Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail

We propose a model for the hard X-ray (> 10 keV) emission observed from the supernova remnant Cas A. Lower hybrid waves are generated in strong (mG) magnetic fields, generally believed to reside in this remnant, by shocks reflected from density inhomogeneities. These then accelerate electrons to energies of several tens of keV. Around 4% of the x-ray emitting plasma electrons need to be in this accelerated distribution, which extends up to electron velocities of order the electron Alfven speed, and is directled along magnetic field lines. Bremsstrahlung from these electrons produces the observed hard x-ray emission. Such waves and accelerated electrons have been observed in situ at Comet Halley, and we discuss the viability of the extrapolation from this case to the parameters relevant to Cas A.