Signatures of Relativistic Helical Motion in the Rotation Measures of AGN Jets

TL;DR

This paper demonstrates that including relativistic helical motion in jet models explains complex rotation measure profiles in AGN jets, revealing insights into their magnetic field structure and velocity components.

Contribution

It introduces a relativistic helical motion model that accounts for observed RM profile asymmetries, frequency dependence, and time variability in AGN jets.

Findings

Reproduces bent RM profiles observed in jets.

Explains frequency-dependent RM morphologies.

Predicts RM profile reversals at high frequencies.

Abstract

Polarization has proved an invaluable tool for probing magnetic fields in relativistic jets. Maps of the intrinsic polarization vectors have provided the best evidence to date for uniform, toroidally dominated magnetic fields within jets. More recently, maps of the rotation measure (RM) in jets have for the first time probed the field geometry of the cool, moderately relativistic surrounding material. In most cases, clear signatures of toroidal magnetic field are detected, corresponding to gradients in RM profiles transverse to the jet. However, in many objects these profiles also display marked asymmetries which are difficult to explain in simple helical jet models. Furthermore, in some cases the RM profiles are strongly frequency and/or time dependent. Here we show that these features may be naturally accounted for by including relativistic helical motion in the jet model. In particular, we are able to reproduce bent RM profiles observed in a variety of jets, frequency dependent RM profile morphologies and even the time dependence of the RM profiles of knots in 3C 273. Finally, we predict that some sources may show reversals in their RM profiles at sufficiently high frequencies, depending upon the the ratio of the components of jet sheath velocity transverse and parallel to the jet. Thus, multi-frequency RM maps promise a novel way in which to probe the velocity structure of relativistic outflows.