Annual Cycles in the Interstellar Scintillation Timescales of PKS
  B1519-273 and PKS B1622-253
  We have used the University of Tasmania's 30 m radio telescope at Ceduna in
South Australia to regularly monitor the flux density of a number of southern
blazars. We report the detection of an annual cycle in the variability
timescale of the centimetre radio emission of PKS B1622-253. Observations of
PKS B1519-273 over a period of nearly two years confirm the presence of an
annual cycle in the variability timescale in that source. These observations
prove that interstellar scintillation is the principal cause of inter-day
variability at radio wavelengths in these sources. The best-fit annual cycle
model for both sources implies a high degree of anisotropy in the scattering
screen and that it has a large velocity offset with respect to the Local
Standard of Rest. This is consistent with a greater screen distance for these
"slow'" IDV sources than for rapid scintillators such as PKS B0405-385 or
J1819+3845.
