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The University of Tasmania Ceduna radio telescope has been used toinvestigate rapid variability in the radio flux density of the BL Lac objectPKS B1144-379 at 6.7 GHz. High-cadence monitoring of this extreme scintillatorwas carried out over a period of approximately nine years, between 2003 and2011. We have used structure functions created from the intensity time seriesto determine the characteristic timescale of the variability. Thecharacteristic timescale is consistently observed to increase during certainperiods of each year, demonstrating the annual cycle expected for scintillationthrough an interstellar scattering screen. The best-fitting annual cycle modelfor each year suggests that the scintillation pattern has an anisotropicstructure and that the upper limit of its scattering screen is at a distance of~0.84 kpc. Higher anisotropy in some of the annual cycle fits suggests thatchanges in the intrinsic source structure might be influencing the variabilitytimescale. We found a prominent annual cycle is only present in the variabilitytimescale for certain years where other evidence suggests that the core iscompact. From our measurements, we calculated that the core angular size variedbetween 5.65-15.90 $\mu$as (0.05-0.13 pc). The core component was found to beat its most compact during two flares in the total flux density, which wereobserved in 2005 and 2008. We conclude that the long-term variability in theradio flux density of PKS B1144-379 is due to intrinsic changes in the sourceand that these affect our ability to measure an annual cycle in its variabilitytime scale.

Interstellar Scintillation of an Extreme Scintillator: PKS B1144-379