[Paper Review] MOJAVE: XVIII. Kinematics and Inner Jet Evolution of Bright Radio-Loud Active Galaxies

M. L. Lister, D. C. Homan+5 more|arXiv (Cornell University)|Aug 30, 2021

Astrophysics and Cosmic Phenomena1 references4 citations

TL;DR

This study analyzes the kinematics and inner jet evolution of 447 bright radio-loud AGN using 15 GHz VLBA data from 1994 to 2019, tracking 1,923 jet features over five or more epochs. It reveals that 60% of well-sampled features exhibit acceleration or non-radial motion, with 13 AGN showing systematic PA changes indicating precessing flow instabilities near the jet base, suggesting jet base dynamics are more complex than previously thought.

ABSTRACT

We have analyzed the parsec-scale jet kinematics of 447 bright radio-loud AGN, based on 15 GHz VLBA data obtained between 1994 August 31 and 2019 August 4. We present new total intensity and linear polarization maps obtained between 2017 January 1 to 2019 August 4 for 143 of these AGN. We tracked 1923 bright features for five or more epochs in 419 jets. A majority (60%) of the well-sampled jet features show either accelerated or non-radial motion. In 47 jets there is at least one non-accelerating feature with an unusually slow apparent speed. Most of the jets show variations of 10 to 50 deg in their inner jet position angle (PA) over time, although the overall distribution has a continuous tail out to 200 deg. AGN with SEDs peaked at lower frequencies tend to have more variable PAs, with BL Lacs being less variable than quasars. The Fermi LAT gamma-ray associated AGN also tend to have more variable PAs than the non-LAT AGN in our sample. We attribute these trends to smaller viewing angles for the lower spectral peaked and LAT-associated jets. We identified 13 AGN where multiple features emerge over decade-long periods at systematically increasing or decreasing PAs. Since the ejected features do not fill the entire jet cross-section, this behavior is indicative of a precessing flow instability near the jet base. Although some jets show indications of oscillatory PA evolution, we claim no bona fide cases of periodicity since the fitted periods are comparable to the total VLBA time coverage.

Motivation & Objective

• To investigate the long-term kinematic behavior of parsec-scale jets in bright radio-loud AGN using extended VLBA monitoring.
• To determine whether observed position angle (PA) variations in inner jets are due to intrinsic instabilities or viewing angle effects.
• To identify and characterize jet features with non-radial or accelerated motion, particularly those indicating precessing flows near the jet base.
• To examine correlations between jet PA variability, spectral energy distribution (SED) peak frequency, and Fermi LAT gamma-ray association.
• To improve kinematic modeling by incorporating new VLBA data up to 2019, refining previous fits and excluding poorly constrained sources.

Proposed method

• Utilized 15 GHz VLBA data spanning 25 years (1994–2019) for 447 bright radio-loud AGN, including 143 new total intensity and linear polarization maps from 2017–2019.
• Tracked 1,923 bright jet features across five or more epochs using model-fitting techniques to determine apparent speeds, position angles, and accelerations.
• Applied vector and acceleration models to distinguish between radial outflow and non-radial or accelerated motions in jet features.
• Used position angle (PA) evolution over time as a diagnostic for jet base instability, particularly systematic trends indicating precession.
• Correlated PA variability with SED peak frequency and Fermi LAT association to infer viewing angle effects and jet orientation.
• Excluded sources with uncertain core locations or unresolved structure (e.g., NGC 1052, compact symmetric objects) to ensure kinematic reliability.

Experimental results

Research questions

• RQ1 What fraction of well-sampled jet features in bright AGN exhibit non-radial or accelerated motion, and what does this imply about jet dynamics?
• RQ2 Are systematic changes in inner jet position angle (PA) over time indicative of precessing flow instabilities near the jet base?
• RQ3 How does the variability of inner jet PA correlate with the spectral energy distribution (SED) peak frequency and Fermi LAT gamma-ray association?
• RQ4 Can long-term PA evolution patterns be attributed to viewing angle effects, particularly for jets with lower SED peak frequencies?
• RQ5 What evidence exists for jet base instabilities, and how do they manifest in the kinematic evolution of multiple features over decades?

Key findings

• 60% of well-sampled jet features (1,923 features in 419 jets) exhibit either accelerated or non-radial motion, indicating complex dynamics beyond simple radial outflow.
• In 47 jets, at least one feature shows unusually slow apparent speed without acceleration, suggesting possible deceleration or complex flow structures.
• Most jets exhibit PA variations between 10° and 50° over time, with a continuous tail extending to 200°, indicating significant and variable jet orientation changes.
• Jets with lower SED peak frequencies and Fermi LAT gamma-ray associations show significantly higher PA variability, consistent with smaller viewing angles.
• Thirteen AGN display systematic PA trends in successive features—increasing or decreasing over time—indicating a precessing flow instability near the jet base.
• No definitive periodicity was found in PA evolution, as fitted periods were comparable to the total observation baseline, limiting claims of true oscillatory behavior.