Nonstationary Phenomena in the Radiation of Young Supernova Remnants

Abstract

The time variation of the radio emission from the supernova remnants Cassiopeia A, the Crab Nebula and Tycho Brahe (SN 1572) is investigated. There is a frequency dependence on the rate of decrease in the flux density of Cassiopeia A for the period 1957 to 1984. The (positive) spectral index has a secular decrease and also, for frequencies above 320 MHz, slight oscillations with a six-year period. The radio emission from the Crab Nebula was constant from 1953 to 1975 but has since decreased accompanied by a change in spectral index. The average decrease of the flux density of SN 1572 from 1963 to 1983 was 0·52% per year, close to the value predicted for adiabatic expansion.