Opacity in parsec-scale jets of active galactic nuclei: VLBA study from
  1.4 to 15 GHz
  In extragalactic jets, the apparent position of the bright/narrow end (the
core) depends on the observing frequency, owing to synchrotron self-absorption
and external absorption. The effect must be taken into account in order to
achieve unbiased results from multi-frequency VLBI data on AGN jets.
Multi-frequency core shift measurements supplemented by other data enable
estimating the absolute geometry and a number of fundamental physical
properties of the jets and their environment. We have previously measured the
shift between 13 and 3.6 cm in a sample of 29 AGNs to range between 0 and 1.4
mas. In these proceedings, we present and discuss first results of our
follow-up study using VLBA between 1.4 and 15.4 GHz.
