Intraday variability of active galaxies

In this thesis I have undertaken a detailed investigation of the BL-Lac object: PKS B1144‚Äöv†v¿379. The centimetre wavelength radio flux density of this source is known to vary on a range of time-scales from a few days up to several months. The rapid (time-scales of less than a day), apparent variability of this compact extragalactic radio source at radio wavelength is known as intraday variability (IDV). The current investigation shows conclusively that the rapid variability is the result of a propagation effect known as interstellar scintillation (ISS), which originates in the inhomogeneous interstellar medium (ISM). On the other hand, the long-term variability (months to years) suggests that the source also exhibits substantial intrinsic evolution. This project utilised high-cadence flux density monitoring of ISS with the University of Tasmania Ceduna 30-m radio telescope at a frequency of 6.7 GHz. A complementary kinematic study of the parsec-scale jet of PKSB1144‚Äöv†v¿379 has been undertaken, using 8.6 GHz Very Long Baseline Interferometry (VLBI) data. The results show that the IDV of PKSB1144‚Äöv†v¿379 is well explained as originating from ISS. I have fitted an annual cycle model for each of the years 2003 to 2011, the results of which suggest that the scattering screen has an anisotropic structure and that evolution of the source structure influences the interstellar scintillation pattern. Changes in the size of the microarcsecond-scale, scintillating component of the source were inferred from the yearly changes in the characteristic time-scale of scintillation. Changes in the source inferred from the scintillation measurements are broadly consistent with VLBI observations, although scintillation probes a much smaller angular scale. The scintillating component was found to be at its most compact during two flares in total flux density (2005 and 2008). I have performed a kinematic study of the parsec-scale jet of PKSB1144‚Äöv†v¿379 using VLBI data obtained between 1997 and 2018. Quasi-periodic flarings with a period of ~ 3‚Äö-4 years were observed. Over the 20 years interval, the average jet position angle was found to be ~150 o. The Ceduna single-dish data complements the VLBI monitoring by providing a direct comparison between changes in the size of the scintillating component (˜í¬¿as-scale) and the linear size of the emitting region (parsec-scale) with the existence of the jet component in the VLBI image (mas-scale). A distribution analysis technique was utilised to study the underlying processes that produced the observed data. This technique showed that changes in the modality of the flux density distribution appear to be related to intrinsic changes in source structure, and are consistent with inference from interstellar scattering observations. This approach also complements investigations of the temporal evolution of PKSB1144‚Äöv†v¿379 which used interstellar scintillation and VLBI, and the combination of these techniques provides a crucial starting point for understanding the system.