Based on five years of observations with the 40-foot telescope at Green Bank Observatory ( GBO ) , \citet r00 found that the radio source Cassiopeia A had either faded more slowly between the mid-1970s and late 1990s than \citet b77 had found it to be fading between the late 1940s and mid-1970s , or that it had rebrightened and then resumed fading sometime between the mid-1970s and mid-1990s , in L band ( 1.4 GHz ) .
Here , we present 15 additional years of observations of Cas A and Cyg A with the 40-foot in L band , and three and a half additional years of observations of Cas A , Cyg A , Tau A , and Vir A with GBO ’ s recently refurbished 20-meter telescope in L and X ( 9 GHz ) bands .
We also present a more sophisticated analysis of the 40-foot data , and a reanalysis of the \citet b77 data , which reveals small , but non-negligible differences .
We find that overall , between the late 1950s and late 2010s , Cas A faded at an average rate of 0.670 \pm 0.019 % /yr in L band , consistent with \citet r00 .
However , we also find , at the 6.3 \sigma credible level , that it did not fade at a constant rate .
Rather , Cas A faded at a faster rate through at least the late 1960s , rebrightened ( or at least faded at a much slower rate ) , and then resumed fading at a similarly fast rate by , at most , the late 1990s .
Given these differences from the original \citet b77 analysis , and given the importance of their fitted spectral and temporal models for flux-density calibration in radio astronomy , we update and improve on these models for all four of these radio sources .
In doing so , we additionally find that Tau A is fading at a rate of 0.102 ^ { +0.042 } _ { -0.043 } % /yr in L band .