MOJAVE: 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
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.
