Extrinsic Radio Variability of JVAS / CLASS Gravitational Lenses

We present flux-ratio curves of the fold and cusp (i.e. close multiple) images of six JVAS/CLASS gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi-Element Radio-Linked Interferometer Network (MERLIN) at 5–GHz with 50 mas resolution, as part of a MERLIN Key-Project. Even though the time delays between the fold and cusp images are small (1 d) compared to the timescale of intrinsic source variability, all six lens systems show evidence, based on their flux-ratio curves, that might suggest the presence of extrinsic variability. In particular, the cusp images of B2045+265 – regarded as the strongest case of the violation of the cusp relation – show extrinsic variations in their flux-ratios up to ∼40% peak-to-peak on time scales of several months. Its low Galactic latitude of b ≈ −10 • and a line-of-sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+231 at b ≈ +69 • do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems (up to 10% rms), even though these relations remain violated when averaged over a 1 year time baseline.