  Analyses of the X-ray data of the five young shell-type supernova remnants
Cas A, Kepler, Tycho, SN 1006, and RCW 86 suggest that some of the X-ray
emission of these sources is non-thermal. This non-thermal emission is
qualitatively consistent with models of the broad-band (radio-to-X-ray)
synchrotron spectra of remnants and does not seem to be consistent with other
non-thermal X-ray emission processes. If this emission is produced by
synchrotron radiation, the radio-to-X-ray synchrotron spectra imply that the
electron spectra have differential spectral indices of about 2.2 and
exponential cut offs at energies ~ 10 TeV. If the remnants also accelerate
cosmic-ray nuclei, the total energies of the cosmic rays in the remnants are
estimated to be ~ 1-5 X 10^49 erg. Therefore, the shapes of the cosmic-ray
electron spectra, the maximum energies of the cosmic-ray electrons, and the
total cosmic-ray energies of the five young remnants seem to be consistent with
the idea that Galactic cosmic rays are predominantly accelerated in the shocks
of supernova remnants.
