Linear polarization in the nucleus of M87 at 7 mm and 1.3 cm
  We report on high angular resolution polarimetric observations of the nearby
radio galaxy M87 using the Very Long Baseline Array at 24 GHz ($\lambda=$1.3
cm) and 43 GHz ($\lambda=$7 mm) in 2017-2018. New images of the linear
polarization substructure in the nuclear region are presented, characterized by
a two-component pattern of polarized intensity and smooth rotation of the
polarization plane around the 43 GHz core. From a comparison with an analogous
dataset from 2007, we find that this global polarization pattern remains stable
on a time interval of 11 yr, while showing smaller month-scale variability. We
discuss the possible Faraday rotation toward the M87 nucleus at centimeter to
millimeter wavelengths. These results can be interpreted in a scenario where
the observed polarimetric pattern is associated with the magnetic structure in
the confining magnetohydrodynamic wind, which also serves as the source of the
observed Faraday rotation.
