Annual Cycles in the Interstellar Scintillation Timescales of PKS B 1519 – 273 and PKS B 1622 – 253 Steven

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 PKSB1622–253. Observations of PKSB1519–273 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 PKSB0405-385 or J1819+3845.