A VLBA survey of the core shift effect in AGN jets I. Evidence for
  dominating synchrotron opacity
  The effect of a frequency dependent shift of the VLBI core position (known as
the "core shift") was predicted more than three decades ago and has since been
observed in a few sources, but often within a narrow frequency range. This
effect has important astrophysical and astrometric applications. To achieve a
broader understanding of the core shift effect and the physics behind it, we
conducted a dedicated survey with NRAO's Very Long Baseline Array (VLBA). We
used the VLBA to image 20 pre-selected sources simultaneously at nine
frequencies in the 1.4-15.4 GHz range. The core position at each frequency was
measured by referencing it to a bright, optically thin feature in the jet. A
significant core shift has been successfully measured in each of the twenty
sources observed. The median value of the core shift is found to be 1.21 mas if
measured between 1.4 and 15.4 GHz, and 0.24 mas between 5.0 and 15.4 GHz. The
core position, r, as a function of frequency, n, is found to be consistent with
an r n^-1 law. This behavior is predicted by the Blandford & Koenigl model of a
purely synchrotron self-absorbed conical jet in equipartition. No systematic
deviation from unity of the power law index in the r(n) relation has been
convincingly detected. We conclude that neither free-free absorption nor
gradients in pressure and/or density in the jet itself and in the ambient
medium surrounding the jet play a significant role in the sources observed
within the 1.4-15.4 GHz frequency range. These results support the
interpretation of the parsec-scale core as a continuous Blandford-Koenigl type
jet with smooth gradients of physical properties along it.
