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^⋆

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: mivan@chalmers.se
² Departament d’Astronomia i Astrofísica, Universitat de València, C/Dr. Moliner 50, 46100 Burjassot, Spain
³ Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedrático José Beltrán 2, 46980 Paterna, València, Spain
⁴ Instituto de Astrofísica de Andalucía (IAA-CSIC), Apdo. 3004, 18080 Granada, Spain
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 17 January 2016
Accepted: 12 July 2016

Abstract

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 ten 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 μas 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.