Broad band flux-density monitoring of radio sources with the Onsala twin telescopes

The Onsala twin telescopes (OTT) are two 13 m telescopes located at the Onsala Space Observatory in Sweden. With dual linear polarized broad-band (3-14 GHz) receivers, they are part of the next generation Very Long Baseline Interferometry (VLBI) Global Observing System (VGOS) for geodesy and astrometry. In addition to purely geodetic data products, VGOS will regularly produce full-polarisation images of hundreds of radio sources. These rich monitoring data will be valuable for both astronomy and geodesy. In this pilot study we aimed at monitoring ten bright radio sources to search for flares or similar activity, and at testing the instrument calibration on long (months) and short (hours) time scales. Using the OTT as a standalone instrument, we observed and analysed 91 short (<30 min) sessions spanning seven months. We monitored seven potentially variable radio sources (0059+581, 0552+398, 1144+402, 1156+295, 1617+229, 3C418, OJ287) and three reference calibrators (3C147, 3C286, 3C295). We used the Common Astronomy Software Applications (CASA) package to fringe-fit, bandpass-correct and scale the data to obtain flux densities in the four standard VGOS bands: 3.0-3.5 GHz (band 1), 5.2-5.7 GHz (band 2), 6.3-6.8 GHz (band 3), and 10.2-10.7 GHz (band 4). We obtained simultaneous multi-frequency light curves for ten radio sources. A bright multi-frequency flare is observed in the radio source 0059+581. OJ287 and 1156+295 show significant long-term variability in their light curves. After correcting for instrumental biases, we determine the empirical flux density uncertainty as ∼ 5