Multi-epoch parsec-scale observations of the blazar PKS 1510-089
  (Abridged) We investigate the flux density variability and changes in the
parsec-scale radio structure of the flat spectrum radio quasar PKS 1510-089.
This source was target of multi-epoch VLBI and Space-VLBI observations at 4.8,
8.4 and 22 GHz carried out between 1999 and 2001. The comparison of the
parsec-scale structure observed at different epochs shows the presence of a
non-stationary jet feature moving with a superluminal apparent velocity of
16.2c+-0.7c. Over three epochs at 8.4 GHz during this period the core flux
density varies of about 50%, while the scatter in the jet flux density is
within 10%. The polarization percentage of both core and jet components
significantly change from 2 to 9 per cent, while the polarization angle of the
core shows an abrupt change of about 90 degrees becoming roughly perpendicular
to the jet direction, consistent with a change in the opacity. To complete the
picture of the physical processes at work, we complemented our observations
with multi-epoch VLBA data at 15 GHz from the MOJAVE programme spanning a time
baseline from 1995 to 2010. Since 1995 jet components are ejected roughly once
per year with the same position angle and an apparent speed between 15c and
20c, indicating that no jet precession is taking place on a timescale longer
than a decade in our frame. The variability of the total intensity flux density
together with variations in the polarization properties may be explained
assuming either a change between the optically-thick and -thin regimes produced
by a shock that varies the opacity, or a highly ordered magnetic field produced
by the compression of the relativistic plasma by a shock propagating along the
jet. Taking into account the high gamma-ray emission observed from this source
by the AGILE and Fermi satellites we investigated the connection between the
radio and gamma-ray activity during 2007-2010.
