Little, L.T.; White, G. J. and Riley, P.W.
Time variations of interstellar water masers: strong sources in HII regions.
Monthly Notices of the Royal Astronomical Society, 180 pp. 639–656.
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The strong water-line sources W3OH, Orion A, W49N and W51S have been monitored over the period 1974 September - 1976 October. Variations on a variety of timescales from years to days have been observed. W49 and W51S exhibit more rapid variations than W30H or Orion A. A comparison of the present results with those of Sullivan during 1969-70 suggests that the flux from the main components of W49 has systematically decreased, whereas that from W51 has systematically in creased in the 5-yr interval between the two sets of observations. This fact is best explained if there are changes in the flux of radiation from a common exciting object, incident on all of the masing components. High-velocity features in W49 have on two occasions been brighter than any of the main components, supporting a Doppler shift rather than scattering explanation for their origin.
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