THE EVOLUTION OF CASSIOPEIA A AT LOW RADIO FREQUENCIES

We have used archival 74 MHz Very Large Array data spanning the last 15 years in combination with new data from the Long-Wavelength Demonstrator Array 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.