Multi-Frequency VLBA Polarimetry and the Twin-Jet Quasar 0850+581
  We present the first multi-frequency VLBA study of the quasar 0850+581 which
appears to have a two-sided relativistic jet.Apparent velocity in the
approaching jet changes from 3.4c to 7c with the separation from the core. The
jet-to-counter-jet ratio of about 5 and apparent superluminal velocities
suggest that the observing angle of the inner jet is $\leq33^\circ$. It is
likely that this orientation significantly changes downstream due to an
interaction of the jet with the surrounding medium, signs of this are seen in
polarization. A dense inhomogeneous Faraday screen is detected in the innermost
regions of this quasar. We suggest that there is a presence of ionized gas in
its nucleus, which might be responsible for the free-free absorption of the
synchrotron emission in the jet and counter-jet at frequencies below 8.4~GHz.
The experiment makes use of slowly varying instrumental polarisation factors
(polarization leakage or D-terms) in time. We report application of the "D-term
connection" technique for the calibration of an absolute orientation of
electric vector position angle (EVPA) observed by VLBA at 4.6, 5.0, 8.1, 8.4,
15.4, 22.3, and 43.3 GHz bands during the 2007--2011.
