Opacity, variability and kinematics of AGN jets
  Synchrotron self-absorption in active galactic nuclei (AGN) jets manifests
itself as a time delay between flares observed at high and low radio
frequencies. It is also responsible for the observing frequency dependent
change in size and position of the apparent base of the jet, aka the core shift
effect, detected with very long baseline interferometry (VLBI). We measure the
time delays and the core shifts in 11 radio-loud AGN to estimate the speed of
their jets without relying on multi-epoch VLBI kinematics analysis. The 15$-$8
GHz total flux density time lags are obtained using Gaussian process
regression, the core shift values are measured using VLBI observations and
adopted from the literature. A strong correlation is found between the apparent
core shift and the observed time delay. Our estimate of the jet speed is higher
than the apparent speed of the fastest VLBI components by the median
coefficient of 1.4. The coefficient ranges for individual sources from 0.5 to
20. We derive Doppler factors, Lorentz factors and viewing angles of the jets,
as well as the corresponding de-projected distance from the jet base to the
core. The results support evidence for acceleration of the jets with bulk
motion Lorentz factor $\Gamma\propto R^{0.52\pm0.03}$ on de-projected scales
$R$ of 0.5$-$500 parsecs.
