MOLECULAR TORT AND H2O MEGAMASER VARIABILITY

Five prominent extragalactic water masers have been observed at roughl y monthly intervals from 1984 until 1993 to study their temporal chara cteristics. The observing program at Haystack Observatory comprised fi ve sources: NGC 1068, NGC 3079, NGC 4258, the irregular galaxy IC 10, and IC 133, an H II region complex in M33. The data on two sources, NG C 4258 and IC 10SE, have been presented earlier. All galaxies show str ong variability in line strength, line velocity, and line width. The o bserved variability characteristics support a model with inverted mole cular gas located in front of the nuclear source that experiences a va riable pump rate and in part amplifies the background continuum. The e xtragalactic sources have been classified as class I for H II region e mission sources in nearby galaxies or class II for mostly powerful emi ssions from galactic nuclei. In analogy with NGC 4258, the spectral fe atures of class II sources NGC 1068 and NGC 3079 display a drift rate consistent with a compact molecular torus for the emission region.