Ultra-High Energy Cosmic Rays from Young Neutron Star Winds

Abstract

The long-held notion that the highest-energy cosmic rays are of distant extragalactic origin is challenged by observations that events above $\sim 10^{20}$ eV do not exhibit the expected high-energy cutoff from photopion production off the cosmic microwave background. We suggest that these unexpected ultra-high-energy events are due to iron nuclei accelerated from young strongly magnetized neutron stars through relativistic MHD winds. We find that neutron stars whose initial spin periods are shorter than $\sim 4 (B_S/10^{13}{\rm G})^{1/2}$ ms, where $B_S$ is the surface magnetic field, can accelerate iron cosmic rays to greater than $\sim 10^{20}$ eV. These ions can pass through the remnant of the supernova explosion that produced the neutron star without suffering significant spallation reactions. For plausible models of the Galactic magnetic field, the trajectories of the iron ions curve sufficiently to be consistent with the observed arrival directions of the highest energy events.