A cut-off in the TeV gamma-ray spectrum of the SNR Cassiopeia A
MAGIC Collaboration, M. L. Ahnen, S. Ansoldi, L. A. Antonelli, C. Arcaro, A. Babić, B. Banerjee, P. Bangale, U. Barres de Almeida, J. A. Barrio, J. Becerra González, W. Bednarek, E. Bernardini, A. Berti, W. Bhattacharyya, B. Biasuzzi, A. Biland, O. Blanch, S. Bonnefoy, G. Bonnoli, R. Carosi, A. Carosi, A. Chatterjee, S. M. Colak, P. Colin, E. Colombo, J. L. Contreras, J. Cortina, S. Covino, P. Cumani, P. Da Vela, F. Dazzi, A. De Angelis, B. De Lotto, E. de Oña Wilhelmi, F. Di Pierro, M. Doert, A. Domínguez, D. Dominis Prester, D. Dorner, M. Doro, S. Einecke, D. Eisenacher Glawion, D. Elsaesser, M. Engelkemeier, V. Fallah Ramazani, A. Fernández-Barral, D. Fidalgo, M. V. Fonseca, L. Font, C. Fruck, D. Galindo, R. J. García López, M. Garczarczyk, M. Gaug, P. Giammaria, N. Godinović, D. Gora, D. Guberman, D. Hadasch, A. Hahn, T. Hassan, M. Hayashida, J. Herrera, J. Hose, D. Hrupec, T. Inada, K. Ishio, Y. Konno, H. Kubo, J. Kushida, D. Kuveždić, D. Lelas, E. Lindfors, S. Lombardi, F. Longo, M. López, C. Maggio, P. Majumdar, M. Makariev, G. Maneva, M. Manganaro, K. Mannheim, L. Maraschi, M. Mariotti, M. Martínez, D. Mazin, U. Menzel, M. Minev, R. Mirzoyan, A. Moralejo, V. Moreno, E. Moretti, V. Neustroev, A. Niedzwiecki, M. Nievas Rosillo, K. Nilsson, D. Ninci, K. Nishijima, K. Noda, et al. (44 additional authors not shown)
aa r X i v : . [ a s t r o - ph . H E ] J u l MNRAS , 000–000 (0000) Preprint July 7, 2017 Compiled using MNRAS L A TEX style file v3.0
A cut-off in the TeV gamma-ray spectrum of the SNRCassiopeia A
MAGIC Collaboration: M. L. Ahnen , S. Ansoldi , , L. A. Antonelli , C. Arcaro ,A. Babi´c , B. Banerjee , P. Bangale , U. Barres de Almeida , J. A. Barrio , J. Be-cerra Gonz´alez , W. Bednarek , E. Bernardini , , A. Berti , W. Bhattacharyya ,B. Biasuzzi , A. Biland , O. Blanch , S. Bonnefoy , G. Bonnoli , R. Carosi ,A. Carosi , A. Chatterjee , M. Colak , P. Colin , E. Colombo , J. L. Contreras ,J. Cortina , S. Covino , P. Cumani , P. Da Vela , F. Dazzi , A. DeAngelis , B. De Lotto , E. de O˜na Wilhelmi ⋆ , F. Di Pierro , M. Doert ,A. Dom´ınguez , D. Dominis Prester , D. Dorner , M. Doro , S. Einecke ,D. Eisenacher Glawion , D. Elsaesser , M. Engelkemeier , V. Fallah Ramazani ,A. Fern´andez-Barral , D. Fidalgo , M. V. Fonseca , L. Font , C. Fruck ,D. Galindo , R. J. Garc´ıa L´opez , M. Garczarczyk , M. Gaug , P. Giammaria ,N. Godinovi´c , D. Gora , D. Guberman , D. Hadasch , A. Hahn , T. Hassan ,M. Hayashida , J. Herrera , J. Hose , D. Hrupec , T. Inada , K. Ishio ,Y. Konno , H. Kubo , J. Kushida , D. Kuveˇzdi´c , D. Lelas , E. Lindfors ,S. Lombardi , F. Longo , M. L´opez , C. Maggio , P. Majumdar , M. Makariev ,G. Maneva , M. Manganaro , K. Mannheim , L. Maraschi , M. Mariotti ,M. Mart´ınez , D. Mazin , , U. Menzel , M. Minev , R. Mirzoyan , A. Moralejo ,V. Moreno , E. Moretti , V. Neustroev , A. Niedzwiecki , M. Nievas Rosillo ,K. Nilsson , D. Ninci , K. Nishijima , K. Noda , L. Nogu´es , S. Paiano ,J. Palacio , D. Paneque , R. Paoletti , J. M. Paredes , G. Pedaletti ,M. Peresano , L. Perri , M. Persic , , P. G. Prada Moroni , E. Prandini , I. Puljak ,J. R. Garcia , I. Reichardt , W. Rhode , M. Rib´o , J. Rico , C. Righi ,T. Saito , K. Satalecka , S. Schroeder , T. Schweizer , S. N. Shore , J. Sitarek ,I. ˇSnidari´c , D. Sobczynska , A. Stamerra , M. Strzys , T. Suri´c , L. Takalo ,F. Tavecchio , P. Temnikov , T. Terzi´c , D. Tescaro , M. Teshima , , N. Torres-Alb`a , A. Treves , G. Vanzo , M. Vazquez Acosta , I. Vovk , J. E. Ward , M. Will and D. Zari´c (Affiliations can be found after the references) July 7, 2017c (cid:13)
MAGIC Collaboration
ABSTRACT
It is widely believed that the bulk of the Galactic cosmic rays are accelerated insupernova remnants (SNRs). However, no observational evidence of the presence ofparticles of PeV energies in SNRs has yet been found. The young historical SNRCassiopeia A (Cas A) appears as one of the best candidates to study accelerationprocesses. Between December 2014 and October 2016 we observed Cas A with theMAGIC telescopes, accumulating 158 hours of good-quality data. We derived thespectrum of the source from 100 GeV to 10 TeV. We also analysed ∼ F ermi -LAT to obtain the spectral shape between 60 MeV and 500 GeV. The spectra measuredby the LAT and MAGIC telescopes are compatible within the errors and show a clearturn off (4.6 σ ) at the highest energies, which can be described with an exponentialcut-off at E c = 3 . (cid:0) +1 . − . (cid:1) stat (cid:0) +0 . − . (cid:1) sys TeV. The gamma-ray emission from 60 MeVto 10 TeV can be attributed to a population of high-energy protons with spectralindex ∼ ∼
10 TeV. This result indicates that Cas A is notcontributing to the high energy ( ∼ PeV) cosmic-ray sea in a significant manner atthe present moment. A one-zone leptonic model fails to reproduce by itself the multi-wavelength spectral energy distribution. Besides, if a non-negligible fraction of the fluxseen by MAGIC is produced by leptons, the radiation should be emitted in a regionwith a low magnetic field (B / µ G) like in the reverse shock.
Key words: gamma rays: general – cosmic ray physics – stars: supernovae: individual:Cassiopeia A – supernova remnants – acceleration of particles
Supernova remnants (SNRs) are widely believed to beable to accelerate cosmic rays (CRs) to PeV energies, andof being the main contributors to the galactic CR sea(Berezhko et al. 2003; Bell et al. 2013; O’C. Drury 2014).Two arguments support this belief. On one hand SNRscan explain the observed energy density of CRs if one as-sumes that around 10% of the kinetic energy of the super-nova (SN) explosion goes into CR acceleration and a su-pernova explosion rate of ∼ knee of the CR spectrum, a feature observed at around 3 PeV.In fact this represented an important theoretical challengefor decades, because standard DSA was unable to explainacceleration beyond 100 TeV. It has been later realised (Bell2004) that the magnetic field upstream of the shock of youngSNR can be amplified due to instabilities produced by CRsthemselves. The missing part to solve the paradox is theobservational evidence: as of today no SNR has been found ⋆ Corresponding authors: Daniel Guberman ([email protected]), Emma de O˜na Wilhelmi ([email protected])and Daniel Galindo ([email protected]) where hadronic CR acceleration up to PeV energies can befirmly established.Cassiopeia A (Cas A) is one of the few good candidatesfor these studies. The precise knowledge of the age of thiscore-collapse SNR (330 yrs), the remnant of a historical su-pernova in AD1680, allows the determination of many oth-erwise free parameters when studying its morphology andspectral shape. It is located at a distance of 3.4 +0 . − . kpc andhas an angular diameter of 5 ′ (Reed et al. 1995). It is thebrightest radio source outside our solar system and it is infact bright all over the electromagnetic spectrum, offeringan excellent opportunity to study particle acceleration.Cas A has been extensively observed in radio wave-lengths (Lastochkin et al. 1963; Medd & Ramana 1965;Allen & Barrett 1967; Parker 1968; Braude et al. 1969;Hales et al. 1995). Most of the emission comes from a brightradio ring of ∼ ∼ ∼ ∼ F ermi -LAT detected the source atGeV energies (Abdo et al. 2010) and later derived a spec-trum that displays a low energy spectral break at 1.72 ± F ermi -LAT energy range to the TeV bands. Thephoton indices measured by HEGRA, MAGIC, and VER-ITAS are seemingly larger than the
F ermi -LAT index of
MNRAS , 2–8 (2017) cut-off in the TeV gamma-ray spectrum of the SNR Cassiopeia A ± F ermi -LAT spectrum at ∼ ∼ and tem-perature of 97 K, Mezger et al. 1986), is more significant ina region of lower magnetic field, as otherwise it would besuppressed due to fast cooling of electrons. Hard X-ray ob-servations (Grefenstette et al. 2015; Siegert et al. 2015), ifof synchrotron origin, prove the presence of relativistic elec-trons with Lorentz factor γ e ≥ ∼
60 MeV to ∼
10 TeV is investigated here to determine theunderlying mechanisms powering the young remnant, con-straining the maximum energy of the accelerated particlesand their nature.
MAGIC is a system of two 17 m diameter Imaging Atmo-spheric Cherenkov Telescopes (IACTs), located at an alti-tude of 2200 m a.s.l. at the Roque de los Muchachos Ob-servatory on the Canary Island of La Palma, Spain (28 ◦ N, Table 1.
Effective observation time of the different hardware andsky brightness conditions under wich Cas A samples were taken.Observation conditions Time [h]Dark and Nominal HV 42.2Moon and Nominal HV 77.7Moon and Reduced HV 38.1All configurations 158.0 ◦ W). The telescopes are equipped with photomultipliertubes (PMTs) that can detect the flashes of Cherenkov lightproduced by extensive air showers initiated in the upper at-mosphere by gamma-ray photons with energies &
50 GeV. Inthe absence of moonlight and for zenith angles less than 30 ◦ MAGIC reaches an energy threshold of ∼
50 GeV at triggerlevel, and a sensitivity above 220 GeV of 0 . ± .
04% of theCrab Nebula flux (C.U., Aleksi´c et al. 2016).Observations were performed between December 2014and October 2016, for a total observation time of 158 hoursafter data quality cuts. They were carried in the so-calledwobble mode (Fomin et al. 1994), with a standard wobbleoffset of 0 . ◦ . The data correspond to zenith angles between28 and 50 degrees and most of them ( ∼ ∼ ∼ ∼
100 GeV during dark con-ditions to ∼
300 GeV in the brightest scenario considered.As achieving a low energy threshold was not critical for thisproject, Moon observations provided a unique way to ac-cumulate observation time. A detailed study of the perfor-mance of the MAGIC telescopes under moonlight is reportedin (MAGIC Collaboration 2017).The data have been analysed using the standardtools used for the analysis of the MAGIC telescope data,MARS (Zanin et al. 2013) following the optimised moon-light analysis described in (MAGIC Collaboration 2017).For the spectrum reconstruction a point-like source was as-sumed and typical selection cuts with 90% and 75% γ -rayefficiency for the γ -ray/hadron separation and sky signal re-gion radius, respectively (Aleksi´c et al. 2016). Three OFFregions were considered for the background estimation. F ermi -LAT
The GeV emission of Cas A was revisited using 3.7 yr ofLAT observations (Yuan et al. 2013). The spectrum derivedis well-represented by a broken power-law with a break of6.9 σ significance at ∼ ◦ × ◦ re- MNRAS , 2–8 (2017)
MAGIC Collaboration gion around the position of Cas A . We selected eventswith energy between 60 MeV and 500 GeV and applied theusual filters and corrections recommended by the F ermi -LAT collaboration (removing intervals when the rockingangle of the LAT was greater than 52 ◦ or when parts ofthe region-of-interest, ROI, were observed at zenith angleslarger than 90 ◦ , as well as enabling the energy dispersion).In order to derive the energy spectrum we applied a max-imum likelihood estimation analysis in 12 independent en-ergy bins from 60 MeV to 500 GeV, modelling the Galacticand isotropic diffuse emission using the templates providedby the F ermi -LAT collaboration . During the broad-bandfit, all sources in the third F ermi catalog (3FGL) within theROI were included. A source located ∼ ◦ away from Cas Aat (l,b)=(113.6 ◦ ,1.1 ◦ ) was added during the fitting processto account for a significant residual excess (with TS= 45 . ◦ of the candidate location and the normalisation of thetwo diffuse background components. Following the resultsobtained by Yuan et al. (2013) we used a smoothly brokenpower-law function to fit the broadband spectrum of Cas A( dN/dE = N o ( EE o ) Γ (1 + ( EE b ) (Γ − Γ ) /β ) − β ) with the pa-rameter β fixed to 1 and the energy break to E b =1.7 GeV.E o is the normalisation energy, fixed to 1 GeV. The data setwas reduced and analysed using Fermipy , a set of pythontools which automatise the Pass 8 analysis. We analysed thefour EDISP event types separately and combined them laterby means of a joint likelihood fit. The SED was obtained byfitting the source normalisation factor in each energy bin in-dependently using a power-law spectrum with a fixed spec-tral index of 2. For each spectral point we required at least aTS of 4, otherwise upper limits at the 95% confidence levelwere computed.
Figure 1 shows the reconstructed SED obtained with theMAGIC telescopes (black solid points). Red solid line is thecurve obtained that best fits the MAGIC data assuming apower-law with an exponential cut-off (EPWL): dNdE = N (cid:18) EE (cid:19) − Γ exp (cid:18) − EE c (cid:19) (1)with a normalisation constant N = (1 . ± . stat ± . sys ) × − TeV − cm − s − at a normalisation energy E = 433 GeV, a spectral index Γ = 2 . ± . stat ± . sys and a cut-off energy E c = 3 . (cid:0) +1 . − . (cid:1) stat (cid:0) +0 . − . (cid:1) sys TeV. Thespectral parameters of the tested models θ = { N , Γ , E c } are obtained via a maximum likelihood approach. The datainputs are the numbers of recorded events (after backgroundsuppression cuts) in each bin of estimated energy E i est , botharound the source direction ( N ON i ) and in the three OFFregions ( N OFF i ). An additional set of nuisance parameters µ i for modelling the background are also optimized in the The analysis on a 30 ◦ × ◦ region yields compatible results. gll iem v06.fits and iso P8R2 ULTRACLEANVETO V6 v06.txt ,http://fermi.gsfc.nasa.gov/ssc/data/analysis/documentation/Cicerone http://fermipy.readthedocs.org/en/latest/ likelihood calculation. In each step of the maximisationprocedure the expected number of gammas in a given binof estimated energy ( E est ) is calculated by folding thegamma spectrum with the MAGIC telescopes response(energy-dependent effective area and energy migrationmatrix). The background nuisance parameters and thestatistical uncertainties in the telescopes response aretreated as explained in (Rolke et al. 2005).The probability of the EPWL fit is 0.42. We tested themodel against the null hypothesis of no cut-off, which isdescribed with a pure power-law (PWL). The probability ofthe PWL fit is 6 × − . A likelihood ratio test between thetwo tested models favours the one that includes a cut-off at ∼ . σ significance.Figure 2 compares the fit residuals for the two testedmodels: PWL and EPWL. The residuals are here defined as N obsON /N expON −
1, where N obsON is the number of observed events(including background) in the ON region and N expON is thenumber of events predicted by the fit in the same region. Allthe bins in estimated energy which contain events are usedin the fits, but only those with a 2 σ significance gamma-rayexcess are shown as SED points in upper panel of Fig. 1.The systematic uncertainty due to an eventual mis-match on the absolute energy scale between MAGIC dataand MC simulations was constrained to be below 15% inAleksi´c et al. (2016). By conservatively modifying the ab-solute calibration of the telescopes by ± average Cherenkov light yield was overestimated by 15%relative to the MC, by applying the corresponding correctionthe resulting spectrum is still better fit by an EPWL at thelevel of 3.1 σ . In the also unlikely scenario in which the lightyield was underestimated, the EPWL is preferred over thePWL at the 6.5 σ level. The systematic uncertainties in theflux normalization and spectral index were retrieved fromthe publication reporting the performance of the MAGICtelescopes during moonlight (MAGIC Collaboration 2017).The systematic errors in the cut-off energy were estimatedfrom the values of E c obtained when modifying the absolutelight scale by ± F ermi -LAT analysis, a broken power-law function with normalisation N o = (8 . ± . × − TeV − cm − s − , indices Γ = 0 . ± .
08 andΓ = 2 . ± .
04 is obtained and showed in Fig. 1, blue solidsquares. The light gray shaded area shows the statistical er-rors of the obtained broken power-law fit whereas the darkone marks the uncertainty coming from the imperfectnessin the Galactic diffuse emission modelling, dominating theCas A flux uncertainties at low energies. The later were ob-tained by modifying the galactic diffuse flux by ± MAGIC observations of the youngest GeV- and TeV-brightknown SNR have allowed us to obtain the most precise spec-
MNRAS , 2–8 (2017) cut-off in the TeV gamma-ray spectrum of the SNR Cassiopeia A E [GeV] −
10 1 10 ] - s - [ e r g c m d E d A d t d N E − − − MAGIC Statistical uncertaintyFermi Systematic uncertainty
Figure 1.
Spectral energy distribution measured by the MAGICtelescopes (black dots) and
F ermi (blue squares). The red solidline shows the result of fitting the MAGIC spectrum with Eq. 1.The black solid line is the broken power-law fit applied to the
F ermi spectrum. F i t r e s i dua l s − − − − − EPWL
Estimated Energy [GeV] F i t r e s i dua l s − − − − − PWL
Figure 2.
Relative fit residuals for the two tested models fit-ting the MAGIC spectrum: power-law with exponential cut-off(EPWL, upper panel) and power law (PWL, lower panel). Theerror bars are calculated such that they correspond to the totalcontribution of each estimated energy bin to the final likelihoodof the fit. trum of Cas A to date, extending previous results obtainedwith Cherenkov instruments up to ∼
10 TeV. In the MAGICenergy range, the spectrum is best-fit with a power-law withexponential cut-off function with index ∼ E c ∼ ∼ ∼ F ermi -LAT falls quickly with a photon index of ∼ naima (Zabalza 2015), derivingthe present-age particle distribution. The code uses theparametrisation of neutral pion decay by Kafexhiu et al.(2014), the parametrization of synchrotron radiation byAharonian et al. (2010) and the analytical approximationsto IC up-scattering of blackbody radiation and non-thermalbremsstrahlung developed by Khangulyan et al. (2014) andBaring et al. (1999), respectively.We first considered the possibility that the gamma-rayemission was originated by an electron population, describedwith a power-law with an exponential cut-off function, pro-ducing Bremsstrahlung and IC radiation in the gamma-rayrange, and synchrotron radiation at lower energies. The pho-ton fields that contribute to the inverse Compton compo-nent are the ubiquitous 2.7 K cosmic microwave background(CMB) and the large far infrared (FIR) field measured inCas A, with a value of ∼ . Themulti-wavelength SED is shown in Fig. 3, with the radioemission displayed in purple dots (Lastochkin et al. 1963;Medd & Ramana 1965; Allen & Barrett 1967; Parker 1968;Braude et al. 1969; Hales et al. 1995; Planck Collaboration2014), soft SUZAKU X-rays are marked in red (Maeda et al.2009) and hard INTEGRAL X-rays in blue (Wang & Li2016). In the gamma-ray regime, the LAT points are shownin cyan and the MAGIC ones in green. The MAGIC pointscan be described by an electron population with amplitudeat 1 TeV of 2 · eV − , spectral index 2.4 and cut-off en-ergy at 8 TeV up-scattering the FIR (brown dash-dot line)and the CMB photons (green dashed line). The comparisonwith the low energy part of the SED constraints the mag-netic field to B / µ G. The resulting emission from theleptonic model is shown in Fig. 3. A relatively low magneticfield and a large photon field could be fulfilled in a reverseshock evolving in a thin and clumpy ejecta medium whichprovides a moderate amplification of the magnetic field andlarge photon fields in the clumps which are observed as opti-cal knots. The same population of electrons would unavoid-ably produce Bremsstrahlung radiation below a few GeV(see green dotted line in Fig. 3 ). The emission observedwith F ermi
LAT at the lowest energies constrain the den-sity to n ∼ − , still compatible with the smooth ejectadensity (Micelotta et al. 2016). The model is generally com-patible with the X-ray points and with MAGIC spectrumabove a few TeV, it is consistent with the radio measure-ments, but fails to reproduce the γ -ray spectrum between 1GeV and 1 TeV, being a factor 2-3 below the measured LATspectrum. In addition, to accommodate a magnetic field ofthe order of ∼ https://github.com/zblz/naima This constraint is due to the fact that, as reported in section 1,several emission regions, likely associated to different particle pop-ulations, were identified at those wavelengths. Note that the structure in the spectral shape around 2 MeVis due to the transition between the two asymptotic regimes de-scribed in Baring et al. (1999), used in the naima code.MNRAS , 2–8 (2017)
MAGIC Collaboration to be decreased at least by a factor 100, rendering a negligi-ble IC contribution at the highest energies.Indeed the GeV-TeV emission of Cas A is usually at-tributed to accelerated protons. Assuming a population ofCRs characterised with a power-law function with an expo-nential cut-off to fit the gamma-ray data from 60 MeV to15 TeV, and a target density of 10 cm − (Laming & Hwang2003). The proton spectrum is best-fit with a hard indexof 2.21 and an exponential cut-off energy of 12 TeV, whichimplies a modest acceleration of CRs to VHE, well belowthe energy needed to explain the CR knee . The proton en-ergy above 1 TeV is 5.1 · erg, which is only ∼ sn = 2 · erg(Laming & Hwang 2003). The total energy stored in protonsabove 100 MeV amounts to 9 . · erg.The flat spectral index is in agreement with the stan-dard theory of diffuse shock acceleration, but the low cut-offenergy implies that Cas A is an extremely inefficient in theacceleration of CRs at the present moment. The character-istic maximum energy of these accelerated protons can beexpressed, for standard parallel shock acceleration efficiency(see e. g. Lagage & Cesarsky 1983), as: E pc ≃ B t
100 yr )( u s / s ) η − TeV , (2)where u s ∼ km / s is the speed of the forward shock, t ∼
330 yrs is the age of Cas A and η ≥ ∼ µ G,a poor acceleration efficiency η ≫
10 has to be invoked toaccommodate the low cut-off energy found. Alternatively,Cas A may also be located in a very diffusive region of theGalaxy, resulting in a very fast escape of protons of TeV andhigher energies.
We report for the first time in VHE, observational evidencefor the presence of a cut-off in the VHE spectrum of CasA. The spectrum measured with the MAGIC telescopes canbe described with an EPWL with a cut-off at ∼ σ signifi-cance. This result implies that even if all the TeV emissionwas of hadronic origin, Cas A could not be a PeVatron atits present age.Several emission regions must be active to explain theradio, X-ray, GeV and TeV emission of Cas A. A purelyleptonic model cannot explain the GeV-TeV spectral shapederived using LAT and MAGIC data, as previously sug-gested based on observations at lower energies (Atoyan et al.2000a,b; Zirakashvili et al. 2014; Saha et al. 2014). A lep-tonic population is undoubtedly necessary to explain theemission at radio and X-ray energies. Indeed the brightsteep-spectrum radio knots and the bright radio ring, de-mand an average magnetic field of ∼ γ -rays must beof hadronic origin. Cas A is most likely accelerating CRs,although to a rather low energy of a few TeV. Even if someleptonic contribution at VHE produced by IC cannot be ex-cluded, this would not affect our conclusion that accelerationin Cas A falls short of the energies of the knee of the CRspectrum.A detailed study of the cut-off shape is crucial to un-derstand the reason behind this low acceleration efficiency,displaying different characteristics if due to escape of CRs,to the maximum energy of the accelerated CRs, or someother mechanism. Observations with the future CherenkovTelescope Array (CTA, Actis et al. 2011), with a superiorangular resolution and sensitivity, will allow detailed spec-troscopic investigation on the cut-off regime, providing newinsights on the acceleration processes in Cas A. ACKNOWLEDGEMENTS
We would like to thank the Instituto de Astrof´ısica deCanarias for the excellent working conditions at the Ob-servatorio del Roque de los Muchachos in La Palma.The financial support of the German BMBF and MPG,the Italian INFN and INAF, the Swiss National FundSNF, the ERDF under the Spanish MINECO (FPA2015-69818-P, FPA2012-36668, FPA2015-68378-P, FPA2015-69210-C6-2-R, FPA2015-69210-C6-4-R, FPA2015-69210-C6-6-R, AYA2015-71042-P, AYA2016-76012-C3-1-P, ESP2015-71662-C2-2-P, CSD2009-00064), and the Japanese JSPS andMEXT is gratefully acknowledged. This work was also sup-ported by the Spanish Centro de Excelencia “Severo Ochoa”SEV-2012-0234 and SEV-2015-0548, and Unidad de Exce-lencia “Mar´ıa de Maeztu” MDM-2014-0369, by the CroatianScience Foundation (HrZZ) Project 09/176 and the Univer-sity of Rijeka Project 13.12.1.3.02, by the DFG CollaborativeResearch Centers SFB823/C4 and SFB876/C3, and by thePolish MNiSzW grant 745/N-HESS-MAGIC/2010/0.
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