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 ≈320-year-old supernova remnant Cassiopeia A, relative to the flux density of Cygnus A, made between 1995 and 1999. When compared to measurements made between 1957 and 1976, we find that the rate at which Cassiopeia A has been fading at this and nearby frequencies has changed from ≈0.9% yr-1 in the 1960s to ≈0.6%-0.7% yr-1 now. Furthermore, we have collected from the literature measurements of this fading rate at lower (38-300 MHz) and higher (7.8-16.5 GHz) frequencies. We show that the fading rate has dropped by a factor of ≈3 over the past 50 years at the lower frequencies, while remaining relatively constant at the higher frequencies, which is in agreement with the findings of others. Our findings at 1405 MHz, in conjunction with a measurement of the fading rate at the nearby frequency of 927 MHz by Vinyajkin, show an intermediate behavior at intermediate frequencies. We also find that Cassiopeia A, as of ≈1990, was fading at about the same rate, ≈0.6%-0.7% yr-1, at all of these frequencies. Future measurements are required to determine whether the fading rate will continue to decrease at the lower frequencies, or whether Cassiopeia A will now fade at a relatively constant rate at all of these frequencies.