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22nd of June 2022
Rapid Intra-Day Variability in PMN J1726+0639
by Bignall et al.
Bignall et al. report the discovery of rapid intra-day variability in the radio source PMN J1726+0639 at GHz frequencies, during a survey to search for such variability with the Australia Telescope Compact Array. Follow-up observations were conducted over two years and revealed a clear, repeating annual cycle in the rate, or characteristic timescale, of variability, showing that the observed variations can be attributed to scintillations from interstellar plasma inhomogeneities (analogous to the twinkling of starlight). The authors fit kinematic models to the data, allowing for finite anisotropy in the scintillation pattern. The parameters are inconsistent with expectations from a previously proposed model of scattering associated with plasma filaments radially oriented around hot stars. We note that evidence for a foreground interstellar cloud causing anomalous Ca II absorption towards the nearby star Rasalhague (alpha Oph) has been previously reported, and Bignall et al. speculate that the interstellar scintillation of PMN J1726+0639 might be associated with this nearby cloud.

The image above shows light curves for PMN J1726+0639 during an epoch of rapid scintillation, at frequencies between 4500 MHz and 10600 MHz, averaged over ∼100 MHz intervals and 150-second time intervals. The lowest frequencies are in red, and the highest are shown in violet. The rapid intra-day variability in flux density is clear.

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