MOJAVE XXI. Decade-long linear polarization variability in AGN jets at parsec scales

Using stacking, we studied the variability properties of linear polarization of active galactic nucleus (AGN) jets on parsec-scales. Our sample is drawn from the MOJAVE programme, which consists of 436 AGNs manifesting core-jet morphology and has at least five VLBA observing epochs at 15 GHz from January 1996 through August 2019, with some additional archival VLBA data reduced by us. We employed a stacking procedure and constructed maps of (i) standard deviation of fractional polarization and electric vector position angle (EVPA) over epochs as the measure of variability and (ii) stacked median polarization degree to quantify typical values in time. The distributions of these values along and across the jet were analysed for the whole sample for the first time. We found that EVPA variability in the core is typically higher than in the jet, most likely due to component blending and outflow bends in the core region. The EVPA becomes more stable down the jet. Most of the sources showing this trend have a time coverage of more than 12 years and at least 15 epochs. The possible cause could be the increase of stability in the magnetic field direction. The majority of AGNs exhibit insignificant trends of relative fractional polarization variability along the ridgeline or across the jet width. There are no significant optical-class-dependent or spectral-class-dependent relations in the properties of EVPA and relative fractional polarization variability in AGN jets.