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) and the distance of the radio core to the jet origin (r), can be calculated from these measurements. For the source Mrk 501, we find a magnetic field strength of 0.15± 0.04 G in the 8.4-GHz core at a distance of 0.8±0.2 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 Bcore(4.8 GHz) = 0.11±0.02 G and rcore(4.8 GHz) = 20±5 pc. The phasereferencing observations of this source at 8.4 and 43 GHz by Jiménez-Monferrer et al. (2008) imply Bcore(43 GHz) = 1.0±0.4 G and rcore(43 GHz) = 2.0±0.9 pc.