Detection of the Velocity Drift of High-Velocity Water Maser Features of a LINER NGC 4258: Evidence of a Spiral Maser Disk

Abstract We monitored the velocities of high-velocity features for a H₂ O 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^{-1 \, yr^-1} on the average for 10 red-shifted features. The drift rate of a blue-shifted feature was a = 0. 20 0. 10 \, km \, s^-1 \, yr^-1. 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 ₏ = 13{^ { }} 7{^ { }} for the blue-shifted feature from the measured drift rates.