Radio spectra and `light curves'

Chevalier (1982, 1984) presents a model for the radio emission from Type II (and Type Ib) supernovae in which the rapidly-moving supernova ejecta interacts with a slow-moving wind from pre-supernova mass loss. Such a model can reproduce much of the observed late-time evolution of the radio flux density and spectral index (though recent results for SN 1993J (van Dyk et al. 1994) show that the Chevalier model does not adequately describe the early behaviour of radio light curves for this supernova, perhaps due to variations in the mass-loss rate and clumping in the stellar wind, which are not yet accounted for in the model).

The radio emission usually peaks later, and is stronger, at lower frequencies (Weiler et al. 1986), so most RSNe have a steep radio spectrum at late times and are most easily detected at frequencies below 1GHz.

With two or more radio images of a galaxy taken at different epochs, it is possible to search for RSNe using a template subtraction method analogous to the `plate blinking' method used in optical supernova searches. The main difference (Figure 1) is that the optical decay time is only a few months, while the radio decay time is many years. An optical supernova search requires that each target field be observed at least every few weeks to be sure of detecting any supernova which appears. The much longer decay time in the radio, however, means that observations a few years apart are all that is needed for complete coverage. For example, Ryder et al. (1993) were able to get a reasonable radio `light curve' spanning 10 years for SN1978K from two archival images of NGC1313 taken in 1982 and 1986, plus a third observation made in 1992.

  
Figure 1: A comparison of the expected optical (B) and radio (843 MHz) light curves for a radio supernova. L/L is the ratio of the observed luminosity to that at maximum. The optical line shows the mean type IIP supernova light curve tabulated by Doggett & Branch (1985), while the radio light curve is based on the model calculated for SN 1986J by Weiler et al. (1996).

© Copyright Astronomical Society of Australia 1997