Time variations of interstellar water masers: strong sources in H II regions

Abstract

The strong water-line sources W3OH, Orian 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 W5lS exhibit more rapid variations than W3OH 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 W5l has systematically increased 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.