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Session 78 - Seyfert Galaxies.
Display session, Friday, January 09
Exhibit Hall,
The H_2O masers in NGC\,4258 trace a thin, nearly edge-on Keplerian disk bound by a supermassive central engine. Masers with line-of-sight velocities close to the systemic velocity of the galaxy (\approx 470 km\,s^-1) all lie at about the same radii on the near side of the disk. These masers drift in velocity at \approx 9 km\,s^-1\,yr^-1 as a result of centripetal acceleration, which is directed nearly along the line of sight. Monitoring of these masers for \approx 15 years up until 1996 has shown variability in individual features of tens of percent on time scales of several weeks. Typical flux densities are a few Jy. We report the discovery of two 20 Jy flares, separated by \sim 1 year, and each lasting for on the order of one month. During the second outburst, the flare drifted in velocity along with the other maser features. Nonetheless, the velocities of both flares were \approx 493 km\,s^-1. We suggest that the flare phenomenon is related at least in part to the intrinsic structure of the disk, rather than to chance fluctuations in the emission rate of a clump of maser material.
