Polarimetric Observations of the Masers in NGC 4258: An Upper Limit on   the Large-Scale Magnetic Field 0.2 pc from the Central Engine
  We report VLA 1 sigma upper limits of 1.5% and 3% on the intrinsic circular and linear fractional polarizations, respectively, of the water vapor maser emission 0.2 pc from the central engine of NGC 4258. A corresponding 0.5% upper limit on any Zeeman-splitting-induced circular polarization translates to a 1 sigma upper limit on the parallel, or toroidal, component of the magnetic field of 300 mG. Assuming magnetic and thermal pressure balance in the disk, this magnetic field upper limit corresponds to a model-dependent estimate of the accretion rate through the molecular disk of 10^-1.9 alpha solar masses per year for the case where the magnetic field lies along the line of sight. 
J. R. Herrnstein, J. M. Moran, L. J. Greenhill, E. G. Blackman, and P.   J. Diamond