Shocks and particle acceleration in SNRs: theoretical aspects

Energetic particles are an essential component in supernova remnants (SNRs). Non-thermal particle acceleration in supernova shocks is expected to be an efficient process at different evolutional stages of SNRs. Non-linear wave–particle interactions being the governing process of the SNR collisionless shock formation are responsible for both shock heating and compression of the thermal gas as well as for creation of energetic particle population. The gas temperature behind such a shock could depart strongly from that predicted by the standard Rankine–Hugoniot law. We overview current models of collisionless supernova shocks with an emphasis on their confrontation with observed multi-wavelength spectra of SNRs from the radio to γ-rays. Simple scaling relations for post-shock ion temperature in the shocks with efficient particle acceleration are presented. A special attention is paid to the connections with high resolution SNR spectra currently obtained with XMM-Newton and Chandra. We discuss X-ray line emission from fast moving supernova ejecta fragments, arguing that they could contribute substantially to the observed line emission of the galactic ridge. We also consider the non-thermal appearance of SNRs interacting with molecular clouds.