Parsec-Scale Blazar Monitoring: Proper Motions
  We present proper motions obtained from a dual frequency, six-epoch, VLBA polarization experiment monitoring a sample of 12 blazars. The observations were made at 15 GHz and 22 GHz at bi-monthly intervals over 1996. Ten of the eleven sources for which proper motion could be reliably determined are superluminal. Only J2005+77 has no superluminal components. Three sources (OJ287, J1224+21, and J1512-09) show motion faster than 10h^{-1}c, requiring $\gamma_{pattern}$ of at least 10h^{-1}c (H_0 = 100h km/s/Mpc). We compare our results to those in the literature and find motions outside the previously observed range for four sources. While some jet components exhibit significant non-radial motion, most motion is radial. In at least two sources there are components moving radially at significantly different structural position angles. In five of six sources (3C120, J1224+21, 3C273, 3C279, J1512-09, and J1927+73) that have multiple components with measurable proper motion, the innermost component is significantly slower than the others, suggesting that acceleration occurs in the jet. In the motions of individual components we observe at least one decelerating motion and two ``bending'' accelerations which tend to align their motions with larger scale structure. We also discuss in detail our procedures for obtaining robust kinematical results from multi-frequency VLBI data spanning several epochs. 
D. C. Homan (1), R. Ojha (1), J. F. C. Wardle (1), D. H. Roberts (1),   M. F. Aller (2), H. D. Aller (2), and P. A. Hughes (...