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 time–scale of the centimetre radio emission of PKS B1622−253. Observations of PKS B1519−273 over a period of nearly 2 yr confirm the presence of an annual cycle in the variability time–scale 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–fitting 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’ intra–day variability (IDV) sources than for rapid scintillators such as PKS B0405−385 or J1819+3845.
Keywords: scattering; ISM: general; galaxies: active; galaxies: nuclei; radio continuum: general
Journal Article. 7286 words. Illustrated.
Subjects: Astronomy and Astrophysics
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