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Gamma Rays from the Tycho Supernova Remnant: Leptonic or Hadronic?

Recent Fermi and VERITAS observations of the prototypical Type Ia supernova remnant (SNR) Tycho have discovered gamma-rays with energies E in the range from ~0.4 GeV to 10 TeV. Crucial for the theory of Galactic cosmic-ray origin is whether the gamma-rays from SNRs are produced by accelerated hadrons (protons and ions), or by relativistic electrons. Here we show that the broadband radiation spectrum of Tycho can be explained within the framework of a two-zone leptonic model, which is likely to apply to every SNR. A model with hadrons can also fit the radiation spectrum. The hadronic origin of gamma-rays can be confirmed by Fermi spectral measurements of Tycho and other SNRs at energies below ~300 MeV.