VLBI observations of helical jets: hints on the nature of radio-jets
  We make use of VLBI observations of the radio jet in the quasar S5 0836+710
at different frequencies and epochs to study its properties. The jet shows
helical structure at all frequencies. The ridge-line of the emission in the jet
coincides at all frequencies and epochs, within the errors. We conclude that
the helicity is a real, physical structure. Small differences between epochs
reveal wave-like motion of the ridge-line transversal to the jet propagation
axis. These transversal motions are measured to be superluminal. This
unphysical result could correspond to a possible small amplitude oscillation of
the ridge-line at the radio-core and to large errors in the determination of
the positions. In addition, higher resolution images at 15 GHz show that the
ridge-line does not coincide exactly with the centre of the radio jet. At
arc-second scales, this powerful jet shows non-collimated, irregular structure
and a lack of a hot-spot. Following this collection of evidence, we conclude
that the ridge-line could be related to a pressure maximum within the jet
cross-section, associated with the observed helical pattern that could lead to
jet disruption at longer scales. We discuss the implications regarding the
nature of the instability and the jet flow itself.
