Multi-epoch parsec-scale observations of the blazar PKS 1510-089

We investigate the flux density variability and changes in the pc-scale radio structure of the flat spectrum radio quasar PKS 1510-089. This source was the target of multi-epoch very long baseline interferometry (VLBI) and space-VLBI observations at 4.8, 8.4 and 22 GHz carried out between 1999 and 2001. Comparison of the pc-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 varied by about 50 per cent, while the scatter in the jet flux density was within 10 per cent. The polarization percentage of both the core and the jet components significantly changed from 2 to 9 per cent, while the polarization angle of the core showed an abrupt change of about 90°, 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 Very Long Baseline Array data at 15 GHz from the MOJAVE programme spanning a time-baseline from 1995 to 2010. Since 1995 jet components were ejected roughly once per year with the same position angle and an apparent speed between 15c and 20c, indicating that no jet precession has taken place on a time-scale 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 optically-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 γ-ray emission from this source observed by the AGILE and Fermi satellites, we investigated the connection between the radio and γ-ray activity during 2007-10. Multiwavelength flux and polarization observations suggest that during some γ-ray-flaring episodes the emission at high (γ-ray) and low (radio) energies has origin in the same region.