The existence of a “knee” at energy $\sim 1\mathrm{PeV}$ in the cosmic-ray spectrum suggests the presence of Galactic PeV proton accelerators called “PeVatrons.” Supernova remnant (SNR) $\mathrm{G}106.3+2.7$ is a prime candidate for one of these. The recent detection of very high energy (0.1–100 TeV) gamma rays from $\mathrm{G}106.3+2.7$ may be explained either by the decay of neutral pions or inverse Compton scattering by relativistic electrons. We report an analysis of 12 years of Fermi-LAT gamma-ray data that shows that the GeV-TeV gamma-ray spectrum is much harder and requires a different total electron energy than the radio and x-ray spectra, suggesting it has a distinct, hadronic origin. The nondetection of gamma rays below 10 GeV implies additional constraints on the relativistic electron spectrum. A hadronic interpretation of the observed gamma rays is strongly supported. This observation confirms the long-sought connection between Galactic PeVatrons and SNRs. Moreover, it suggests that $\mathrm{G}106.3+2.7$ could be the brightest member of a new population of SNRs whose gamma-ray energy flux peaks at TeV energies. Such a population may contribute to the cosmic-ray knee and be revealed by future very high energy gamma-ray detectors.

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• Cosmic ray sources
• Cosmic rays & astroparticles
• Gamma ray astronomy
• Supernova remnants