A new method for accurate calibration of solar disk emission in the radio band

Abstract

The SunDish project aims to map and monitor the Sun at radio frequencies with the Sardinia and Medicina Italian Radio telescopes by performing single-dish imaging observations in K-band (18-26 GHz) at present. In the future it will even be possible to exploit Ka (32.5 GHz) and X-band (8.5-9 GHz) simultaneous observations and go up to 100 GHz in order to fill the observational gap in literature and contributing to Space Weather networks. This technical note is focusing on the original and innovative procedure for the absolute calibration of the Sun maps that we developed for the SunDish project that adopted the Supernova Remnant Cassiopeia A as a flux reference. This is a challenging procedure since this calibrator is an extended source (resolved at our frequencies) and it is currently in expansion. However, due to its strong flux, Cassiopeia A presents several advantages compared to other standard calibrators and its brightness variation is well studied in literature. We found that this Supernova Remnant is a well suited and very reliable calibrator for solar radio observations that require high signal attenuation in the instrumentation chain and consequently strong flux calibrators. Thanks to the Cassiopeia A visibility (circumpolar source) at the telescopes latitude, it is possible to obtain accurately calibrated Sun maps throughout the day with typical errors <3% in K-band.