The Radio Spectra of Supernova Remnants

Abstract

The radio emission from supernova remnants (SNRs) is synchrotron radiation by relativistic electrons in magnetic fields. One electron will produce radiation in a series of harmonics of its cyclotron frequency around a magnetic-field line but these harmonics will smear into a continuum with the frequency of peak emission dependent upon the energy of the electron. The radio frequencies of interest are generally well above the peak frequency. A collection of electrons with a power law distribution in energy, N(E) - CE^–γ dE where N(E) is the number of electrons with energy E, will then produce continuum radiation with a power-law spectrum of the form S_f = const f^α, where S_f is the observed flux density (in Janskys where 1 Jy - 10^–26w m^–2 Hz^–1) at frequency f and the power, a, is called the spectral index where α = –(γ-l)/2. Typical values of α range between -0.8 and -0.3 for shell SNRs. This means that they are brighter at lower frequencies.