2005PASJ...57..861Y

We monitored the velocities of high-velocity features for a H₂O megamaser, NGC 4258, using the 45-m telescope of the Nobeyama Radio Observatory. The spiral shock model of maser disks predicts that the red-shifted features of observed maser spectra decelerate and the blue-shifted features accelerate for trailing arms. To confirm this model, we measured the velocity drift rates. Using the data of 1992-2005, the drift rate was detected to be a = -0.036 ± 0.007 km/s/yr on the average for 10 red-shifted features. The drift rate of a blue-shifted feature was a = 0.20 ± 0.10 km/s/yr. These results are consistent with a prediction of the spiral shock model. The pitch angles of the spirals were obtained to be  = 2° ± 1° on the average for the red-shifted features and θ_p = 13° ± 7° for the blue-shifted feature from the measured drift rates.