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