Magnetic field strength and spectral distribution of six parsec-scale active galactic nuclei jets

We use observations of six 'blazars' with the Very Long Baseline Array (VLBA), at eight frequencies (4.6, 5.1, 7.9, 8.9, 12.9, 15.4, 22.2 and 43.1 GHz), to investigate the frequency-dependent position of their VLBI cores ('core-shift') and their overall jet spectral distribution. By cross-correlating the optically thin jet emission, we are able to accurately align the multifrequency images of three of the jets (1418+546, 2007+777, 2200+420), whose core-shifts and spectra we find consistent with the equipartition regime of the Blandford & Konigl conical jet model, where the position of the radio core from the base of the jet follows r(core)(nu) proportional to nu-1. For the jet of 0954+658, we align the higher frequency images using our lower frequency measurements assuming equipartition in the radio core from 4.6-43 GHz. The jet emission of the other two sources in our sample (1156+295, 1749+096) is too sparse for our alignment technique to work. Using our measured core-shifts, we calculate equipartition magnetic field strengths of the order of 10s to 100s of mG in the radio cores of these four AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disc and black hole jet-launching distances, we find magnetic field strengths consistent with those expected from theoretical models of magnetically powered jets.