Relative Astrometry of Compact Flaring Structures in Sgr A* with   Polarimetric VLBI
  We demonstrate that polarimetric interferometry can be used to extract precise spatial information about compact polarized flares of Sgr A*. We show that, for a faint dynamical component, a single interferometric baseline suffices to determine both its polarization and projected displacement from the quiescent intensity centroid. A second baseline enables two-dimensional reconstruction of the displacement, and additional baselines can self-calibrate using the flare, enhancing synthesis imaging of the quiescent emission. We apply this technique to simulated 1.3-mm wavelength observations of a "hot spot" embedded in a radiatively inefficient accretion disk around Sgr A*. Our results indicate that, even with current sensitivities, polarimetric interferometry with the Event Horizon Telescope can achieve ~5 microarcsecond relative astrometry of compact flaring structures near Sgr A* on timescales of minutes. 
Michael D. Johnson, Vincent L. Fish, Sheperd S. Doeleman, Avery E.   Broderick, John F. C. Wardle, and Daniel P. Marrone
astro-ph.HE astro-ph.IM