Radio spectrum variations in supernova remnants

Supernova remnants are prominent sources of astronomical radio emission within our Galaxy. The mechanism is synchrotron radiation from relativistic electrons and the observed spectrum depends on the relativistic electron spectrum and propagation effects from the source to the observer. Here the electron spectrum and radio absorption effects are discussed and compared with recent observations of galactic supernova remnants.

I. Introduction

The radio emission from supernova remnants (SNR) is caused by synchrotron emission from relativistic electrons moving in a magnetic field. We give an idea of the electron energies involved for a typical galactic magnetic field of , for example, for three commonly used observing frequencies: electrons with 2.9 GeV energy will have their peak radio emission at a frequency 408 MHz; electrons of 5.4 GeV energy will have peak emission at 1420 MHz; and electrons of 7.5 GeV energy will have peak emission at 2695 MHz. The relativistic electrons are generally understood to been accelerated by the Fermi mechanism in fast moving shock fronts. For older SNR, it is not yet known whether the mechanism for producing the relativistic electrons is local acceleration at a shock front, or compression of the existing population of galactic background relativistic electrons, which have been accelerated in shocks from the general population of SNR.