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G. E. Prochter (University of California, Davis), J. A. Braatz (National Radio Astronomy Observatory)
We investigate rapid flux variability of water masers at 22 GHz. An analysis of archival data confirms the result of Greenhill et al. (1997), who show that the masers in the Circinus galaxy are variable on a 5-minute timescale. The effect is likely caused by interstellar scintillation, but intrinsic variability has not been ruled out. New observations of galactic water masers were taken with the Haystack 37-m telescope to search for similar variability. Flux variations in these sources are evident over the course of several hours, the first reported detection of intraday galactic water maser variablity. We calculated modulation indices based on minute-long integrations taken over several hours and found those of galactic masers (< 0.1) to be smaller than that of the Circinus galaxy (~q 0.5). There is an apparent weak trend for the modulation index to increase with distance among the galactic masers. This may reflect that the angular size of the masers decreases with distance, making the distant ones more susceptible to scintillation. Alternatively, since the distant sources in our survey are necessarily more luminous, we may be detecting a trend for more luminous maser sources to have greater intrinsic variability.
The author(s) of this abstract have provided an email address for comments about the abstract: geprochter@ucdavis.edu