http://arxiv.org/abs/2203.08590
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. The VGOS network is currently being rolled out globally, and regularly observes hundreds of quasars. In contrast to legacy S/X-band geodetic VLBI, VGOS will eventually – in addition to purely geodetic data products – also produce full-polarisation images and flux densities of all observed radio quasars on a regular basis. This will be a rich archive of broad-band quasar monitoring information, of value both for geodesy and astronomy.
We scheduled, correlated and analysed 91 short (<30 min) observing sessions with the OTT spanning 7 months, with 33 sessions placed within a 3 day period to search for short-term variability. We monitored seven quasars (0059+581, 0552+398, 1144+402, 1156+295, 1617+229, 3C418, OJ287) and three absolute flux-density reference calibrators (3C147, 3C286, 3C295). We used the Common Astronomy Software Applications (CASA) package to fringe-fit, bandpass-correct and scale (using auto-correlations, antenna gains and measured system temperatures) the data to obtain simultaneous flux densities in the four standard VGOS bands: A = 3.0-3.5 GHz, B= 5.2-5.7 GHz, C=6.3-6.8 GHz, and D=10.2-10.7 GHz.
After correcting for instrumental biases, we find the OTT capable of monitoring flux densities with 5 % uncertainty in all four VGOS bands for Jansky-level sources. Three quasars show significant variability during our observing campaign: 0059+581, OJ287 and 1156+295. We also find a tentative elevation-dependence of about 5 %, suggesting that more detailed characterisation of the antenna gain curves could further improve the accuracy of the monitoring.
E. Varenius, F. Maio, K. Bail, et. al.
Thu, 17 Mar 22
11/66
Comments: 10 pages, 5 figures. Submitted to Astronomy & Astrophysics