Potential PeVatron supernova remnant G106.3+2.7 seen in the highest-energy gamma rays

Amenomori, Michihiro*; Tsuchiya, Harufumi ; Tibet AS Collaboration*; 93 of others*

Cosmic rays (protons and other atomic nuclei) are believed to gain energies of petaelectronvolts (PeV) and beyond at astrophysical particle accelerators called "PeVatrons" inside our Galaxy. Although a characteristic feature of a PeVatron is expected to be a hard gamma-ray energy spectrum that extends beyond 100 teraelectronvolts (TeV) without a cut-off, none of the currently known sources exhibit such a spectrum owing to the low maximum energy of accelerated cosmic rays or owing to insufficient detector sensitivity around 100 TeV. Here, we report the observation of gamma-ray emission from the supernova remnant G106.3+2.7 above 10 TeV. This work provides flux data points up to and above 100 TeV and indicates that the very-high-energy gamma-ray emission above 10 TeV is well correlated with a molecular cloud rather than with the pulsar PSR J2229+6114. Regarding the gamma-ray emission mechanism of G106.3+2.7, this morphological feature appears to favour a hadronic origin via the decay caused by accelerated relativistic protons over a leptonic origin via the inverse Compton scattering by relativistic electrons. Furthermore, we point out that an X-ray flux upper limit on the synchrotron spectrum would provide important information to firmly establish the hadronic scenario as the mechanism of particle acceleration at the source.