Secular Decrease and Random Variations of Cassiopeia A at 151.5 and 927 MHz

Abstract

Long-term measurements of the radio flux density of Cassiopeia A relative to Cygnus A have been carried out at 927 and 151.5 MHz. It was found the following mean secular decrease rates of the radio emission of Cassiopeia A: $(0.72\pm 0.03)% \hbox{year}^{-1}$ at 927 MHz (for the period 1977--2002) and $(0.88\pm 0.09)% \hbox{year}^{-1}$ at 151.5 MHz (for the period 1980--2002). These values of the secular decrease rate obtained over the period of the last 25 years are substantially less than those of Baars et al. (1977). This indicates to the slowing down of Cassiopeia A radio emission secular decrease. In addition to this large scale time variation of Cassiopeia A flux density the measurements have also shown a small scale (a few years) time variations over the smooth secular decrease.