Spectra of Cosmic Ray Protons and Helium Produced in Supernova Remnants

Abstract

Data obtained in the ATIC-2 (Advanced Thin Ionization Calorimeter), CREAM (Cosmic Ray Energetics and Mass)) and PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics) experiments suggest that elemental interstellar spectra of cosmic rays below the knee at a few times $10^{6}$ GeV are not simple power laws, but they experience hardening at magnetic rigidity above about 240 GV. Another essential feature is the difference between proton and Helium energy spectra, so that the He/p ratio increases by more than 50% in the energy range from $10^{2}$ to $10^{4}$ GV. We consider the concavity of particle spectrum resulting from the nonlinear nature of diffusive shock acceleration in supernova remnants (SNR) as a possible reason for the observed spectrum hardening. Helium-to-proton ratio increasing with energy can be interpreted as a consequence of cosmic ray acceleration by forward and reverse shocks in SNRs. The contribution of particles accelerated by reverse shocks makes the concavity of the produced overall cosmic ray spectrum more pronounced. The spectra of protons and helium nuclei accelerated in SNRs and released into the interstellar medium are calculated. The derived steady state interstellar spectra are in reasonably good agreement with observations.