We have analysed archival VLA 8.4-GHz monitoring data of the gravitational lens system JVAS B1030+074 with the goal of determining the time delay between the two lensed images via the polarization variability .
In contrast to the previously published total intensity variations , we detect correlated variability in polarized flux density , percentage polarization and polarization position angle .
The latter includes a fast ( < 5 d ) 90-degree rotation event .
Our best estimate of the time delay is 146 \pm 6 d ( 1 \sigma ) , considerably longer than that predicted by the lens model presented in the discovery paper .
Additional model constraints will be needed before this system can be used to measure H _ { 0 } , for example through a detection of the lensed source ’ s VLBI jet in image B .
No time delay is visible in total flux density and this is partially due to much greater scatter in the image B measurements .
This must be due to a propagation effect as the radio waves pass through the ISM of the lensing galaxies or the Galaxy .