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, measured with very long baseline interferometry (VLBI). We probe this by measuring and comparing the 15$-$8 GHz total flux density time delays and core shifts in 11 radio-loud AGN. The peak-to-peak time delays are measured using Gaussian process regression, the core shift values are obtained from the literature as well as with our own VLBI observations. These measurements are used to estimate the speed of jet plasma flow without relying on multi-epoch VLBI kinematics analysis. A strong correlation is found between the multi-frequency VLBI core shift and variability time delay confirming their common nature. Our estimate of the plasma flow speed is higher than 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. Our results imply an 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.