The observed polarization direction depending on geometrical and kinematic parameters of relativistic jets

TL;DR

This paper models how the polarization direction in relativistic jets depends on geometrical and kinematic parameters, revealing complex dependencies and ambiguities in magnetic field topology interpretation.

Contribution

It provides a detailed analysis of polarization distributions considering different magnetic field configurations and jet parameters, highlighting complexities and ambiguities in polarization interpretation.

Findings

Polarization distribution shape depends on jet and velocity angles.

Ambiguity exists in determining the twist of helical magnetic fields.

Both magnetic field topologies can explain observed polarization structures.

Abstract

The study of the polarization direction is crucial in the issue of restoring the spatial structure of the magnetic field in the active galaxy parsec-scale jets. But, due to relativistic effects, the magnetic field projected onto the celestial sphere in the source reference frame cannot be assumed to be orthogonal to the observed direction of the electric vector in the wave. Moreover, the local axis of the jet component may not coincide with its motion direction, which affects the observed polarization direction. In this article, we analyze the transverse to jet distributions of the electric vector in the wave, obtained as a result of modeling with different jet kinematic and geometrical parameters for a helical magnetic field with a different twist angle and for a toroidal magnetic field in the center, surrounded by a varying thickness sheath, penetrated by a poloidal field. We obtained: 1) the shape of the electric vector transverse distribution depends in a complex way on the angles of the jet axis and the velocity vector with the line of sight; 2) ambiguity in determining the twist direction of the helical magnetic field under using only the distributions of the electric vector in the wave; 3) both considered magnetic field topologies can reproduce both the ``spine-sheath'' polarization structure and individual bright details with the longitudinal to the jet axis polarization direction.