The fading of supernova remnant Cassiopeia A from 38 MHz to 16.5 GHz from 1949 to 1999 with new observations at 1405 MHz

We report 1405 MHz measurements of the flux density of the approximate to 3 20-year-old supernova remnant Cassiopeia A, relative to the flux density of Cygnus A, made between 1995 and 1999. When compared to measurements made b etween 1957 and 1976, we find that the rate at which Cassiopeia A has been fading at this and nearby frequencies has changed from approximate to 0.9% yr(-1) in the 1960s to approximate to 0.6%-0.7% yr(-1) now. Furthermore, we have collected from the literature measurements of this fading rate at low er (38-300 MHz) and higher (7.8-16.5 GHz) frequencies. We show that the fad ing rate has dropped by a factor of approximate to 3 over the past 50 years at the lower frequencies, while remaining relatively constant at the highe r frequencies, which is in agreement with the findings of others. Our findi ngs at 1405 MHz, in conjunction with a measurement of the fading rate at th e nearby frequency of 927 MHz by Vinyajkin, show an intermediate behavior a t intermediate frequencies. We also find that Cassiopeia A, as of approxima te to 1990, was fading at about the same rate, approximate to 0.6%-0.7% yr( -1), at all of these frequencies. Future measurements are required to deter mine whether the fading rate will continue to decrease at the lower frequen cies, or whether Cassiopeia A will now fade at a relatively constant rate a t all of these frequencies.