Past Galactic GRBs, and the origin and composition of ultrahigh-energy cosmic rays

Recent results from the Pierre Auger Observatory show energy dependent chemical composition of ultrahigh-energy cosmic rays (UHECR) with a growing fraction of heavy elements at high energies. This points to a non-negligible contribution of the Galactic sources, such as past GRBs and other rare but powerful stellar explosions in the Milky Way. The effects of diffusion in the Galactic magnetic fields alter the observed composition and render the flux of UHECR isotropic, up to a few per cent anisotropy in the direction of the Galactic Center, as well as some small-scale anisotropy with “hot spots” due to the locations of the most recent/closest bursts.