Browsing by Author "Bignall, Hayley"
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Item Interstellar scintillation, ISS, and intrinsic variability of radio AGN(Elsevier, 2019-05-13) Jauncey, David L.; Koay, Jun Yi; Bignall, Hayley; Macquart, Jean-Pierre; Pursimo, Tapio; Giroletti, Marcello; Hovatta, Talvikki; Kiehlmann, Sebastian; Rickett, Barney; Readhead, Anthony; Max-Moerbeck, Walter; Vedantham, Harish; Reynolds, Cormac; Lovell, James; Ojha, Roopesh; Kedziora-Chudczer, LucynaWe investigate the relationship between the 5 GHz interstellar scintillation (ISS) and the 15 GHz intrinsic variability of the compact, radio-selected active galactic nuclei (AGN) common to the Microarcsecond Scintillation-Induced Variability (MASIV) Survey and the Owens Valley Radio Observatory blazar flux density monitoring program. As part of this investigation, we also re-examine the reported intrinsic nature of the February 1990 VLA observations of the blazar S5 0716+714. We are also examining the presence of IDV/ISS in the Owens Valley 15 GHz flux density monitoring data. We find a significant relationship between the Owens Valley 15 GHz modulation index and the MASIV modulation index. We also discuss the implications of these findings for RadioAstron.Item The Radio Variability of the Gravitational Lens PMN J1838−3427(IOP, 2004-12-01) Winn, Joshua N.; Lovell, James E. J.; Bignall, Hayley; Gaensler, Bryan M.; Getts, Tracy J.; Kedziora-Chudczer, Lucyna; Ojha, Roopesh; Reynolds, John E.; Tingay, Steven J.; Tzioumis, TassoWe present the results of a radio variability study of the gravitational lens PMN J1838-3427. Our motivation was to determine the Hubble constant by measuring the time delay between variations of the two quasar images. We monitored the system for 4 months (approximately 5 times longer than the expected delay) using the Australia Telescope Compact Array at 9 GHz. Although both images were variable on a timescale of a few days, no correlated intrinsic variability could be identified, and therefore no time delay could be measured. Notably, the fractional variation of the fainter image (8%) was greater than that of the brighter image (4%), whereas lensed images of a point source would have the same fractional variation. This effect can be explained, at least in part, as the refractive scintillation of both images due to the turbulent interstellar medium of the Galaxy.Item An update on MASIV: How intrinsic properties influence interstellar scintillation of AGN(Proceedings of Science, 2012-04-17) Bignall, Hayley; Koay, Jun Yi; Lovell, Jim; Jauncey, Dave; Macquart, Jean-Pierre; Pursimo, Tapio; Ojha, Roopesh; Rickett, Barney; Kedziora-Chudczer, Lucyna; Reynolds, Cormac; Khoo, SueMany flat-spectrum AGN exhibit short timescale variations when observed at radio wavelengths, due to interstellar scintillation (ISS). The observed variations depend on source angular size and "core dominance", as well as on the scattering properties of the intervening interstellar medium, both of which are highly non-uniform when considering large samples. ISS offers a powerful probe of source structure on micro-arcsecond angular scales. However, in order to use the statistics of ISS as a probe, it is necessary to understand the selection effects in the AGN samples under study. One initially surprising result of the MASIV 5 GHz VLA Survey was a drop-off in the ISS amplitudes of sources above z=2. In order to determine the cause of this effect, follow-up VLA observations were conducted at two frequencies. Our sample shows on average a steepening of source spectral index with redshift. Careful analysis shows that, after accounting for the redshift-spectral index correlation, the redshift dependence of ISS can be successfully modelled by the expected (1 + z)⁰˙⁵ scaling of intrinsic angular sizes of a flux- and intrinsic brightness temperature-limited sample. The MASIV follow-up observations allow limits to be placed on angular broadening due to interstellar and intergalactic scattering, beyond the resolution achievable with ground-based VLBI.