The evolution of supernova remnants as radio sources

Abstract

The acceleration of relativistic electrons by hydromagnetic turbulence in shell-type supernova remnants (SNRs) is examined in relation to their structural development through interaction with the interstellar medium. The transport equation governing the energy spectrum of the electrons is analytically solved, enabling study of the evolution of their synchrotron radio emission. The sudden emergence of SNRs as long-lived radio sources, several decades after the supernova event, is then explained. Their subsequent radio evolution, with spectral changes and structural details as exemplified by the young remnant Cassiopeia A, follows naturally. The absence of younger radio remnants in the Galaxy implies a supernova rate lower than that inferred from extragalactic observations. The collective properties of galactic SNRs suggest a mild acceleration of electrons throughout the adiabatic phase of evolution. SNRs may thus contribute significantly to cosmic-ray electrons in the Galaxy.