Ultra-High-Energy Cosmic Rays from Young Neutron Star Winds

The long-held notion that the highest energy cosmic rays are of distant extragalactic origin is challenged by observations that events above ~10²⁰ 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 ~10 ms and whose surface magnetic fields are in the 10¹²-10¹⁴ G range can accelerate iron cosmic rays to greater than ~10²⁰ eV. These ions can pass through the remnant of the supernova explosion that produced the neutron star without suffering significant spallation reactions or energy loss. For plausible models of the Galactic magnetic field, the trajectories of the iron ions curve sufficiently to be consistent with the observed, largely isotropic arrival directions of the highest energy events.