First hints of pressure waves in a helical extragalactic jet:
  S5~0836+710
  One of the open questions in extragalactic jet Astrophysics is related to the
nature of the observed radio jet, namely whether it traces a pattern or the
flow structure itself. In this paper I summarize the evidence collected for the
presence of waves in extragalactic jets. The evidence points towards the peak
of emission in helical jets corresponding to pressure-maxima of a wave that is
generated within the core region and propagates downstream. Making use of a
number of very long baseline interferometry (VLBI) observations of the radio
jet in the quasar S5~0836+710 at different frequencies and epochs, Perucho et
al. (2012) were able to observe wave-like behavior within the observed
radio-jet. The ridge-line of the emission in the jet coincides within the
errors at all frequencies. Moreover, small differences between epochs at 15 GHz
reveal wave-like motion of the ridge-line transversal to the jet propagation
axis. The authors conclude that the helicity is a real, physical structure. I
report here on those results and discuss this result in the light of new
results recently announced by other authors that confirm the presence of waves
in the close-by object BL Lac (Cohen et al., in preparation), and the possible
magnetic or hydrodynamic nature.
