2012ApJ...749L..26A

Recent Fermi and VERITAS observations of the prototypical Type Ia supernova remnant (SNR) Tycho have discovered γ-rays with energies E in the range 0.4 GeV ≲ E ≲ 10 TeV. Crucial for the theory of Galactic cosmic-ray origin is whether the γ-rays from SNRs are produced by accelerated hadrons (protons and ions) or by relativistic electrons. Here we show that strong constraints on the leptonic model imposed in the framework of the commonly used single-zone model are essentially removed if the analysis of the broadband radiation spectrum of Tycho is done in the two-zone (or, in general, multi-zone) approach, which is likely to apply to every SNR. Importantly, we show that the single-zone approach may underpredict the γ-ray fluxes by an order of magnitude. A hadronic model can, however, also fit the detected γ-ray spectrum. The difference between γ-ray fluxes of hadronic and leptonic origins becomes significant only at ≲ 300 MeV, which could be revealed by spectral measurements of Tycho and other SNRs at these energies.