Swinging jets and the variability of active galactic nuclei

Abstract

We consider a kinematic model for the rapid variability of AGN in both total and polarized flux density. This picture emphasizes the changes in relativistic aberration produced when the trajectories of the shocks propagating down relativistic jets deviate slightly from linearity. This simple model can quantitatively explain the reported correlation (and anticorrelation) between the varying total (S) and polarized flux densities as well as the offsets in time recorded between the extrema of these quantities. A key prediction is that changes in the apparent transverse velocity will almost always be positively correlated with changes in the fractional polarization Pi, although the sense of their correlation with the variation in total flux will depend on the viewing angle. It is further predicted that anticorrelated variations in S and Pi should be more common among the most core-dominated radio sources. We compare several features of the model with the widely discussed two-component model for AGN variability and indicate how future very long baseline array observations could distinguish between them.