Relativistic Doppler beaming and misalignments in AGN jets

Abstract

Radio maps of AGNs often show linear features, called jets, both on pc as well as kpc scales. These jets supposedly possess relativistic motion and are oriented close to the line of sight of the observer and accordingly the relativistic Doppler beaming makes them look much brighter than they really are in their respective rest-frames. The flux boosting due to the relativistic beaming is a very sensitive factor of the jet orientation angle, as seen by the observer. Quite often large bends are seen in these jets, with misalignments being $90^\circ$ or more and might imply a change in the orientation angle that could cause a large change in the relativistic beaming factor. Such large bends should show high contrasts in the brightness of the jets, before and after the misalignments, if relativistic beaming does play an important role in these jets. It needs to be kept in mind that sometimes a small intrinsic change in the jet angle might appear as a much larger misalignment due to the geometrical projection effects, especially when seen close to the line of sight. Of course what really matters is the final orientation angle of the jet with respect to the observer's line of sight. Taking the geometrical projection effects properly into account, we calculate the consequences of the presumed relativistic beaming and demonstrate that there ought to be large brightness ratios in jets before and after the observed misalignments.