The influence of the jet opening angle on the appearance of relativistic jets

TL;DR

This paper investigates how the opening angle of relativistic jets affects their observed velocities and brightness, proposing a new criterion based on the brightest jet zone, with implications for AGN models.

Contribution

It introduces a new criterion for apparent velocities and Doppler factors considering jet brightness, and analyzes the impact of jet opening angles on observed superluminal motions.

Findings

High superluminal velocities are only possible for narrow collimation angles.

Jets with larger opening angles show maximal apparent velocities less than gamma.

The relation between observing angle and Doppler factor is complex without the homokinetic assumption.

Abstract

We reinvestigate the problem of the appearance of relativistic jets when geometrical opening is taken into account. We propose a new criterion to define apparent velocities and Doppler factors, which we think being determined by the brightest zone of the jet. We numerically compute the apparent velocity and the Doppler factor of a non homokinetic jet using different velocity profiles. We argue that if the motion is relativistic, the high superluminal velocities beta_{app} ~ gamma, expected in the case of an homokinetic jet, are only possible for geometrical collimation smaller than the relativistic beaming angle 1/gamma. This is relatively independent of the jet velocity profile. For jet collimation angles larger than 1/gamma, the apparent image of the jet will always be dominated by parts of the jet traveling directly towards the observer at lorentz factors < gamma resulting in maximal apparent velocities smaller than gamma}. Furthermore, getting rid of the homokinetic hypothesis yields a complex relation between the observing angle and the Doppler factor, resulting in important consequences for the numerical computation of AGN population and unification scheme model.