The time delay of JVAS B1030+074 from VLA polarization monitoring

TL;DR

This study measures a 146-day time delay in polarization variability between lensed images of JVAS B1030+074 using VLA data, revealing polarization changes not seen in total flux density, and highlighting propagation effects.

Contribution

It presents the first polarization-based time delay measurement for JVAS B1030+074, providing new constraints for lens modeling and H0 determination.

Findings

Polarization variability shows a 146-day delay between images.

Total flux density shows no clear delay due to scatter and propagation effects.

A fast 90-degree polarization rotation event was observed.

Abstract

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 ($<$5d) 90-degree rotation event. Our best estimate of the time delay is $146\pm6$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.