On the source of Faraday rotation in the jet of the radio galaxy 3C120
  The source of Faraday rotation in the jet of the radio galaxy 3C120 is
analyzed through Very Long Baseline Array observations carried out between 1999
and 2007 at 86, 43, 22, 15, 12, 8, 5, 2, and 1.7 GHz. Comparison of
observations from 1999 to 2001 reveals uncorrelated changes in the linear
polarization of the underlying jet emission and the Faraday rotation screen:
while the rotation measure (RM) remains constant between approximately 2 and 5
mas from the core, the RM-corrected electric vector position angles (EVPAs) of
two superluminal components are rotated by almost 90 degrees when compared to
other components moving through similar jet locations. On the other hand, the
innermost 2 mas experiences a significant change in RM -- including a sign
reversal -- but without variations in the RM-corrected EVPAs. Similarly,
observations in 2007 reveal a double sign reversal in RM along the jet, while
the RM-corrected EVPAs remain perpendicular to the jet axis. Although the
observed coherent structure and gradient of the RM along the jet supports the
idea that the Faraday rotation is produced by a sheath of thermal electrons
that surrounds the emitting jet, the uncorrelated changes in the RM and
RM-corrected EVPAs indicate that the emitting jet and the source of Faraday
rotation are not closely connected physically and have different configurations
for the magnetic field and/or kinematical properties. Furthermore, the
existence of a region of enhanced RM whose properties remain constant over
three years requires a localized source of Faraday rotation, favoring a model
in which a significant fraction of the RM originates in foreground clouds.
