MOJAVE XVI: Multi-Epoch Linear Polarization Properties of Parsec-Scale
  AGN Jet Cores
  We present an analysis of the core linear polarization properties of 387
parsec-scale active galactic nuclei (AGN) jets. Using 15 GHz VLBA data, we
revisit the conclusions of the first paper in this series with multi-epoch
measurements and more detailed analysis of a larger AGN sample which spans a
broader range of synchrotron peak frequencies. Each AGN has been observed for
at least five epochs between 1996 and 2017. We find that BL Lac objects have
core electric vector position angles (EVPAs) which tend towards alignment with
the local jet direction; compared to flat spectrum radio quasars (FSRQs), their
EVPAs are also less variable over time. The AGN cores which are most
fractionally polarized and least variable in polarization have EVPAs that are
closely aligned with the local jet direction; they also have low variability in
EVPA. These results support the popular model of a standing transverse shock at
the base of the jet which collimates the jet magnetic field perpendicular to
the jet direction, increasing the fractional polarization and leading to
greater polarization stability over time. High-synchrotron-peaked (HSP) BL Lac
objects form a low luminosity, low fractional polarization population. The five
narrow-line Seyfert 1 galaxies in our sample have low fractional polarization
and large EVPA-jet misalignments. Although AGN detected at gamma-rays are
thought to be more Doppler boosted than non-detected AGN, we find no
significant differences in fractional polarization based on detection by
Fermi-LAT; the gamma-loud AGN are, however, more variable in core EVPAs.
