Flux density monitoring at VGOS frequencies with the Onsala Twin Telescopes

Paper in proceeding, 2025

An accurate flux density catalog at the frequencies observed by the VLBI Global Observing System (VGOS) is needed to improve the scheduling of geodetic experiments. We present the setup and results of flux monitoring experiments performed with the Onsala Twin Telescopes (OTT) on a rather regular basis since June 2021. We have obtained time series of flux density in the standard 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. The experiments targeted approximately 200 ICRF3 defining sources that are visible from Onsala. In addition to these dedicated flux monitoring experiments with the OTT, we also analyzed OTT data from several VO sessions. 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). Most sources are found to be variable over time. We estimate a systematic uncertainty of 5 % and a statistical uncertainty of on average 1 % for our flux density results. In the future, we are interested in expanding the sample size, increasing baseline sensitivity and running collaboration observations with other facilities.