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Title:
Magnetic Fields in the Accretion Disks of Agn: NGC 4258
Authors:
Modjaz, Maryam; Moran, Jim M.; Greenhill, Lincoln J.; Kondratko, Paul T.
Affiliation:
AA(Harvard University), AB(Harvard-Smithsonian Center for Astrophysics), AC(Harvard-Smithsonian Center for Astrophysics), AD(Harvard University)
Publication:
Quasar Cores and Jets, 25th meeting of the IAU, Joint Discussion 18, 23-24 July 2003, Sydney, Australia
Publication Date:
00/2003
Origin:
IAU
Bibliographic Code:
2003IAUJD..18E..49M

Abstract

NGC 4258 is a low-luminosity Seyfert II galaxy in which water maser emission arises in a very thin nearly edge-on accretion disk. The molecular portion of the disk has an outer radius of about 0.3 pc. We present polarimetric observations of some of the systemic and redshifted high-velocity maser features obtained with the VLA at 22 GHz. Having searched for a Zeeman-induced splitting of the maser lines in circular polarization we report 1 sigma upper limits on the line-of-sight component of the magnetic field at different locations in the disk. 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. Attempts to extend this Zeeman experiment with GBT data that have a higher sensitivity (by a factor of 2 - 4) and larger bandwidth (by a factor of 130) are underway. Assuming equipartition of thermal and magnetic energy an upper limit on the mass accretion rate can be estimated. We delineate the ramifications of our results on current accretion theories; those include standard thin-disk models and Advection-Dominated Accretion Flows which are highly debated candidates for describing sub-Eddington-luminosity AGNs.
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