Particle Acceleration in Shell Supernova Remnants: Observational Evidence

The origin of Galactic cosmic-ray electrons and ions up to energies of order a few thousand TeV (the knee in the cosmic-ray spectrum) is generally attributed to shell supernova remnants (SNRs). Until recently, the only direct observational evidence for this conclusion was the presence in SNRs of synchrotron radio emission, indicating electrons with a power-law energy distribution between roughly 0.1 and 100 GeV (in the best cases). Further arguments are necessary to rule out the possibility that these electrons are just borrowed from the ambient Galactic pool and compressed in the remnant shock wave. In the last few years, however, striking evidence at much higher photon energies has emerged: X-ray synchrotron emission in several objects, indicating electrons with energies up to 100 TeV, and TeV photon emission in a few, most likely due to inverse-Compton upscattering of cosmic microwave background photons by these same electrons. Evidence for cosmic-ray ions is so far inconclusive; the most unmistakable form would be detecting the expected bump at a few hundred MeV due to photons from the decay of neutral pions produced in inelastic collisions between cosmic-ray ions and interstellar gas. I shall review these various lines of evidence, and the constraints they impose on theoretical models.