A survey for H2O megamasers.: III.: Monitoring water vapor masers in active galaxies

We present single-dish monitoring of the spectra of 13 extragalactic water megamasers taken over a period of 9 years and a single epoch of sensitive spectra for seven others. The primary motivation is a search for drifting line velocities analogous to those of the systemic features in NGC 4258, which are known to result from centripetal acceleration of gas in an edge-on, subparsec molecular disk. We detect a velocity drift analogous to that in NGC 4258 in only one source, NGC 2639. Another, the maser source in NGC 1052, exhibits erratic changes in its broad maser profile over time. Narrow maser features in all of the other disk galaxies discussed here either remain essentially constant in velocity over the monitoring period or are sufficiently weak or variable in intensity that individual features cannot be traced reliably from one epoch to the next. In the context of a circumnuclear, molecular disk model, our results suggest that either (a) the maser lines seen are systemic features subject to a much smaller acceleration than present in NGC 4258, presumably because the gas is farther from the nuclear black hole, or (b) we are detecting "satellite" lines for which the acceleration is in the plane of the sky. Our data include the first K-band science observations taken with the new 100 in Green Bank Telescope (GBT). The GBT data were taken during testing and commissioning of several new components and so are subject to some limitations; nevertheless, they are in most cases the most sensitive H2O spectra ever taken for each source and cover 800 MHz (similar or equal to10,800 km s(-1)) of bandwidth. Many new maser features are detected in these observations. Our data also include a tentative and a clear detection of the megamaser in NGC 6240 at epochs a year and a few months, respectively, prior to the detections reported by Hagiwara et al. and Nakai et al. We also report a search for water vapor masers toward the nuclei of 58 highly inclined (i > 80degrees), nearby galaxies. These sources were selected to investigate the tendency that H2O megamasers favor inclined galaxies. None were detected, confirming that megamasers are associated exclusively with active galactic nuclei.