγ-rays from the source (Mariotti, 2011). In this paper we presentthe results of the January-February 2011 observations. We also

1 http://users.utu.fi/kani/1m/2 http://nedwww.ipac.caltech.edu/

present the previous observations of 1ES 1215+303 with theMAGIC telescopes performed in January-February 2010 andMay-June 2010 that resulted only in a hint of signal. For allepochs we present simultaneous and quasi-simultaneous multi-wavelength data from radio, optical, X-ray, and GeVγ-rays.

2. Observations and Data Analysis

The observations of 1ES 1215+303 were performed in a broadwavelength range (from radio to VHEγ-rays) by 5 different in-struments. This is the first time that such a broad wavelengthrange is covered for this source in quasi-simultaneous observa-tions.

2.1. MAGIC

MAGIC consists of two 17 m Imaging Air CherenkovTelescopes (IACTs) sensitive toγ-rays with energy above50 GeV in standard trigger mode (which is the lowest trigger en-ergy threshold among the existing IACTs). The system is locatedin the Canary Island of La Palma, 2200 m above sea level. Sincefall 2009 the telescopes are working together in stereoscopicmode which ensures a sensitivity of< 0.8% of the Crab Nebulaflux above 300 GeV in 50 hrs of observations (Aleksic et al.,2012c). The field of view of the each MAGIC camera has a di-ameter of 3.5◦.

1ES 1215+303 was observed by MAGIC in January-February 2010, May-June 2010, and January-February 2011, fora total of 48 hrs. The observations were done in the so-calledwobble mode (i.e. with the source offset by 0.4◦ from the cam-era center), which allows a simultaneous estimation of the back-ground from the same data set (Fomin et al., 1994). While mostof the data were taken in dark night conditions, a small amountwere taken in presence of moderate moon light. The data span azenith angle range from 1◦ to 40◦ with most of the data taken be-low 25◦ (in 2010 the mean zenith angle was∼ 13◦, and in 2011∼ 8◦).

The data were analyzed using the standard MAGIC softwareand analysis package (Aleksic et al., 2012c). Another VHEγ-ray emitter, 1ES 1218+304 (Albert et al., 2006a) is present in thesame field of view as 1ES 1215+303. The sources are separatedby ∼ 0.8◦, which is much larger than the point spread function(PSF) of the MAGIC telescopes (∼ 0.1◦), thus source confusionor contamination are excluded. On the other hand, since bothsources have nearly the same Right Ascension, in the standardwobble setup used in the January-February 2010 observations,the background estimation region partially overlapped with the1ES 1218+304 position. This would result in an overestimationof the background, thus this region was excluded from the back-ground estimation. In the later observations (May-June 2010,and January-February 2011) the wobbling offset direction wasmodified in order to have the standard background estimationregions far away from the second source.

After the data quality selection, mainly based on the rate ofstereo events, the data samples of January-February 2010, May-June 2010 and January-February 2011 contain 19.4, 3.5, and20.6 hrs of good quality data respectively. Because of differentpositions of the source in the camera, and the variable nature ofAGN, we decided to split the analysis into these 3 periods.

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J. Aleksic et al.: 1ES 1215+303

2.2. Fermi-LAT

TheFermi-LAT is a pair conversion telescope designed to coverthe energy band from 20 MeV to greater than 300 GeV. It oper-ates in all-sky survey mode and therefore can provide observa-tions of 1ES 1215+303 simultaneous to MAGIC. In this paperthe standard LAT Science Tools (version v9r23p0) were used toanalyze the data collected in the time interval from August 5th,2008 to March 22th, 2011. For this analysis, only events belong-ing to the “Diffuse” class (which have the highest probability ofbeing photons) and located in a circular Region Of Interest (ROI)of 7◦ radius, centered at the position of 1ES 1215+303, were se-lected (using Pass 6 event selection). In addition, we applied acut on the zenith angle (< 100◦) limb γ-rays and a cut on therocking angle (> 52◦) to limit contamination from Earth Limb.

The data analysis of 1ES 1215+303 is very challenging dueto the existence of severalγ-ray emitting sources in the sameROI. 1ES 1218+304 is located at a distance of just 0.8◦ fromthe source of interest. Furthermore, another well known VHEemitter, W Comae, is located at∼ 2◦ from the latter source.For this reason the LAT analysis was restricted to energiesabove 1 GeV where theFermi-LAT PSF is sufficient3 to sep-arate 1ES 1215+303 from the other sources in the ROI. Theunbinned likelihood method was applied to events in the en-ergy range from 1 GeV to 300 GeV. All point sources from the2FGL catalog (Nolan & (the Fermi-LAT Collaboration), 2012)located within 12◦ of 1ES 1215+303 were included in the modelof the region. Sources located within a 5◦ radius centered on1ES 1215+303 position had flux and photon index left as freeparameters. The diffuse galactic and isotropic components (in-cluding residual instrumental background) were modeled withthe publicly available filesgll iem v02 P6 V11 DIFFUSE.fitand isotropiciem v02 P6 V11 DIFFUSE.txt 4. The normaliza-tions of the components comprising the total background modelwere allowed to vary freely during the spectral point fitting.The instrument response functionsP6 V11 DIFFUSE were used.The successful separation of flux between 1ES 1215+303 and1ES 1218+304 was verified by the absence of any significantcorrelation between their light curves. The systematic uncer-tainty in the flux is estimated as 5% at 560 MeV and 20% at10 GeV and above5.

2.3. Swift

The Swift satellite (Gehrels et al., 2004) is equipped with threetelescopes, the Burst Alert Telescope (BAT; Barthelmy et al.,2005) covering the 15-150keV energy range, the X-ray tele-scope (XRT; Burrows et al., 2005) covering the 0.2-10keV en-ergy band, and the UV/Optical Telescope (UVOT; Roming et al.,2005) covering the 180–600nm wavelength range.

A Swift ToO request was submitted on January 3rd, 2011.The Swift observations started on January 4th until January12th with four ∼ 5 ks exposures in photon counting mode.The data were processed with standard procedures using theFTOOLS task XRTPIPELINE (version 0.12.6) distributed byHEASARC within the HEASoft package (v.6.10). Events withgrades 0–12 were selected for the data (see Burrows et al., 2005)and the latest response matrices available in theSwift CALDB(v.20100802) were used. For the spectral analysis, we extracted

3 http://www.slac.stanford.edu/exp/glast/groups/canda/archive/pass6v11/lat Performance.htm

4 http://fermi.gsfc.nasa.gov/ssc/data/access/lat/BackgroundModels.html5 http://fermi.gsfc.nasa.gov/ssc/data/analysis/LAT caveats.html

the source events in the 0.3-10keV range within a circle witharadius of 20 pixels (∼ 47 arcsec). The background was extractedfrom an off-source circular region with a radius of 40 pixels.

The spectra were extracted from the corresponding eventfiles and binned using GRPPHA to ensure a minimum of 25counts per energy bin, in order to get a reliableχ2 statistics.Spectral analysis was performed using XSPEC version 12.6.0.The absorption hydrogen-equivalentcolumn density was fixed tothe Galactic value in the direction of the source 1.74· 1020 cm−2

(Kalberla et al., 2005).Swift/UVOT observed the source with all filters (V, B, U,

UVW1, UVM2, UVW2) for four nights. UVOT source countswere extracted from a circular region 5 arcsec-sized centered onthe source position, while the background was extracted from alarger circular nearby source–free region. These data werepro-cessed with theuvotmaghist task of the HEASOFT package.The observed magnitudes have been corrected for Galactic ex-tinction EB−V = 0.024 mag (Schlegel et al., 1998), applying theformulae by Pei (1992) and finally converted into fluxes follow-ing Poole et al. (2008).

2.4. KVA

The KVA optical telescopes are located in La Palma, but areoperated remotely from Finland. The two telescopes are attachedto the same fork. The larger telescope has a mirror diameter of60 cm and the smaller 35 cm.

The 35 cm telescope is used for simultaneous photomet-ric observations with MAGIC, but also to monitor potentialVHE γ-ray candidate AGN in order to trigger MAGIC obser-vations if the sources are in high optical states. The observa-tions are performed in the R-band and the magnitude of thesource is measured from CCD images using differential pho-tometry. The comparison star magnitudes for 1ES 1215+303are from Fiorucci & Tosti (1996), and the magnitudes are con-verted to flux using the formula and values from Bessell (1979).1ES 1215+303 has been part of the Tuorla blazar monitoringprogram from its beginning and has been observed regularlysince 2002.

The 60 cm telescope is used for polarimetric observa-tions (see e.g. Piirola et al., 2005; Aleksic et al., 2011b). For1ES 1215+303 polarimetric observations were performed on 6nights from January 7th, 2011 to January 17th, 2011. The de-gree of polarization and position angle were calculated from theintensity ratios of the ordinary and extraordinary beams, usingstandard formulae and semiautomatic software specially devel-oped for polarization monitoring purposes.

2.5. Metsahovi Radio Telescope

37 GHz radio observations were made with the 13.7 mMetsahovi radio telescope located in Kylmala, Finland.The tele-scope, the observation methods, and the data analysis procedureare described in e.g. Terasranta et al. (1998). The telescope de-tection limit is∼ 0.2 Jy under optimal conditions and given that1ES 1215+303 is a rather weak source at 37 GHz it can only beobserved under good weather conditions. Typically, an accept-able measurement of the source is obtained approximately oncea month. Data simultaneous to MAGIC observations were ob-tained in June 2010, but in January-February 2011 the weatherdid not allow simultaneous observations with MAGIC, the clos-est points being from December 2010 and March 2011.

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3. Results

3.1. MAGIC results

MAGIC data were divided in three samples corresponding tothree observation epochs: January-February 2010, May-June2010, and January-February 2011. The so-calledθ2 plots (thedistribution of the squared angular distance between the arrivaldirection of the events and the real position), for energiesabove300 GeV, corresponding to the three observation epochs, areshown in Fig. 1. The computation of the number of the ON (sig-nal) and OFF (background) events is performed in a fiducial sig-nal region ofθ2 < 0.01 deg2, and using 5 background regions (4in case of the January-February 2010 data). In January-February2010 (left panel) 194 ON events were detected over 144.6± 6.0OFF events, resulting in a significance level of 3.5σ (using Eq.17 in Li & Ma, 1983). In May-June 2010 (middle panel) theobservation time was much shorter and no excess events werepresent. For 2011 data (right panel) the numbers are 251 ONover 119±4.8 OFF corresponding to∼9.4σ significance, whichrepresents the first detection of VHEγ-rays from this source.

In Fig. 2, we show the significance map of the sky regionabove 300 GeV for the 2010 (January-February and May-Junecombined) and 2011 observations. 1ES 1218+304 is clearly vis-ible in both maps while 1ES 1215+303 is fainter in 2010 than2011. 1ES 1218+304 data analysis and results will be addressedin a separate paper.

For light curve and spectrum softer cuts that have a higherγ-ray efficiency were used. The light curve (in a 5-days bins)above 200 GeV of the 2011 data is well described by a constantflux of (7.7± 0.9) × 10−12 cm−2s−1 (χ2/ndof= 0.56 / 3), whichcorresponds to about 3.5% of the Crab Nebula flux. Assumingthat the hint of signal seen in the 2010 data is aγ-ray excessthe corresponding flux was calculated also for 2010 data, F (>200 GeV)= (3.4 ± 1.0) × 10−12 cm−2s−1, which is over a factortwo below the flux measured in 2011. The hypothesis of constantflux between 2010 and 2011 is excluded at the level of 3.1σ.

The derived VHEγ-ray spectrum for 2011 observations canbe described by a simple power law (χ2/ndof = 5.2/3, see Fig. 3):

dNdE= (2.27±0.25)×10−11

( E300GeV

)(−2.96±0.14)TeV−1cm−2s−1(1)

in the fitting range 70 GeV – 1.8 TeV. Since the spectral indexof 1ES 1215+303 is similar to that of the Crab Nebula and thesource is relatively bright we can directly use the systematic er-rors estimated in Aleksic et al. (2012c). The systematic error ofthe slope is±0.15 and at the energy range of the 1ES 1215+303spectrum, the error in the flux normalization (without the energyscale uncertainty) was estimated to be 11%. The systematic errorin the energy scale is 15%. Finally, the MAGIC spectrum was de-absorbed using different EBL models (Domınguez et al., 2011;Kneiske & Dole, 2010; Franceschini et al., 2008; Primack et al.,2005) and the maximum high UV EBL model described inAlbert et al. (2008) forz = 0.130. The results are shown inFig. 3.

3.2. Fermi-LAT results

The light curve of 1ES 1215+303 was derived between 1 and100 GeV, in 14–day bins from August 2008 to March 2011(Fig. 4). It shows the major flare reported in Abdo et al. (2009)in the beginning of the Fermi mission. There is a hint of en-hanced flux in November 2010 (MJD 55500, duration only onebin, i.e. 14 days) but very little variability outside that.Especially

in the two MAGIC observation epochs (January-June 2010 andJanuary-February 2011) the flux varies very little. In ordertomaximize the number of photons the spectral energy distributionis derived from the whole MAGIC observation epochs (January-June 2010 and January-February 2011). The spectral energy dis-tributions are shown in Fig. 5. In January-June 2010 the inte-gral flux, F (1− 100 GeV) is (4.9 ± 0.7) × 10−9 cm−2s−1 andthe photon index 2.1 ± 0.1, while in January-February 2011F (1 − 100 GeV)= (7.3 ± 1.6) × 10−9 cm−2s−1 and the photonindex 2.0 ± 0.2. The mean flux is measured∼ 50% higher inJanuary-February 2011 than in January-June 2010, althoughdueto large error bars the increase is not statistically significant. Thespectral index is constant within the error bars.

3.3. Swift results

The results of theSwift/XRT observations are summarizedin Table 1. The source showed the highest flux on January8th, 2011 (MJD 55569.1) and previous/subsequent observa-tions from December 2009 (MJD 55168.7)/ April 2011 (MJD55674.2) show significantly lower flux. For the X-ray spectraboth log parabola (in the form∼ E−a−b∗log(E), with E being theenergy in keV) and a simple power-law fit were tested. The bestfit was acchieved with a log parabola law model in the range0.3–10 keV for four observations while a simple power law, inthe range 0.5–10 keV, provided a better fit for three of the obser-vations. Generally, a log parabola fit suggests that there iscurva-ture in the X-ray spectra, but in the case of 1ES 1215+303, thedifference between log parabola and power law fit is small andtherefore no strong conclusions can be drawn. Due to differentfits the comparison between the spectral slopes is difficult, butfor the highest flux night the spectral index is marginally harderthan for low state observations.

The Swift/UVOT results from January 2011 ToO observa-tions show constant brightness with V-band magnitude= 15.06±0.10, B= 15.38± 0.10, U= 14.53± 0.08, UVW1=14.43± 0.08,UVM2=14.35± 0.06, and UVW2=14.46± 0.06. However, inall bands the source is clearly brighter than in the previousob-servation (December 2009: V= 15.60± 0.10, B= 15.95± 0.10,U= 15.12± 0.08, UVW1= 15.07± 0.08, UVM2= 15.00± 0.06,and UVW2= 15.15± 0.06).

3.4. KVA and Metsahovi results

In the optical R-band the source is clearly variable on dailyandyearly time-scales. The host galaxy contributes to the flux with0.99± 0.09 mJy (Nilsson et al., 2007) and when this contribu-tion was subtracted from the measured flux, the AGN core wasfound to be∼ 40% brighter in January-February 2011 (aver-age total flux 3.64 mJy) than in January-June 2010 (average totalflux 2.55 mJy). Similarly, it was found that within the January-February 2011 observations the flux varied by∼ 25% (core fluxbetween 3.2 mJy and 4.1 mJy).

During January 2011 the optical polarization degree was∼9% while the follow up observation in April 2011 revealed ahigher degree of∼ 15%. The polarization angle (PA) was onlyslightly variable between 140 and 150 degrees.

In the radio band the source is rather weak and does notshow strong variability. The 37 GHz flux from the Metsahoviradio telescope has a similar level (0.3-0.4Jy) both in 2010and2011, although there is unfortunately no radio observationfromJanuary-February 2011.

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J. Aleksic et al.: 1ES 1215+303

]2 [ deg2θ0 0.05 0.1 0.15 0.2 0.25

even

tsN

0

20

40

60

80

100

120Time = 18.5 h

6.0± = 144.6 off

= 194; NonN

15.2± = 49.4 exN

σSignificance (Li&Ma) = 3.5

]2 [ deg2θ0 0.05 0.1 0.15 0.2 0.25

even

tsN

0

5

10

15

20

25

30

35 Time = 3.5 h 2.5± = 32.1

off = 29; NonN

5.9± = -3.1 exN

σSignificance (Li&Ma) = -0.5

]2 [ deg2θ0 0.05 0.1 0.15 0.2 0.25

even

tsN

0

20

40

60

80

100

120

140

160

180Time = 20.3 h

4.8± = 119.0 off

= 251; NonN

16.5± = 132.0 exN

σSignificance (Li&Ma) = 9.4

Fig. 1. Distributions of theθ2 parameter for 1ES 1215+303 signal (black histograms) and background estimation (gray histograms)for the three observation periods: January-February 2010 (left), May-June 2010 (middle), and January-February 2011 (right). Thevertical dashed line corresponds to the apriori defined signal regionθ2 < 0.01 deg2.

MJD Obs. Time [ks] Flux (2-10 keV) [10−12 erg/cm2/s] a (Γ for PL) b χ2red/ ndof

55168.6799 4.99 1.21± 0.19 2.56± 0.10 0.34± 0.34 1.19/2555565.0340 4.39 2.74± 0.25 2.41± 0.08 0.37± 0.24 0.87/4255569.1281 2.38 3.02± 0.40 2.29± 0.16 – 1.23/18 (PL)55571.1327 4.07 1.69± 0.17 2.65± 0.14 – 1.23/18 (PL)55572.1361 4.27 1.45± 0.20 2.64± 0.09 0.28± 0.27 1.15/3255573.1396 2.99 1.73± 0.25 2.46± 0.11 0.66± 0.37 1.26/2555674.2438 2.34 1.30± 0.30 2.67± 0.25 – 0.48/8 (PL)

Table 1. Data summary and results for theSwift/XRT ToO observations. The datasets in the first/last rows are prior/subsequent tothe MAGIC observations and are reported for comparison. Foreach dataset the following quantities are reported: the MJDtimeof the beginning of the observations; the exposure time; theintegral flux in the 2-10 keV band; thea andb parameters for the logparabola fit (or the photon indexΓ in case a simple power-law is used, see text); the reducedχ2 with number of degrees of freedomndof. PL indicates when the simple power law is used instead of thelog parabola.

4. Interpretation

The quasi-simultaneous light curves are studied to establish con-nections between different energy regimes and locate the emis-sion region. The spectral energy distribution is reconstructedfor the first time from radio frequencies to TeV energies for1ES 1215+303, allowing us to study the capability of the one-zone synchrotron self Compton model to reproduce the con-structed SED.

4.1. Multi-wavelength behavior

The long-term multi-wavelength light curve from radio to VHEγ-rays is shown in Fig. 6. The MAGIC light curve shows a lowerflux in 2010 (January-February and May-June) than in 2011 (bya factor of 2). The large uncertainty of theFermi-LAT measure-ment do not allow to draw a conclusion if a similar flux enhance-ment occurred also in the 1−100 GeV energy range (see section3.2). In X-rays the source was in a high state (by a factor of 2)in January 2011 compared to previous observations. In the opti-cal band the average flux during MAGIC observations in 2010 is3.5 mJy, while in 2011 it is 4.6 mJy. So, the source was clearlyundergoing an outburst in early 2011 at least in VHEγ-rays, X-rays, and the optical band. In the radio band simultaneous obser-vations are missing, but both the previous and consequential ob-servations show low flux, suggesting that the outburst mighthavetaken place rather close to the central engine where the emissionregion is opaque for radio emission.

Within the January-February 2011 observations (Fig. 7), theMAGIC light curve is consistent with a constant flux. The sourcewas in a rather low state in theFermi-LAT energy range andno short term variability is detected. In X-rays and opticalthesource is variable within the MAGIC observation period: thefirst

two X-ray exposures show clearly a higher flux than the latterthree. The X-ray spectra show hints of hardening with higherflux, they are however compatible within the error. The MAGICobservations started when the optical flux was decreasing, butduring January 2011 the optical light curve showed several smallflares. In the nightly time-scale the X-ray and optical lightcurvesseem to follow the same pattern confirming that they originatefrom the same emission region.

In addition to multi-wavelength light curves, also the op-tical polarization measurements have proven to be a powerfultool to analyze the emission scenarios in the blazar jets (e.g.Marscher et al., 2008). The polarization traces the magnetic fieldof the jet. A net polarization oriented either parallel or perpen-dicular to the projected jet axis can be confused with shockwaves and the signatures are visible in optical polarization. Theoptical polarization measurements from January 2011 show onlylittle variability in polarization degree (average∼ 9%) or PA(varying between∼ 140◦ - 150◦) during the MAGIC observa-tions, but the follow-up observations from April 2011 (Fig.6)show a higher polarization degree of∼ 15%. Unfortunately,the polarization measurements missed the peak of the first op-tical outburst and our data sample is very small. Ikejiri et al.(2011) monitored the photo-polarimetric behavior of the sourcein 2008-2009 and their observations seem to show similar polar-ization trends (i.e. a decreasing polarization degree during out-bursts). They also find that the PA is almost constant at∼ 150◦,which agrees with our observations and with the historical datafrom 1981-1989 (Wills et al., 2011) showing PA values from∼ 130− 170◦. Such preferred polarization position angles havebeen observed for several BL Lac objects (e.g. Jannuzi et al.,1994) and implies long-term stability of the geometry of theregion producing the polarized emission e.g. the existenceofa optical polarization core. In first order, if the optical out-

5

J. Aleksic et al.: 1ES 1215+303

Fig. 2. Significance maps (> 300 GeV) from MAGIC observa-tions performed during January-February 2010 and May-June2010 (combined together, total time 22.0 hrs, top) and January-February 2011 (total time 20.3 hrs, bottom).

Energy, E [GeV]70 100 200 300 400 500 1000 2000

]-1

s-2

cm

-1 [T

eVdE

dA

dt

dN

-1310

-1210

-1110

-1010

-910

z=0.13

Observed spectrum

Deabsorbed spectrum

EBL models spread

Fig. 3. Observed and deabsorped VHEγ-ray spectra for a red-shift of 0.130. The EBL model of Domınguez et al. (2011) wasused, the blue area shows the spread of the EBL models.

burst was produced by a shock traveling in the jet, one wouldexpect the polarization degree to increase during the outburst.However, if a standing shock component (optical polarizationcore) is present, a shock with a different magnetic field orien-tation colliding with the standing component could produceanoutburst in the total flux, but decrease the observed polariza-tion degree (Villforth et al., 2010). A detailed photo-polarimetric

1

2

3

4

5

6

54800 55000 55200 55400 55600

30 Nov 08 18 Jun 09 04 Jan 10 23 Jul 10 08 Feb 11

Flu

x [1

0-8 p

h/cm

2 /s]

Time[MJD]

Time[date]

Fermi (1-100 GeV)

Fig. 4. Long-term light curve of 1ES 1215+303 fromFermi-LATbetween 1 and 100 GeV.

Energy (GeV)1 10 210

)-1

s-2

dN/d

E (

erg

cm2

E

-1310

-1210

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-1010

Energy (GeV)1 10 210

)-1

s-2

dN/d

E (

erg

cm2

E

-1310

-1210

-1110

-1010

Fig. 5. Spectral energy distribution fromFermi-LAT derived forJanuary-June 2010 (top) and January-February 2011 (bottom).The upper limits have been computed when the test statistics(seee.g. Mattox et al., 1996) in the energy band were lower than 4.The bow-ties are derived from the unbinned likelihood analysis.

study based on more data would be needed to further test this hy-pothesis.

4.2. Spectral Energy Distribution

The SED of 1ES 1215+303 in both MAGIC observation epochsis shown in Fig. 8. The 2011 high energy bump is constructedusing MAGIC deabsorbed spectrum (using the EBL model ofDomınguez et al., 2011) and the simultaneousFermi-LAT spec-trum (collecting all photons from January-February 2011).Asstated in section 4.1, the low energy bump is variable duringthe period and is constructed for the night MJD 55569 which

6

J. Aleksic et al.: 1ES 1215+303

0.3

0.6

0.9

55200 55300 55400 55500 55600 55700

Flu

x [J

y]

Time[MJD]

f) Metsähovi 37 GHz

0

5

10

15

140

145

150

155

160

P[%

]

PA

[deg

rees

]e) KVA Polarization

3

4

5

6

7

Flu

x [m

Jy] d) KVA R−band

1

2

3

Flu

x [1

0−12

erg

/cm

2 /s]

c) Swift/XRT (2−10keV)

1

2

3

F

lux

[10−

8 ph/

cm2 /s

]

b) Fermi (1−100 GeV)

0 2 4 6 8

10 12

04 Jan 10 14 Apr 10 23 Jul 10 31 Oct 10 08 Feb 11 19 May 11

Flu

x [1

0−12

/cm

2 /s]

Time[date]

a) MAGIC(>200 GeV)

Fig. 6. Long-term multi-wavelength light curve of1ES 1215+303 from December 2009 to May 2011. Thevertical line shows the beginning of the MAGIC 2011 obser-vation campaign.a) In the MAGIC light curve 2011 data arebinned in 5-day intervals. 2010 data are divided in January-February and May-June bins.b) The Fermi-LAT light curve(1 − 100GeV) has bins of 14 days and the points with arrowsare upper limits.c) The Swift/XRT light curve is derived fromthe target of opportunity observations performed during theMAGIC observations and archival data.d) The R-band lightcurve shows hourly average flux of the source, the error barsare in most cases smaller than the symbols.e) The opticalpolarization degree (filled circles, left axis) and polarizationangle (triangles, right axis) are hourly averages.f) 37 GHz radiolight curve from the Metsahovi radio observatory.

presents the highestSwift flux and has simultaneous KVA andUVOT observations. The contribution of the host galaxy is sub-tracted from the R-band flux following Nilsson et al. (2007).Thehost galaxy contributes also in V, B, and U bands of the UVOTdata, but its contribution should be negligible in the UV-bands.As we have no direct measurements of the host galaxy contribu-tion in V, B and U bands we extrapolated the magnitudes fromthe R-band value using the galaxy colors of elliptical galaxies atz = 0.2 (Fukugita et al., 1995).

As for 2010 MAGIC data the low significance of the sig-nal did not allow us to derive spectra, we report the flux be-tween 300 GeV and 1 TeV (assuming the same spectral index asin 2011). The simultaneousFermi-LAT spectrum is calculatedin the whole period between January and June 2010. There is nosimultaneous X-ray observation, while in optical we use theav-erage (host galaxy subtracted) flux from nights when MAGICobserved. This “low state SED” is determined for illustrativepurposes only and it is not modeled, as both synchrotron andIC peaks are poorly constrained.

0

5

10

15

55565 55570 55575 55580 55585 55590 55595 55600

140

145

150

155

160

P[%

]

PA

[deg

rees

]

Time [MJD]

f) KVA Polarization

4

5

6

7

Flu

x [m

Jy]

e) KVA R−band

14

15

16

mag

nitu

de

d) UVOT

VB

U+UV

1

2

3

4

Flu

x [1

0−12

erg

/cm

2 /s]

c) Swift/XRT (2−10keV)

1

2

F

lux

[10−

8 ph/

cm2 /s

]

b) Fermi (1−100 GeV)

0 2 4 6 8

10 12 14

04 Jan 11 09 Jan 11 14 Jan 11 19 Jan 11 24 Jan 11 29 Jan 11 03 Feb 11 08 Feb 11

Flu

x [1

0−12

/cm

2 /s]

Time[date]

a) MAGIC(>200 GeV)

Fig. 7. Multi-wavelength light curve of 1ES 1215+303 fromJanuary to February 2011.a) In the MAGIC light curve, the 2011data are binned in 5-day intervals.b) TheFermi-LAT light curve(1− 100 GeV) has bins of 14 days.c) Swift/XRT light curve.d)UVOT optical and UV light curves.e) The R-band light curveshows hourly average flux of the source, the error bars are inmost cases smaller than the symbols.f) The optical polarizationdegree (filled circles, left axis) and polarization angle (triangles,right axis) are hourly averages.

The SED of 2011 shows two peaks, with the synchrotronpeak frequency slightly above the optical band, like for manyother VHEγ-ray emitting BL Lac objects. The X-ray spectralindex is also typical for a BL Lac source. The second peakseems to be located between theFermi-LAT and MAGIC points(∼1 GeV) like in many of the VHEγ-ray emitting BL Lacs. Thelocations of the synchrotron and IC peaks are in agreement withvalues derived in Abdo et al. (2010a) for this source, but thesyn-chrotron peak luminosity is slightly higher than in the previousobservation by Giommi et al. (2012).

The emission of BL Lac objects is generally well repro-duced by the one-zone leptonic model, in which a populationof relativistic electrons inside a region moving down the jet emitthrough synchrotron and synchrotron self-Compton mechanisms(Bloom & Marscher, 1996; Tavecchio et al., 1998). The spec-tral energy distribution in 2011 is modelled with the one-zoneleptonic model fully described in Maraschi & Tavecchio (2003).The emission region is assumed to be spherical, with radiusR,filled with a tangled magnetic field of intensityB and relativisticelectrons, emitting synchrotron and synchrotron self-Comptonradiation. The electrons follow a smoothed broken power lawenergy distribution with normalizationK betweenγmin andγmax,with slopesn1 andn2 below and above the break atγb. The rel-ativistic boosting is fully accounted for by the Doppler factor δ.We recall that one-zone models cannot reproduce the emissionat the smallest frequencies, since the emission is self-absorbed

7

J. Aleksic et al.: 1ES 1215+303

Fig. 8. Spectral energy distribution of January-February 2011data (red symbols) modeled with the one-zone SSC model ofMaraschi & Tavecchio (2003). From high to low energies: thedeabsorbed MAGIC spectra (asterisk, see text), theFermi-LATdata (filled squares), Swift and UVOT data (triangles: MJD55569, blue for Swift MJD 55565) and simultaneous KVA data(squares, host galaxy subtracted, see text). The cyan symbols andline report the January-June 2010Fermi-LAT (filled circles) andMAGIC (line) data. The green open squares represents historicaldata. The dashed line is the model fit using the extreme Dopplerfactor δ = 60, while the solid line is the model fit with highγmin and the long dashed line reports the model parameters thatresults with smallestχ2 (see text and Table 2).

below the millimeter band. It is generally assumed that thispartof the SED is due to outer regions of the jet, not important forthe modeling of the high-energy emission.

The optical-UV and X-ray data define a narrow synchrotroncomponent peaking around 1015 Hz. At high energies, the SSCbump is well constrained by theFermi-LAT and MAGIC datato peak at about 10 GeV. This particular structure of the SEDis not easy to reproduce. In particular, the relatively widesep-aration between the two peaks inevitably implies a large valueof the Doppler factor if standard parameters are used for theelectron energy distribution (e.g. Georganopoulos & Kazanas,2003; Tavecchio & Ghisellini, 2008). Our best attempt to repro-duce the data in the standard framework provides the parametersreported in Table 2 and is shown by the dashed line of Fig.8.As expected from the discussion above, a large value of theDoppler factor,δ = 60, is derived, well above the typical rangeof Doppler factors obtained from the modeling of the emissionof similar sources (e.g. Tavecchio et al., 2010) and in conflictwith the smaller values required by the FR I-BL Lac6 unificationscheme (Urry & Padovani, 1995). However, a viable possibilityto reproduce the observed SED using smaller Doppler factorsexists if a relatively large minimum Lorentz factor of the emit-ting electrons,γmin = 8 × 103, is assumed. This, coupled witha steep electron distribution at high energies (n2 = 4.85), al-

6 Fanaroff-Riley I radio galaxies (FR I)

lows us to properly reproduce the narrow synchrotron bump andto locate the SSC peak at high enough energies using a moder-ately large boosting,δ = 30. This solution resembles that dis-cussed for the case of BL Lacs showing hard spectra in the softX-ray and TeV band (Katarzynski et al., 2005; Tavecchio et al.,2009; Kaufmann et al., 2011; Lefa et al., 2011). Interestingly,such parameters (largeγmin, steep slope) are broadly consis-tent with the prediction of some simulations of particle ac-celeration by relativistic shocks (e.g. Virtanen & Vainio,2003;Sironi & Spitkovsky, 2011). For example, in the case of aproton-electron composition, it is expected that the electrons areheated when crossing the shock to a typical Lorentz factor ofΓ=Γrel mp/me, wheremp/me = 1836 is the proton to electronmass ratio andΓrel = 2− 3 is the relative Lorentz factor betweenthe upstream and the downstream flows. From thisΓ, whichplays the role of our parameterγmin, electrons are subsequentlyaccelerated forming a non-thermal tail, well approximatedby asteep (n = 3.5) power law.

The goodness of the fit can be judged by “eyeball procedure”or by χ2-minimization procedure. For the fits above the formerwas used, as for the latter the systematic errors of the data fromdifferent instruments are in the key role. However, also the au-tomaticχ2-minimization procedure of Mankuzhiyil et al. (2011)is tested with estimated systematical errors of 2%, 10% and 40%for optical-X-ray, GeVγ-rays and VHEγ-rays respectively. Theγmin is fixed to same value as in our highδ model (103) to alloweasier comparison. The resulting parameters are shown in Table2 and the fit with long-dashed (dark green) line in Fig. 8. Theminimal χ2 fit results in lowerδ, but in a highγmax and ratherlarge emission region radiusR compared to other fits, but stillcompatible with the day scale variability observed in X raysandoptical.

5. Summary and Conclusions

In this paper the first detection of VHEγ-rays from1ES 1215+303, resulting from MAGIC observations triggeredby an optical outburst of the source in January 2011, has beenreported. In those data, the source is clearly detected at a 9.4σsignificance level. Also simultaneous multi-wavelength data arepresented from radio to HEγ-rays and compared to results fromearlier MAGIC observations in 2010, when the source was in alower optical state. The VHEγ-ray flux in 2011 was higher com-pared to 2010, suggesting that the activity in these two bandsis connected. This conclusion is further supported by the factthat 1ES 1215+303 is already the fifth discovery at VHEγ-raysachieved after the MAGIC observations were triggered by an op-tical outburst.

The collected multi-wavelength data set, the most extensiveenergy coverage for 1ES 1215+303 up to date, allows also acomparison between different wavelengths. The optical-VHEγ-ray outburst seems to be accompanied by an X-ray outburst,while in theFermi-LAT band the flux increase is only marginallysignificant. The optical photo-polarimetric data seems to suggestthat the high state could be caused by a shock traveling down thejet colliding with a standing shock with different magnetic fieldorientation. The X-ray and VHEγ-ray high states could thenalso originate from this collision.

The SED of 1ES 1215+303 in January 2011 was modelledusing a one-zone SSC model since it has been found to providea good description of the SED of many VHEγ-ray emitting BLLac objects. However, in the case of 1ES 1215+303 the syn-chrotron and IC peaks is narrow and the separation between thetwo peaksis wide and a simple one-zone SSC model with typical

8

J. Aleksic et al.: 1ES 1215+303

model γmin γb γmax n1 n2 B K R δ χ2/d.o.f[103] [104] [106] [G] [cm−3] [1016 cm]

high δ (dashed) 1 3 1.0 2.0 4.2 0.02 8× 103 0.8 60 3.36highγmin (solid) 8 9.2 2.5 3.0 4.85 0.055 1.3× 108 1.0 30 6.94min χ2 (long dashed) 1 1.6 16.1 1.8 3.7 0.01 3.22×102 3.75 36 1.04

Table 2. Input model parameters for the three models shown in Fig.8 The following quantities are reported: the minimum, break,and maximum Lorentz factors and the low and high energy slopeof the electron energy distribution, the magnetic field intensity,the electron density, the radius of the emitting region and its Doppler factor. In addition in the last column we report the χ2/d.o.fassuming 2%, 10% and 40% systematical errors for optical-X-ray, GeVγ-rays and VHEγ-rays respectively.

parameters failed to reproduce the observed SED. In order tofitthe SED a high Doppler factor or a narrow electron energy distri-bution is required. While high Doppler factors are disfavored bythe unification models, the highγmin value could be a viable solu-tion in the light of simulations modeling the acceleration of elec-trons in a relativistic shock in a proton-electron jet. Thisshouldbe further investigated in future work, e.g. using the approach ofWeidinger et al. (2010); Weidinger & Spanier (2010).

Given the rather extreme conditions needed for the one-zone model, the presence of a velocity structure in thejet (Georganopoulos & Kazanas, 2003; Ghisellini et al., 2005)could also be possible and should be tested for modeling theSED. The narrow synchrotron and IC peaks should be ableto constrain the model well. Also the more complex emis-sion scenario suggested by the photo-polarimetric behavior of1ES 1215+303 could be tested in future (e.g. using the approachof Marscher, 2011). Therefore, future observations of thisex-ceptional VHEγ-ray emitting BL Lac are encouraged.

6. Acknowledgments

We would like to thank the Instituto de Astrofısica de Canariasfor the excellent working conditions at the Observatorio delRoque de los Muchachos in La Palma. The support of theGerman BMBF and MPG, the Italian INFN, the Swiss NationalFund SNF, and the Spanish MICINN is gratefully acknowl-edged. This work was also supported by the Marie Curie pro-gram, by the CPAN CSD2007-00042 and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 pro-gramme, by grant DO02-353 of the Bulgarian NSF, by grant127740 of the Academy of Finland, by the YIP of the HelmholtzGemeinschaft, by the DFG Cluster of Excellence “Originand Structure of the Universe”, by the DFG CollaborativeResearch Centers SFB823/C4 and SFB876/C3, and by the PolishMNiSzW grant 745/N-HESS-MAGIC/2010/0. The Metsahoviteam acknowledges the support from the Academy of Finland toour observing projects (numbers 212656, 210338, 121148, andothers).

The Fermi-LAT Collaboration acknowledges generous on-going support from a number of agencies and institutes that havesupported both the development and the operation of the LATas well as scientific data analysis. These include the NationalAeronautics and Space Administration and the Department ofEnergy in the United States, the Commissariat a l’EnergieAtomique and the Centre National de la Recherche Scientifique/ Institut National de Physique Nucleaire et de Physique desParticules in France, the Agenzia Spaziale Italiana and theIstituto Nazionale di Fisica Nucleare in Italy, the Ministry ofEducation, Culture, Sports, Science and Technology (MEXT),High Energy Accelerator Research Organization (KEK) andJapan Aerospace Exploration Agency (JAXA) in Japan, and the

K. A. Wallenberg Foundation, the Swedish Research Counciland the Swedish National Space Board in Sweden.

Additional support for science analysis during the operationsphase is gratefully acknowledged from the Instituto Nazionale diAstrofisica in Italy and the Centre National d’Etudes Spatiales inFrance.

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1 IFAE, Edifici Cn., Campus UAB, E-08193 Bellaterra, Spain2 Universidad Complutense, E-28040 Madrid, Spain3 INAF National Institute for Astrophysics, I-00136 Rome, Italy4 Universita di Siena, and INFN Pisa, I-53100 Siena, Italy5 Technische Universitat Dortmund, D-44221 Dortmund, Germany6 Max-Planck-Institut fur Physik, D-80805 Munchen, Germany7 Universita di Padova and INFN, I-35131 Padova, Italy8 Inst. de Astrofısica de Canarias, E-38200 La Laguna, Tenerife,

Spain9 University of Łodz, PL-90236 Lodz, Poland

10 Depto. de Astrofısica, Universidad de La Laguna, E-38206 LaLaguna, Spain

11 Deutsches Elektronen-Synchrotron (DESY), D-15738 Zeuthen,Germany

12 ETH Zurich, CH-8093 Zurich, Switzerland13 Universitat Wurzburg, D-97074 Wurzburg, Germany14 Universitat de Barcelona (ICC/IEEC), E-08028 Barcelona, Spain15 Universita di Udine, and INFN Trieste, I-33100 Udine, Italy16 Institut de Ciencies de l’Espai (IEEC-CSIC), E-08193 Bellaterra,

Spain17 Inst. de Astrofısica de Andalucıa (CSIC), E-18080 Granada, Spain18 Croatian MAGIC Consortium, Rudjer Boskovic Institute,

University of Rijeka and University of Split, HR-10000 Zagreb,Croatia

19 Universitat Autonoma de Barcelona, E-08193 Bellaterra, Spain20 Tuorla Observatory, University of Turku, FI-21500 Piikki¨o, Finland21 Inst. for Nucl. Research and Nucl. Energy, BG-1784 Sofia, Bulgaria22 Japanese MAGIC Consortium, Division of Physics and Astronomy,

Kyoto University, Japan23 INAF/Osservatorio Astronomico and INFN, I-34143 Trieste, Italy24 Universita dell’Insubria, Como, I-22100 Como, Italy25 Universita di Pisa, and INFN Pisa, I-56126 Pisa, Italy26 ICREA, E-08010 Barcelona, Spain27 now at Ecole polytechnique federale de Lausanne (EPFL),

Lausanne, Switzerland28 supported by INFN Padova29 now at: Centro de Investigaciones Energeticas, Medioambientales y

Tecnologicas (CIEMAT), Madrid, Spain30 now at: KIPAC, SLAC National Accelerator Laboratory, USA31 now at: Institut fur Experimentalphysik, University of Hamburg,

Germany32 Finnish Centre for Astronomy with ESO (FINCA), University of

Turku, Finland33 Aalto University Metsahovi Radio Observatory, Metsahovintie 114,

02540, Kylmala, Finland34 Department of Physics, Stockholm University, AlbaNova, SE-106

91 Stockholm, Sweden35 The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, SE-

106 91 Stockholm, Sweden36 Department of Astronomy, Stockholm University, SE-106 91

Stockholm, Sweden37 *Corresponding authors: jbecerra@iac.es, colin@mppmu.mpg.de,

elilin@utu.fi, saverio.lombardi@pd.infn.it, jsitarek@ifae.es

10