Abstract
The origin of PeV cosmic rays in the Galaxy is a long-standing puzzle. Superbubble or massive star cluster (MSC) has been suggested as powerful cosmic-ray accelerators. Cygnus Cocoon, an extended gamma-ray source, is likely associated with the MSC Cygnus OB2. Previous observations have shown that its spectrum extends up to 100\,TeV with a break around 10\,TeV. The spatial coincidence between the gamma-ray emission and molecular clouds in the region implies the hadronic origin of the gamma-ray emission, supporting Cygnus cocoon as a cosmic-ray proton accelerator. However, lack of a precise measurement of the spectrum beyond 100\,TeV as well as relevant morphology prevent people from concluding that it is the source of PeV cosmic rays. We here report the observation of LHAASO-KM2A on this region. The maximum significance above 25\,TeV is about $21.8\sigma$. A photo with energy up to 1.4\,PeV is detected from this region, which indicates the spectrum can extend up to 1\,PeV. Such a result may be considered as the evidence for cosmic ray accelerated beyond PeV in Cygnus Cocoon.
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