Talk Title: Probing the Magnetic Field in the Accretion Disk of NGC 4258
Abstract: NGC 4258 is a low luminosity Seyfert II galaxy in which water maser emission arises in a very thin accretion disk around a supermassive black hole of 4 x 10^7 M_solar. The detected part of the molecular portion of the disk has an outer radius of about 0.3 pc and is nearly edge-on. We present polarimetric observations of some of the systemic and redshifted high-velocity water maser features obtained with the VLA and the GBT at 22 GHz. We did not detect any circular polarization in the spectrum indicative of Zeeman-induced splitting of the maser lines of water vapor. At a distance of about 0.2 pc from the central black hole, along the diameter perpendicular to the line of sight, the toroidal component of the B field is < 90 mG (from highly redshifted maser components) and the radial component along the line of sight is < 30 mG (from systemic features at 470-510 km/s). Assuming equipartition of thermal and magnetic energy, we estimate an upper limit on the mass accretion rate of ~ 10^-3 alpha M_solar/yr (where alpha is the Shakura-Sunyaev viscosity parameter) for a magnetic field purely along the line-of-sight. We delineate the ramifications of our results on current accretion theories; those include standard thin-disk models and advection-dominated accretion flows, which have been used to explain sub-Eddington-luminosity AGNs.
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