A new Method for Verification of Astronomical Spectral Flux Density Calibrators based on SRT Data

The RadioAstron is an international project of a ground-space VLBI at the wavelengths 92, 18, 6.2 and 1.35 cm. The SRT has a paraboloid antenna with a 10 m diameter. It had been launched on July 18, 2011 and worked in flight up to January 10, 2019 together with many ground-based radio telescopes. The base of the VLBI reached 250 000 km. The results of mass processing of radiometric observations obtained with the RadioAstron Space Radio Telescope (SRT) are presented. A new complex for automated processing of SRT calibration sessions in the single telescope mode has been designed. The main and backup noise signal generators as secondary calibration standards in the wavelength ranges of 6.2, 18 and 92 cm in the calibration sessions for the 4 years of SRT flight in 2015–2019 are calibrated relative to the known primary flux density standard sources - Cassiopeia-A (Cas A) and the Crab Nebula (Crab). Calibration was carried out using the 1977 generally accepted spectral flux density scale. One of the most important and unexpected results of processing is the detection of a 25–35% discrepancy between the noise generator calibrations relative to Crab and Cas A. The analysis showed that the reason for this discrepancy is a real long-term variability of Cas A and Crab, which does not correspond to the known extrapolated flux densities in the standard scale of 1977. The transition from the generally accepted flux density scale of 1977 to the new corrected scales of 2017 and 2014 eliminates the discrepancy found. Based on the analysis above the report proposes a new method for verifying astronomical spectral flux density calibrators. A noise generator of a telescope is used as an indicator. The SRT calibration example is extended to most ground-based radio telescopes, also possibly including the RATAN-600.