Searching for Galactic Cosmic-Ray Pevatrons with Multi-TeV Gamma Rays and Neutrinos

The recent H.E.S.S. detections of supernova remnant shells in TeV γ-rays confirm the theoretical predictions that supernova remnants can operate as powerful cosmic-ray accelerators. If these objects are responsible for the bulk of galactic cosmic rays, then they should accelerate protons and nuclei to 10¹⁵ eV and beyond, i.e., act as cosmic PeVatrons. The model of diffusive shock acceleration allows, under certain conditions, acceleration of particles to such high energies and their gradual injection into the interstellar medium, mainly during the Sedov phase of the remnant evolution. The most energetic particles are released first, while particles of lower energies are more effectively confined in the shell and are released at later epochs. Thus, the spectrum of nonthermal particles inside the shell extends to PeV energies only during a relatively short period of the evolution of the remnant. For this reason, one may expect spectra of secondary γ-rays and neutrinos extending to energies beyond 10 TeV only from T<~1000 yr old supernova remnants. On the other hand, if by chance a massive gas cloud appears in the <~100 pc vicinity of the supernova remnant, ``delayed'' multi-TeV signals of γ-rays and neutrinos arise when the most energetic particles that emerged from the supernova shell reach the cloud. The detection of such delayed emission of multi-TeV γ-rays and neutrinos allows indirect identification of the supernova remnant as a particle PeVatron.