Scintillation in Circinus

This work is an investigation of the Circinus H20 megamasers, especially the unusual rapid variability seen in their flux density. It has been previously hypothesized that this was due to interstellar scintillation and one of the aims of this study was to test this. If scintillation proved to be the cause, then there was the potential application of Earth-Orbit synthesis to study the pas structure of the masers themselves. This was carried out by a monitoring campaign using the Tidbinbilla 70m telescope, joint VLBI imaging / time-delay detection experiments with the LBA (Long Baseline Array) and fullpolarization observations using the ATCA (Australia Telescope Compact Array). The results have shown good but not definitive evidence for the scintillation hypothesis, and specifically for quenched diffractive scintillation. The strongest single piece of evidence is in the detection of an apparent annual cycle at the 3˜ìvâ significance level. The fitted annual cycle is consistent with the non-detections of a time delay between the Tidbinbilla and ATCA telescopes in the simultaneous observations made in 2004. The quenched diffractive scintillation model successfully accounts for the observed properties of the variability by proposing a certain range of size and morphologies within the population of the Circinus megamasers. While consistent with the observations, the models have proved difficult to usefully test due to the extremely high levels of intrinsic variability that the monitoring campaign has revealed. It is hoped that future work will enable a more rigorous testing of the proposed models.