Flux monitoring at VGOS frequencies with the Onsala Twin Telescopes

The VLBI Global Observing System (VGOS) for geodesy and astrometry has been in operation for several years. However, many of the sources of the International Celestial Reference Frame (ICRF3) still have poorly known radio flux densities in the currently observed VGOS frequency bands. To improve the efficiency of observation scheduling, an accurate flux density catalog is urgently needed. We present the results of flux monitoring experiments performed with the Onsala Twin Telescopes (OTT) on a rather regular basis since June 2021. The telescopes have a diameter of 13.2 m and are located at Onsala, Sweden, forming an interferometer with a baseline of 75 m distance. We present time series of flux density in the standard VGOS bands that are used so-far for the VGOS Operational (VO) sessions, i.e. 3.0–3.5 GHz, 5.2–5.7 GHz, 6.3–6.8 GHz and 10.2–10.7 GHz, for over 200 sources, most of which are defining sources of the ICRF3. The first set of flux monitoring experiments, conducted between June and December 2021, were a pilot study focusing on seven radio sources from the standard VGOS catalog and three calibrators (3C147, 3C286 and 3C295). Later experiments extended the source list to all ICRF3 defining sources, of which approximately 200 are visible from Onsala. In addition to these dedicated flux monitoring experiments with the OTT, we also analysed OTT data from several VO sessions. These VO observations have been calibrated by adding calibrator scans with the OTT to the end of the VO-schedules. All data were analyzed using Common Astronomy Software Applications (CASA). Due to local radio frequency interference at Onsala, few radio sources have reliable flux density values in the lowest VO-band (3.0–3.5 GHz). Also VO-band-2 (5.2–5.7 GHz) is somewhat affected by disturbing interference. Most sources are found to be variable over time. We estimate an uncertainty of 5 % for our flux density results. In the future, we plan to extend the method to include other stations, allowing to obtain flux densities for longer baselines as well.