An improved time delay from VLA and ATCA monitoring of the gravitational lens system PKS 1830–211

ABSTRACT We have measured a time delay of $25.3 \pm 2.0$ d (1$\sigma$ confidence) in the Einstein ring gravitational lens system PKS 1830–211 from an analysis of archival Very Large Array (VLA) and Australia Telescope Compact Array (ATCA) monitoring data observed between 1997 and 2004. A small portion of the ATCA data was previously used to determine a time delay and our result is consistent with the previous value, but with an uncertainty that is smaller by more than a factor of 2. The long time-baseline of the monitoring reveals that the flux density ratio is smoothly varying on a time-scale of years, an effect which we attribute to millilensing by massive objects ($\gg 1$ M$_{\odot }$) in the lensing galaxy. Image A is unpolarized in the VLA monitoring, but very long baseline interferometry (VLBI) observations show that this is partly due to beam dilution by an unpolarized counter-jet that is only present in that image. Based on the identification of this feature as a counter-jet, we conclude that its unexpected prominence in image A is a consequence of lensing and that more detailed modelling is required in order to reconcile the VLBI morphology of each image.