First Hints of Pressure Waves in a Helical Extragalactic Jet: S5 0836+710

Abstract

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 dierent frequencies and epochs, Perucho et al. (2012a) 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 them 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).