VLBA Observations of the Masers in NGC 4258: A Unique Window into the Sub-Parsec-Scale World of an Active Galactic Nucleus

VLBA observations of the water maser in NGC4258 reveal a nearly edge-on, slightly warped, extremely thin disk in Keplerian rotation about a central mass of 39 million solar masses. Analysis of these VLBA data has yielded the most precise (fractional uncertainty of about 3%) measurement of an AGN central mass and arguably the most compelling evidence to date for the existence of supermassive blackholes. Because the masers are confined within 8 mas (0.26 pc) of the central mass and span nearly 2200 km/s in velocity, NGC4258 is a challenging target. The outstanding spatial and spectral resolution and broadband capabilities of the VLBA make it uniquely suited for studies of this remarkable AGN laboratory. In this contribution I will summarize past a recent results from our ongoing program of VLBA observations of the NGC4258 water maser. These include: (1) a more refined analysis of the disk rotation curve, leading to a more precise central mass estimate and tighter constraints on alternatives to a central blackhole. (2) A full three dimensional description of the disk geometry and the resulting implications for various models of AGN accretion. And (3) The successful detection of proper motions amongst the NGC4258 masers and the derivation of a purely geometric distance to the source based on these motions.