PERIODIC VARIATIONS OF THE EXTRAGALACTIC H2O MASER IN NGC-4258

The H2O maser emission in the galaxy NGC 4258 has been monitored over the time period 1984 April to 1989 July and has shown rapid variability in the velocity range centered on 465 km s-1. Five distinct flares with a separation of 85 days or 2.8 months were observed during the last period of monitoring of 1 year. The center velocity has shown a systematic change of 10 km s-1 during these five outbursts. The line width varied with the line intensity as I-0.47 during one of these flares. This dependence indicates that the maser is mostly unsaturated during the bursts. Two other emission features at 495 and 525 km s-1 did not vary significantly during the whole monitoring period of 5 years. A foreground molecular region pumped by a variable star and amplifying the compact nuclear radio source is suggested as producing the periodic variations.