Most of the supernova remnants known in the Galaxy have only been detected at radio frequencies. The reason for this is absorption in the Galactic plane at both optical and X-ray wavelengths. All available evidence suggests that the shock fronts which accompany supernova remnants accelerate enough cosmic rays to GeV energies to produce readily detectable radio emission. This is fortunate, for it enables us to study remnants throughout the Galactic disk, although existing catalogues may be anywhere from 50 to 90 % incomplete. Cosmic rays and the magnetic fields in which they gyrate are the essential ingredients for producing the synchrotron radiation which is observed at radio frequencies. Various methods for estimating magnetic field strengths can be applied to a small number of remnants, and produce values not far from those based upon equipartition between the energy contents of particles and fields. From this, the particle energy content is derived for a number of objects.

Introduction

If we could view the heavens with radio eyes, then the majestic sweep of the Galactic disk would dominate the large scale structure we see. This radio version of the Milky Way has greater symmetry about a more dominant Sagittarius than its optical counterpart, as can be seen in the fine 408 MHz map of Haslam et al (1982). The main reason for this difference is simply dust, for although both stars and the cosmic rays responsible for decimeter radio emission are similarly distributed in the Galaxy, great clouds of the stuff obscure our optical vision, especially toward the Galactic center.