An oversized magnetic sheath wrapping around the parsec-scale jet in 3C 273

TL;DR

This paper models an oversized magnetic sheath around the parsec-scale jet in 3C 273, explaining observed Faraday Rotation Measure changes through a wide sheath and jet direction swings, providing insights into AGN jet magnetic environments.

Contribution

It introduces a novel model of an oversized sheath around the jet, linking jet direction changes with RM evolution in 3C 273.

Findings

The sheath is wider than the jet, with a full intrinsic opening angle of 2.1°.

Magnetic field strength at the sheath boundary is approximately 3 μG.

Thermal particle density at the boundary is about 125 cm⁻³.

Abstract

In recent studies, several AGN have exhibited gradients of the Faraday Rotation Measure (RM) transverse to their parsec-scale jet direction. Faraday rotation likely occurs as a result of a magnetized sheath wrapped around the jet. In the case of 3C 273, using Very Long Baseline Array multi-epoch observations at 5, 8 and 15 GHz in 2009--2010, we observe that the jet RM has changed significantly towards negative values compared with that previously observed. These changes could be explained by a swing of the parsec-scale jet direction which causes synchrotron emission to pass through different portions of the Faraday screen. We develop a model for the jet-sheath system in 3C 273 where the sheath is wider than the single-epoch narrow relativistic jet. We present our oversized sheath model together with a derived wide jet full intrinsic opening angle $\alpha_\mathrm{int}=2.1^\circ$ and magnetic field strength $B_{||}=3$ $\mu$G and thermal particle density $N_\mathrm{e}=125~\mathrm{cm}^{-3}$ at the wide jet--sheath boundary 230 pc downstream (deprojected) from its beginning. Most of the Faraday rotation occurs within the innermost layers of the sheath. The model brings together the jet direction swing and long-term RM evolution and may be applicable to other AGN jets that exhibit changes of their apparent jet direction.