EVOLUTION OF LOW-FREQUENCY RADIO ABSORPTION IN 3C461

Using the highly accurate and sensitive observations from July 13 to October 13, 2019, with the Giant Ukrainian Radio Telescope (GURT) in the correlation interferometer mode, we have investigated a monotonic trend of free-free absorption parameters from the absolute integrated spectrum of 3C461 (Cassiopeia A) measured at low frequencies. The form and peak of this spectrum depend on the magnitudes of the emission measure, the electron temperature, and the average number of charges of the ions for the internal and external absorbing ionized gas toward the supernova remnant (SNR). The most interesting information concerns the evolution of unshocked ejecta inside the SNR. Its emission measure, average number of charges of the ions, and temperature can change with time, and the absorption on the two halves of the shell indicates how the ejecta are heating up. The study of the unshocked ejecta is a requisite step toward a better understand- ing of radio absorption evolution in Cassiopeia A. The trends from the GURT radio data were analyzed using the Mann–Kendall test and Sen’s method. This analysis is the first attempt to detect changes in the absorption parameters inside and outside this evolving SNR using continuous, broadband, and highly sensitive observations. We took into account the possible influence of radio interference on the trend detection results. Our study shows no trend in the absorption parameters. A possible reason for this result could be either the relatively short observation interval, together with the very slow change in absorption parameters, or the uneven nature of the changes in the evolu- tion of absorption. Further measurements of the absolute flux-density spectrum obtained from Cassiopeia A with the help of low-frequency correlation interferometers (implemented on GURT, URAN-2, NenuFAR, and other appropriate radio telescopes) may contribute to the elucidation of the astrophysical processes responsible for thermal absorption in the SNR.