The Cosmic-Ray Knee as a Local Signature of Nearby PeVatrons

Abstract A “knee” in the cosmic-ray spectrum, characterized by a sudden steepening of the spectral shape at ∼4 PeV, may be interpreted either as a global feature of Galactic cosmic rays or as a local signature. In the former scenario, cosmic-ray spectra throughout the Galaxy would be similar to that observed in the solar neighborhood, and the knee would be a common feature of the cosmic-ray sea. In the latter scenario, the PeV cosmic-ray flux varies across the Galactic disk, and the knee is dominantly contributed by a small number of nearby sources. By simulating cosmic-ray propagation in the Galactic magnetic field and interstellar medium, we show that the two scenarios correspond to different regimes of the birth rate of PeV proton accelerators and depend on the presence of powerful nearby sources. By comparison with both cosmic-ray and gamma-ray observations, we find that a local knee would be best explained by sources located at distances of order ∼1 kpc and with ages in the range 0.1–1 Myr, with the Cygnus Cocoon being a particularly promising candidate.