Measurements of the flux densities and variability of the standard flux scale ''Artificial Moon'' sources on time scales of 1-8.5 years

We present flux density measurements for the standard radio sources of the ``Artificial Moon'' (AM) absolute flux scale during the time interval of 2017.5–2024.0 at wavelengths $\lambda=3.5$ and $6.2$ cm. The standard sources are weakly variable, and maintaining the accuracy of the scale requires regularly recurring calibrations. Currently, the RT-32 radio telescope at the Svetloe Observatory of the Institute of Applied Astronomy of the Russian Academy of Sciences (IAA RAS) monitors the AM flux scale standard sources to study their variability and maintain the accuracy of the scale. We determined the flux densities of the AM flux scale calibrators at two wavelengths during the time interval of 2017.5–2024.0. The measurement errors for both wavelengths did not exceed $\pm2$%, the measurement data were averaged over time intervals of 1 year at the average epochs 2019.0, 2022.0, and 2023.0. A comparison of the standard source flux densities $S$ for the epochs 2019.0, 2022.0, and 2023.0 with each other as well as with the flux densities $S_0$ at the epoch 2015.5 has shown that at time intervals of more than 1 year, there exists variation in the standard source flux densities that exceeds the standard deviations $\sigma$ and the established flux scale error limits of $\pm3$%. We conclude that it is necessary to recalibrate the standard sources at least every 2 years to maintain the accuracy of the flux scale.