Significant core shift variability in parsec-scale jets of active galactic nuclei

Abstract

The apparent position of jet base (core) in radio-loud active galactic nuclei changes with frequency because of synchrotron self-absorption. Studying this 'core shift' effect enables us to reconstruct properties of the jet regions close to the central engine. We report here results from core shift measurements in AGNs observed with global VLBI at 2 and 8 GHz at epochs from 1994 to 2016. Our sample contains 40 objects observed at least 10 times during that period. The core shift is determined using a new automatic procedure introduced to minimize possible biases. The resulting multiple epoch measurements of the core position are employed for examining temporal variability of the core shift. We argue that the core shift variability is a common phenomenon, as established for 33 of 40 AGNs we study. Our analysis shows that the typical offsets between the core positions at 2 and 8 GHz are about 0.5 mas and they vary in time. Typical variability of the individual core positions is about 0.3 mas. The measurements show a strong dependence between the core position and its flux density, suggesting that changes in both are likely related to the nuclear flares injecting denser plasma into the flow. We determine that density of emitting relativistic particles significantly increases during these flares, while relative magnetic field changes less and in the opposite direction.

Constraining the photon coupling of ultra-light dark-matter axion-like particles by polarization variations of parsec-scale jets in active galaxies

“…[42,49]. Because of variable optical depth [50], Faraday depth and higher turbulence [51], the core components demonstrate, on average, larger EVPA fluctuations [47]. In this work, we therefore concentrate only on the jet features.…”

Inferring the jet parameters of active galactic nuclei using Bayesian analysis of VLBI data with a non-uniform jet model

“…This assumption is weaker than the equipartition assumption. However, it could be violated during flares (Lobanov & Zensus 1999;Lisakov et al 2017;Plavin et al 2019), when region of e.g. higher plasma density is travelling along the jet.…”

“…At the same time even these simplified approaches need measurements of the core shifts. However core shift estimates obtained using the fitting of the VLBI core with a Gaussian are biased with bias that depends on resolution (Plavin et al 2019) and observed jet parameters (Pashchenko et al, in prep.). Finke (2019) used analytical expressions for flux density and core shift of the Blandford & Königl (1979) model to estimate physical jets properties.…”

Realization of a multifrequency celestial reference frame through a combination of normal equation systems

“…This is because the apparent position of the jet base (core) in radio-loud active galactic nuclei changes with frequency due to synchrotron selfabsorption. Plavin et al (2019b) investigated 40 AGN and determined typical offsets between the core positions at 2 and 8 GHz of about 0.5 mas with a variability of the individual core positions of about 0.3 mas over ten years. At higher frequencies the core shift is less pronounced, as it follows in many cases a negative exponential distribution, leading to a core shift between X and K band of about 0.1 mas (Hada et al 2011).…”

“…We are aware that there are issues related to core shift (Plavin et al 2019b), which may cause the matching procedures of identical catalogs at different frequency bands to become insufficient for a variety of candidates. However, in this publication, we demonstrate that the effect is of a random nature for catalog combinations.…”