Measuring line-of-sight shear with Einstein rings: a proof of concept

TL;DR

This paper demonstrates that line-of-sight shear in strong gravitational lensing can be accurately measured from simulated images, even with complex structures and model limitations, suggesting its potential as a cosmological observable.

Contribution

The study shows that line-of-sight shear can be reliably extracted from strong lensing images, including complex scenarios and violations of the tidal approximation, advancing its use in cosmology.

Findings

Line-of-sight shear measurable with percent accuracy in simple simulations.

Complex models still recover shear despite reduced precision.

Violations of tidal approximation do not prevent shear measurement.

Abstract

Line-of-sight effects in strong gravitational lensing have long been treated as a nuisance. However, it was recently proposed that the line-of-sight shear could be a cosmological observable in its own right, if it is not degenerate with lens model parameters. We firstly demonstrate that the line-of-sight shear can be accurately measured from a simple simulated strong lensing image with percent precision. We then extend our analysis to more complex simulated images and stress test the recovery of the line-of-sight shear when using deficient fitting models, finding that it escapes from degeneracies with lens model parameters, albeit at the expense of the precision. Lastly, we check the validity of the tidal approximation by simulating and fitting an image generated in the presence of many line-of-sight dark matter haloes, finding that an explicit violation of the tidal approximation does not necessarily prevent one from measuring the line-of-sight shear.