A new Method for Verification of Astronomical Spectral Flux Density Calibrators based on SRT Data
September 20, 2022
The RadioAstron is the international project of a ground-space VLBI at the wavelengths of 92, 18, 6.2 and 1.35 cm. The SRT has a paraboloid antenna with diameter 10 m. It had been launched on 18 July 2011 and worked in the flight up to 10 Janury 2019 together with many ground radio telescopes. The base of the VLBI was up to 250 000 km. The results of mass processing of radiometric observations on the RadioAstron Space Radio Telescope (SRT) data are presented. A new complex for automated processing of calibration sessions of SRT 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 calibration sessions for 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 discrepancy of 25–35% between the noise generators calibrations relative to Crab and Cas A. The analysis showed that the reason for this discrepancy is the real long-term variability of Cas A and Crab, which does not correspond to known the 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 of 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, including, possibly, the RATAN-600 also.
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