2007ApJ...659.1040A

We report a 3 yr, 18 epoch, VLBI monitoring study of H₂ O masers in the subparsec, warped, accretion disk within the NGC 4258 AGN. Our immediate goals are to trace the geometry of the underlying disk, track rotation via measurement of proper motion, and ascertain the radii of masers for which centripetal acceleration may be measured separately. The monitoring includes ∼4 times as many epochs, ∼3 times denser sampling, and tighter control over sources of systematic error than earlier VLBI investigations. Coverage of a ∼2400 km/s bandwidth has also enabled mapping of molecular material ∼30% closer to the black hole than accomplished previously, which will strengthen geometric and dynamical disk models. Through repeated observation we have also measured for the first time a 5 µas (1 σ) thickness of the maser medium. Assuming that this corresponds to the thickness of the accretion disk, hydrostatic equilibrium requires a disk plane temperature of ~600 K. Our long-term goal is a geometric distance to NGC 4258 that is accurate to ∼3%, a ∼2 times improvement over the current best estimate. A geometric estimate of distance can be compared to distances obtained from analysis of Cepheid light curves, with the intent to recalibrate the extragalactic distance scale with reduced systematic uncertainties. This is the first paper in a series. We present here VLBI observations, data reduction, and temporal and spatial characteristics of the maser emission. Later papers will report estimation of orbital acceleration and proper motion, modeling of disk 3D geometry and dynamics, and estimation of a ``maser distance.'' Estimation of a ``Cepheid distance'' is presented in a parallel paper series.