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Unified model for the gamma-ray emission of supernova remnants

Shocks of supernova remnants (SNRs) are important (and perhaps the dominant) agents for production of the Galactic cosmic rays. Recent $γ$-ray observations of several SNRs have made this case more compelling. However, these broadband high-energy measurements also reveal a variety of spectral shape demanding more comprehensive modeling of emissions from SNRs. According to the locally observed fluxes of cosmic ray protons and electrons, the electron-to-proton number ratio is known to be about 1%. Assuming such a ratio is universal for all SNRs and identical spectral shape for all kinds of accelerated particles, we propose a unified model that ascribes the distinct $γ$-ray spectra of different SNRs to variations of the medium density and the spectral difference between cosmic ray electrons and protons observed at Earth to transport effects. For low density environments, the $γ$-ray emission is inverse-Compton dominated. For high density environments like systems of high-energy particles interacting with molecular clouds, the $γ$-ray emission is $π^0$-decay dominated. The model predicts a hadronic origin of $γ$-ray emission from very old remnants interacting mostly with molecular clouds and a leptonic origin for intermediate age remnants whose shocks propagate in a low density environment created by their progenitors via e.g., strong stellar winds. These results can be regarded as evidence in support of the SNR-origin of the Galactic cosmic rays.