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, 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 multi-frequency 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
$r_{core}\propto\nu^{-1}$. 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 10's to 100's of mG in the radio cores of these four
AGN from 4.6-43 GHz. Extrapolating our results back to the accretion disk and
black hole jet-launching distances, we find magnetic field strengths consistent
with those expected from theoretical models of magnetically powered jets.
