Scintillation in the Circinus Galaxy H2O Megamasers

We present observations of the 22 GHz water vapor megamasers in the Circinus galaxy made with the Tidbinbilla 70 m telescope. These observations confirm the rapid variability seen earlier by Greenhill et al. We show that this rapid variability can be explained by interstellar scintillation, based on what is now known of the interstellar scintillation seen in a significant number of flat-spectrum active galactic nuclei. The observed variability cannot be fully described by a simple model of either weak or diffractive scintillation. © 2005. The American Astronomical Society. All rights reserved.