AGN Disk Diagnostics

The discovery of megamasers in nearby active galactic nuclei (AGN) is an unexpected boon for AGN researchers. The most immediate benefit, brought out in observations of NGC4258 (Miyoshi et al 1995, Moran, these proceedings) is that it allows an accurate mass measurement for the central black hole, in this case of 3.6 x 107 M⊙, with unprecedented accuracy. (I shall not belabor the point that we have not proven rigorously that it is a black hole because this is surely on much firmer footing than what follows!) Masers also allow us to discover second order features of the kinematics, specifically that the disk is warped. Thirdly, and this is what I have been asked to review here, it should enable us to choose between several possible accretion modes by treating masers as diagnostic probes of physical conditions in the disk