Annual Cycles in the Interstellar Scintillation Timescales of PKS B1519-273 and PKS B1622-253

TL;DR

This study detects annual cycles in radio variability timescales of two blazars, confirming interstellar scintillation as the main cause and revealing anisotropic scattering screens with significant velocity offsets.

Contribution

It provides the first evidence of annual cycles in the variability timescales of PKS B1519-273 and PKS B1622-253, highlighting anisotropic scattering and large velocity offsets.

Findings

Annual cycles confirmed in both sources.

Interstellar scintillation identified as primary cause.

Scattering screens are highly anisotropic with large velocity offsets.

Abstract

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.