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

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 first demonstrate that the line-of-sight shear can be accurately measured from a simple simulated strong lensing image with per cent 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.

Automated summary

By simulating strong lensing images, the line-of-sight shear can be accurately measured, independent of lens model parameters. Even with deficient fitting models, the recovery of the line-of-sight shear can escape degeneracies with lens model parameters, although at the cost of precision. Simulating and fitting images with multiple line-of-sight dark matter haloes verifies the validity of the tidal approximation, showing that an explicit violation of the tidal approximation does not necessarily prevent measuring the line-of-sight shear.