AGN jet physics from measurements of the frequency-dependent position of   the VLBI radio core
  Accurate measurement of the frequency-dependent shift of the self-absorbed radio core is required for multi-frequency analysis of VLBI data since absolute positional information is lost as a result of phase self-calibration. We use the cross-correlation technique of Croke & Gabuzda (2008) on the optically thin jet emission to align our VLBA images. Our results are consistent with those obtained from the phase-referencing method, as well as alignment by model-fitted optically thin jet components. Physical parameters of the compact jet regions, such as the magnetic field strength (B_core) and the distance of the radio core to the jet origin (r_core), can be calculated from these measurements. For the source Mrk 501, we find a magnetic field strength of 0.15\pm0.04 G in the 8.4-GHz core at a distance of 0.8\pm0.2 pc from the base of the jet. By extrapolating our 4.6 to 15.4 GHz results for BL Lac (2200+420), we estimate magnetic field strengths of the order of 1 G in the millimetre VLBI core. Using our core-shift measurement between 1.6 and 4.8 GHz for 1803+784, we find B_core(4.8 GHz) = 0.11\pm0.02 G and r_core(4.8 GHz) = 20\pm5 pc. The phase-referencing observations of this source at 8.4 and 43 GHz by Jim\'enez-Monferrer et al. (2008) imply B_core(43 GHz) = 1.0\pm0.4 G and r_core(43 GHz) = 2.0\pm0.9 pc. 
Shane P. O'Sullivan and Denise C. Gabuzda