The Evolution of Cas A at Low Radio Frequencies
  We have used archival 74 MHz VLA data spanning the last 15 years in
combination with new data from the Long Wavelength Demonstrator Array (LWDA)
and data from the literature covering the last 50 years to explore the
evolution of Cas A at low radio frequencies. We find that the secular decrease
of the flux density of Cas A at ~80 MHz is rather stable over five decades of
time, decreasing at a rate of 0.7-0.8% yr^-1. This is entirely consistent with
previous estimates at frequencies as low as 38 MHz, indicating that the secular
decrease is roughly the same at low frequencies, at least between 38 and 80
MHz. We also find strong evidence for as many as four modes of flux density
oscillation about the slower secular decrease with periods of 3.10+/-0.02$ yr,
5.1+/-0.3 yr, 9.0+/-0.2 yr, and 24+/-2 yr. These are also consistent with
fluctuations seen previously to occur on scales of a few years. These results
provide compelling motivation for a thorough low frequency monitoring campaign
of Cas A to constrain the nature and physical origins of these fluctuations,
and to be able to better predict the flux density of Cas A at any given epoch
so that it may be used as a reliable low frequency calibrator.
