Broad band flux-density monitoring of radio sources with the Onsala twin
  telescopes
  Context and aims: 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 aim to monitor 10 bright radio sources to search for flares or similar
activity, and to verify the instrument calibration on long (months) and short
(hours) time scales. Method: We observed and analysed 91 short (<30 min)
sessions spanning 7 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). Results: We obtain 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.
Conclusions: After correcting for instrumental biases, we determine the
empirical flux density uncertainty as $\sim$5%. Future refined analysis methods
will allow regular monitoring of more and fainter sources.
