Green Bank Telescope Observations of the Water Masers of NGC 3079: Accretion Disk Magnetic Field and Maser Scintillation

We present observations of the 22 GHz water megamasers in the circumnucleardisk of NGC 3079 obtained with the Green Bank Telescope. The data are analyzedfor circular polarization due to the Zeeman-induced splitting of the watermaser lines. No circular polarization is detected and we derive a 1 sigma upperlimit of 11 mG for the toroidal magnetic field at ~0.64 pc from the centralblack hole. This is the tightest upper limit for the magnetic field around ablack hole to date. We use the magnetic field limit to derive an estimate ofthe mass accretion onto the central black hole. In addition to the polarimetricresults, we present an observation of rapid variability in the maser lines,which we explain as weak interstellar scintillation. From the scintillationparameters, we estimate an intrinsic size of the mostly saturated maserfeatures of ~12 microarcseconds. This is consistent with models assuming athick, clumpy accretion disk.Comment: 8 pages, 7 figures; accepted for publication in Ap