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23rd of June 2022
Rapid Intra-Day Variability in PMN J1726+0639
by Bignall et al.
Sufficiently compact (sub-milliarcsecond-scale), centimetre-wave radio sources scintillate, as a result of inhomogeneities in the ionized interstellar medium (ISM). Scintillation can be thought of as a spatial flux density pattern – i.e. the source projected through a transparent plasma “screen” – that drifts relative to the Earth. For sources at cosmological distances, the velocity of the pattern is essentially that of the screen, and therefore the change in the velocity of the Earth as it orbits the Sun can strongly affect the characteristic variability timescales. Such an annual modulation has been reported for more than a dozen sources. Broad-band intra-hour variability due to weak or refractive scintillation implies scattering within tens of parsec of the observer. Such nearby scattering offers the possibility to reliably determine the nature and origin of the scattering material.

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 strong annual cycle includes an apparent "standstill" in April and another in September. The image above shows some of the observed light curves of PMN J1726+0639, averaged over 0.5 GHz-wide bands near the bottom and top of the observing bandwidth. The red points are 5.5 GHz data, and the blue points are 10.0 GHz data. The tick marks on the X-axis are at intervals of 0.1 day, or 2.4 hours. Rapid variation is seen in some months, and much slower variation around April and September.

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