Cosmic Rays and Particle Acceleration in Astrophysical Shocks

Abstract

We investigate particle acceleration in non-relativistic astrophysical shocks using two dimensional hybrid (kinetic ions, fluid electrons) simulations. We account for pre-existing cosmic rays in the interstellar medium and compare our results to previous simulations without cosmic rays which show a roughly constant acceleration effi ciency for quasi-parallel shocks that drops sharply for quasi-perpendicular shocks [Caprioli & Spitkovsky 2014a]. We find that the current in cosmic rays reflected at the shock drives Bell's non resonant hybrid (NRH) instability [Bell 2004]. This instability isotropizes and ampli es the upstream magnetic fi eld, creating quasi-parallel pockets in an initially quasi-perpendicular field con guration. For a range of quasi-perpendicular fi eld inclinations, 50 . . 60 , we fi nd these quasi-parallel pockets boost the ion acceleration effi ciency over its ef- ciency without cosmic rays. We thus demonstrate a mechanism through which quasi-perpendicular shocks turn into e fficient ion accelerators. We show that the cosmic ray acceleration e ffciency is largely independent of the field inclination, demonstrating that pre-existing energetic cosmic rays are re-accelerated at shocks regardless of the field inclination. We measure the re effected cosmic ray current at quasi-perpendicular shocks and con rm agreement with theoretical predictions. Using this current, we fi nd that the NRH instability growth time is 10 years, much less than the supernova Sedov time of tSed 103 years [Caprioli et al. 2009]. This indicates the cosmic ray current alone is suffi cient to trigger the NRH instability in supernova remnant shocks. Cosmic rays and ions escaping from the shock scatter of the NRH instability generated turbulence back towards the shock, gaining energy via di ffusive shock acceleration each shock crossing. These results suggest that supernova remnants up to = 60 accelerate ions, and supernova remnants up to = 80 re-accelerate already energetic cosmic rays, making supernova remnants more versatile particle accelerators than previously thought.