Accelerations of Water Masers in NGC 4258

The water masers in NGC4258 delineate the structure and dynamics of asub-parsec-diameter accretion disk around a supermassive black hole. We reportacceleration measurements for 12 systemic-velocity spectral features and 19high-velocity spectral features, using a total of 25 epochs of observationsfrom Effelsberg (5 epochs), the VLA (15 epochs), and the VLBA (5 epochs)spanning the years 1994 to 1997. The measured accelerations of thesystemic-velocity features are between 7.5 and 10.4 km/s/yr, and there is noevidence for a dip in the spectrum at the systemic velocity. Such a dip hasbeen attributed in the past to an absorbing layer of non-inverted H2O (Watson &Wallin 1994; Maoz & McKee 1998). The accelerations of the high-velocityfeatures, measured here for the first time, range from -0.77 to 0.38 km/s/yr. From the line-of-sight accelerations and velocities, we infer the positionsof these high-velocity masers with a simple edge-on disk model. The resultingpositions fall between -13.6 and 9.3 degrees in azimuth (measured from themidline, the diameter perpendicular to the line of sight). A model thatsuggests a spiral shock origin of the masers (Maoz & McKee 1998), in whichchanges in maser velocity are due to the outward motion of the shock wave,predicts apparent accelerations of -0.05 times theta_p/2.5 degrees km/s/yr,where theta_p is the pitch angle of the spiral arms. Our data are notconsistent with these predictions. We also discuss the physical properties ofthe high-velocity masers. Most notably, the strongest high-velocity masers lienear the midline, where the velocity gradient is smallest, thereby providingthe longest amplification path lengths. (Abridged)Comment: 40 pages, latex, uses aaspp4 style file, 10 figures. Accepted for publication in the Astrophysical Journa