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Supernova remnants (SNRs) are believed to be the primary location of theacceleration of Galactic cosmic rays, via diffusive shock (Fermi) acceleration.Despite considerable theoretical work the precise details are still unknown, inpart because of the difficulty in directly observing nucleons that areaccelerated to TeV energies in, and affect the structure of, the SNR shocks.However, for the last ten years, X-ray observatories ASCA, and more recentlyChandra, XMM-Newton, and Suzaku have made it possible to image the synchrotronemission at keV energies produced by cosmic-ray electrons accelerated in theSNR shocks. In this article, we describe a spatially-resolved spectroscopicanalysis of Chandra observations of the Galactic SNR Cassiopeia A to map thecutoff frequencies of electrons accelerated in the forward shock. We set upperlimits on the electron diffusion coefficient and find locations where particlesappear to be accelerated nearly as fast as theoretically possible (the Bohmlimit).

Cosmic-ray diffusion near the Bohm limit in the Cassiopeia A supernova remnant