Higher Resolution VLBI Imaging with Fast Frequency Switching
  Millimetre-VLBI is an important tool in AGN astrophysics, but it is limited by short atmospheric coherence times and poor receiver and antenna performance. We demonstrate a new kind of phase referencing for the VLBA, enabling us to increase the sensitivity in mm-VLBI by an order of magnitude. If a source is observed in short cycles between the target frequency, nu_t, and a reference frequency, nu_ref, the nu_t data can be calibrated using scaled-up phase solutions from self-calibration at nu_ref. We have demonstrated the phase transfer on 3C 279, where we were able to make an 86 GHz image with 90 % coherence compared to self-calibration at nu_t. We have detected M81, our science target in this project, at 86 GHz using the same technique. We describe scheduling strategy and data reduction. The main impacts of fast frequency switching are the ability to image some of the nearest, but relatively weak AGN cores with unprecedented high angular resolution and to phase-reference the nu_t data to the nu_ref core position, enabling the detection of possible core shifts in jets due to optical depth effects. This ability will yield important constraints on jet properties and might be able to discriminate between the two competing emission models of Blandford-Konigl jets and spherical advection-dominated accretion flows (ADAFs) in low-luminosity AGNs. 
E. Middelberg (MPIfR), A. L. Roy (MPIfR), R. C. Walker (NRAO), H.   Falcke (MPIfR), T. P. Krichbaum (MPIfR)