THE MASERING TORUS IN NGC-4258

The galaxy NGC 4258 displays strong spectral variability in its H2O maser emission during a 7 year monitoring program. Variations of line strength and line velocity are particularly prominent. A velocity-epoch diagram of prominent spectral features shows that all features are accelerated systematically with an effective rate of 7.5 km s(-1) yr(-1). Combining this result with a recent VLBI result of an angular gradient for the velocity, we find that the masering clouds are located at the inner edge of a molecular torus/disk or a ring at a radial distance of 0.11 pc and have a Keplerian velocity of 900 km s(-1) around a 2.2 x 10(7) M. nuclear mass. The masering clouds amplify the radio continuum during the 15 years it takes to pass in front of the extended (0.014 pc) nucleus. The pumping of the high density masering clouds does not directly result from the nuclear activity itself but rather from short-lived pump sources within the torus. Searches for periodic behavior have not revealed any incidences besides the one reported earlier. The observed high-velocity features in NGC 4258 originate in the tangential sections of the torus.