AAS 203rd Meeting, January 2004
Session 55 AGN and Starburst Galaxies
Poster, Tuesday, January 6, 2004, 9:20am-6:30pm, Grand Hall

## [55.12] Magnetic Fields in the Accretion Disks of AGN: The Case of NGC 4258

M. Modjaz, J.M. Moran, L.J. Greenhill, P.T. Kondratko (CfA)

NGC 4258 is a low luminosity Seyfert II galaxy in which water maser emission arises in a very thin accretion disk. 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 maser features obtained with the VLA 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, a non-paramagnetic molecule. The 1 sigma upper limits on the line-of-sight component of the magnetic field at different locations in the disk are reported here. 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 < 50 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 ~10-4 \alpha Msolar/yr (where \alpha is the Shakura-Sunyaev viscosity parameter and the magnetic field is assumed to be purely toroidal). We discuss 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.

Attempts to extend this Zeeman experiment with data which have higher sensitivity (by a factor of 2 - 4) and larger velocity coverage are underway: data are about to be obtained with the GBT and will be discussed if available.