Absolute kinematics of radio-source components in the complete S5 polar
  cap sample. IV. Proper motions of the radio cores over a decade and spectral
  properties
  We have carried out a high-precision astrometric analysis of two
very-long-baseline-interferometry (VLBI) epochs of observation of the 13
extragalactic radio sources in the complete S5 polar cap sample. The VLBI
epochs span a time baseline of 10 years and enable us to achieve precisions in
the proper motions of the source cores up to a few micro-arcseconds per year.
The observations were performed at 14.4 GHz and 43.1 GHz, and enable us to
estimate the frequency core-shifts in a subset of sources, for which the
spectral-index distributions can be computed. We study the source-position
stability by analysing the changes in the relative positions of fiducial source
points (the jet cores) over a decade. We find motions of 0.1-0.9 mas among
close-by sources between the two epochs, which imply drifts in the jet cores of
approximately a few tens of micro-as per year. These results have implications
for the standard Active Galactic Nucleus (AGN) jet model (where the core
locations are supposed to be stable in time). For one of our sources, 0615+820,
the morphological and spectral properties in year 2010, as well as the relative
astrometry between years 2000 and 2010, suggest the possibility of either a
strong parsec-scale interaction of the AGN jet with the ISM, a gravitational
lens with ~1 mas diameter, or a resolved massive binary black hole.
