Interstellar Scintillation of an Extreme Scintillator: PKS B1144-379
  The University of Tasmania Ceduna radio telescope has been used to
investigate rapid variability in the radio flux density of the BL Lac object
PKS B1144-379 at 6.7 GHz. High-cadence monitoring of this extreme scintillator
was carried out over a period of approximately nine years, between 2003 and
2011. We have used structure functions created from the intensity time series
to determine the characteristic timescale of the variability. The
characteristic timescale is consistently observed to increase during certain
periods of each year, demonstrating the annual cycle expected for scintillation
through an interstellar scattering screen. The best-fitting annual cycle model
for each year suggests that the scintillation pattern has an anisotropic
structure 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 that
changes in the intrinsic source structure might be influencing the variability
timescale. We found a prominent annual cycle is only present in the variability
timescale for certain years where other evidence suggests that the core is
compact. From our measurements, we calculated that the core angular size varied
between 5.65-15.90 $\mu$as (0.05-0.13 pc). The core component was found to be
at its most compact during two flares in the total flux density, which were
observed in 2005 and 2008. We conclude that the long-term variability in the
radio flux density of PKS B1144-379 is due to intrinsic changes in the source
and that these affect our ability to measure an annual cycle in its variability
time scale.
