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