Expanding Supernova Remnants and Galactic Radio Sources

Abstract

The effect of the expansion of the envelope ejected by a supernova on the interstellar medium is examined. The one-dimensional expansion and hydromagnetic shock propagation is treated first, to investigate the effect of the three constituents (the ordinary “thermal” gas, the cosmic-ray gas and the magnetic field) of the interstellar medium on its compression behind the shock front. A simplification is introduced in the form of an “isothermal” approximation, which is justified a posteriori and this model is generalized for the case of a spherical expansion. It is shown that such an expansion gives rise to a compressed region outside the ejected envelope and that this region acts as a synchrotron radiation source which, in the case of some galactic supernovae, is observable. The theory is applied in some detail to parts of the Cygnus Loop and, less specifically, to IC 443. The results are compared with observations, and satisfactory agreement is found between calculated and measured flux densities. From the applications it is concluded that a significant proportion of the non-thermal radiation from some supernova remnants is emitted in regions formed by the compression process. This process is shown to provide a direct explanation for the well-known near-coincidence of regions of enhanced radio emission with some conspicuous optical features in the Cygnus Loop. The theory predicts a relation between the power frequency spectrum of sources formed by this process and that of the non-thermal galactic background radiation which can be checked by observation.