VLBI monitoring of two distant quasars as a showcase for 'EVN Lite'
Two prominent quasars at extremely high redshifts, J0906+6930 (z=5.47) and J2102+6015 (z=4.57) have been monitored with an ad-hoc array of five very long baseline interferometry (VLBI) stations (Badary, Svetloe, Zelenchukskaya, Yebes, and Sheshan) at 2.3 and 8.6 GHz in the past two years. The primary purpose of this series of 24-hour astrometric-style experiments was to investigate possible changes of the accurate source positions over time. Moreover, the apparent non-variability of the objects allowed us to combine data obtained at multiple epochs and produce the best-quality VLBI images of the quasars at these observing frequencies available so far. Here we present the imaging results. We argue that this type of observations could be attractive for the proposed `EVN Lite' concept in the future where a subset of the telescopes in the European VLBI Network (EVN) would observe more frequently than the regular EVN sessions.