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

TL;DR

This study uses a decade of VLBI observations at two frequencies to measure the proper motions and spectral properties of radio-source cores in the S5 polar cap sample, revealing core drifts and potential complex source structures.

Contribution

It provides high-precision proper motion measurements of radio cores over ten years and investigates spectral shifts, challenging the assumption of core stability in AGN models.

Findings

Core motions of 0.1-0.9 mas over a decade.

Core drifts of a few tens of micro-arcseconds per year.

Possible evidence of jet-ISM interaction, gravitational lensing, or binary black holes.

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 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.