Cosmic Ray Diffusion near the Bohm Limit in the Cassiopeia a Supernova Remnant

Supernova remnants(SNRs) are believed to be the primary location of the acceleration of Galactic cosmic rays, via diffusive shock (Fermi) acceleration. Despite considerable theoretical work the precise details are still unknown, in part because of the difficulty in directly observing nucleons that are accelerated to TeV energies in, and affect the structure of, the SNR shocks. However , for the last ten years, X-ray observatories ASCA, and more recently Chandra, XMM-Newton, and Suzaku have made it possible to image the synchrotron emission at keV energies produced by cosmic-ray electrons accelerated in the SNR shocks. In this article, we describe a spatially-resolved spectroscopic analysis of Chandra observations of the Galactic SNR Cassiopeia A to map the cutoff frequencies of electrons accelerated in the forward shock. We set upper limits on the diffusion coefficient and find locations where particles appear to be accelerated nearly as fast as theoretically possible (the Bohm limit). Supernova remnants (SNRs) have been established as the leading candidate for the acceleration of cosmic rays.[1][2] It has been shown that the mechanism of diffusive shock acceleration in SNR shocks coupled with some understanding of Galactic transport effects can in theory produce the observed power-law spectrum of cosmic rays.[3]-[10]. This model works at least up to the " knee " of the cosmic ray spectrum near 5 × 10 15 eV, and possibly 1 all the way to the " ankle " near 3×10 18 eV.[11] The charged particles scatter off of irregularities in the magnetic field, increasing their momentum by a fraction of the shock velocity v/c with each round-trip shock crossing.[11] Theoretical work in the last several years has suggested that the process is significantly nonlinear.[10]-[15] Higher energy particles have larger diffusion lengths and therefore sample a greater change in velocity and compression ratio across the shock than lower energy particles. This effect introduces a decrease in the particle distribution's spectral index and a flattening of the spectrum with increasing energy.[10] A smoothing effect on the structure of the shock results, predominantly caused by the ions; electrons are almost " test particles. " At high energies the electron spectrum is expected to have the same (curved) spectral shape as the proton spectrum.[10] This correspondence of electron and proton spectra is significant because it is extremely difficult to directly observe the acceleration of cosmic-ray protons in SNR shocks. Their presence is inferred indirectly by detecting π 0 decay γ-rays …