A bias in VLBI measurements of the core shift effect in AGN jets
  The Blandford and K\"{o}nigl model of AGN jets predicts that the position of
the apparent opaque jet base - the core - changes with frequency. This effect
is observed with radio interferometry and is widely used to infer parameters
and structure of the innermost jet regions. The position of the radio core is
typically estimated by fitting a Gaussian template to the interferometric
visibilities. This results in a model approximation error, i.e. a bias that can
be detected and evaluated through simulations of observations with a realistic
jet model. To assess the bias, we construct an artificial sample of sources
based on the AGN jet model evaluated on a grid of the parameters derived from a
real VLBI flux-density-limited sample and create simulated VLBI data sets at
2.3, 8.1 and 15.4 GHz. We found that the core position shifts from the true jet
apex are generally overestimated. The bias is typically comparable to the core
shift random error and can reach a factor of two for jets with large apparent
opening angles. This observational bias depends mostly on the ratio between the
true core shift and the image resolution. This implies that the magnetic field,
the core radial distance and the jet speed inferred from the core shift
measurements are overestimated. We present a method to account for the bias.
