Supernovae and supernova remnants at low frequencies

Abstract

The intrinsic radio emission from supernovae and their remnants is still poorly understood so that observations over the broadest possible frequency range can be very useful. An example of a peculiarity at low frequencies is the 38 MHz “flare” observed from Cas A in the mid-1970's. Such events could provide the key to understanding the radio emission from a remnant like Cas A.

The radio emission from supernovae is so strongly free-free absorbed by circumstellar gas that explosions in a dense wind may never be detectable at low frequencies. However, for explosions in a lower density wind it may be possible to follow the absorption over a large range of shock front radii. Free-free absorption can also affect the radio emission from supernova remnants. Absorption local to the remnants can occur in HII regions created by the supernovae or their progenitors and in the cooling layers of radiative shock fronts. Alternatively, absorption can occur in the ionized interstellar medium. Observations of emission as well as absorption are probably necessary to determine the location of the ionized gas.