Monitoring Of Jets in Active Galactic Nuclei with VLBA Experiments. XVIII. Kinematics and Inner Jet Evolution of Bright Radio-loud Active Galaxies

We have analyzed the parsec-scale jet kinematics of 447 bright radio-loud active active galactic nuclei (AGN), based on 15 GHz Very Long Baseline Array (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 and 2019 August 4 for 143 of these AGN. We tracked 1923 bright features for five or more epochs in 419 jets. The majority (60%) of the well-sampled jet features show either accelerated or nonradial motion. In 47 jets there is at least one nonaccelerating feature with an unusually slow apparent speed. Most of the jets show variations of 10°-50° in their inner jet position angle (PA) over time, although the overall distribution has a continuous tail out to 200°. AGN with spectral energy distributions peaked at lower frequencies tend to have more variable PAs, with BL Lac objects being less variable than quasars. The Fermi Large Area Telescope (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.