VLBI observations of helical jets: hints on the nature of radio-jets

TL;DR

This study uses VLBI observations to analyze the helical structure of the radio jet in quasar S5 0836+710, suggesting the ridge-line is a physical pressure maximum related to jet instability, with implications for jet dynamics.

Contribution

It provides new evidence that the jet's helical ridge-line is a real physical structure and explores its implications for jet stability and disruption.

Findings

Helical structure observed at all frequencies and epochs.

Ridge-line coincides across frequencies, indicating a physical feature.

Transversal motions appear superluminal, likely due to measurement errors.

Abstract

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.