  We present observations of the 22 GHz water megamasers in the circumnuclear
disk of NGC 3079 obtained with the Green Bank Telescope. The data are analyzed
for circular polarization due to the Zeeman-induced splitting of the water
maser lines. No circular polarization is detected and we derive a 1 sigma upper
limit of 11 mG for the toroidal magnetic field at ~0.64 pc from the central
black hole. This is the tightest upper limit for the magnetic field around a
black hole to date. We use the magnetic field limit to derive an estimate of
the mass accretion onto the central black hole. In addition to the polarimetric
results, we present an observation of rapid variability in the maser lines,
which we explain as weak interstellar scintillation. From the scintillation
parameters, we estimate an intrinsic size of the mostly saturated maser
features of ~12 microarcseconds. This is consistent with models assuming a
thick, clumpy accretion disk.

Astrophysics