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A. E. Bragg, L. J. Greenhill, J. M. Moran (CfA), C. Henkel (MPIfR)
The water masers in NGC 4258 provide a rare opportunity to study the structure and dynamics of a sub-parsec diameter accretion disk around a black hole. It is now known that the disk is very thin (H/R \leq 0.0025) but slightly warped, that the central mass is \approx 3.5 \times 107 M\sun, and that the observed rotation velocity at the inner edge of the disk (radius of 0.13 pc) is \approx 32 \muas yr-1. The VLBA observations provide precise information about the positions in the plane of the sky and the three dimensional velocity vectors for the maser spots. However, the maser positions along the line of sight must be inferred from models. For example, past measurements place an upper limit on the accelerations of the high-velocity features of 1 km s-1yr-1, suggesting that they are located near the midline (the diameter perpendicular to the line-of-sight), where they would have zero acceleration. In contrast, the systemic-velocity maser features seen in the front of the disk have accelerations of about 9 km s-1yr-1. Also, deviations from the Keplerian rotation curve fitted to the VLBA data suggest that the high-velocity features have a statistical scatter of ~6\circ in azimuth around the midline of the disk. We report precise acceleration measurements for 19 high-velocity maser features using 24 epochs of observations from Effelsberg (5 epochs), the VLA (14 epochs) , and the VLBA (5 epochs) spanning the years 1994 to 1996. The measured accelerations range from -0.79 to 0.60 km s-1yr-1. From the line-of-sight accelerations and velocities, we infer high-velocity maser positions with a simple edge-on disk model. The resulting positions are clustered around the midline from -12.7\circ to 12.6\circ in azimuth. A model suggesting a spiral shock origin of the masers (Maoz & McKee, 1998, ApJ 494, 218) predicts accelerations of -0.05(\thetap/2.5\circ) km s-1yr-1, where \thetap is the pitch angle of the spiral arms. It also predicts that the red-shifted masers will lie in front of the midline and the blue-shifted masers behind it. Our data is not consistent with these predictions.