Line-of-sight effects in strong gravitational lensing by Pierre Fleury et al. on Wednesday 30 November

While most strong-gravitational-lensing systems may be roughly modelled by a

single massive object between the source and the observer, in the details all

the structures near the light path contribute to the observed images. These

additional contributions, known as line-of-sight effects, are non-negligible in

practice. This article proposes a new theoretical framework to model the

line-of-sight effects, together with very promising applications at the

interface of weak and strong lensing. Our approach relies on the dominant-lens

approximation, where one deflector is treated as the main lens while the others

are treated as perturbations. The resulting framework is technically simpler to

handle than the multi-plane lensing formalism, while allowing one to

consistently model any sub-critical perturbation. In particular, it is not

limited to the usual external-convergence and external-shear parameterisation.

As a first application, we identify a specific notion of line-of-sight shear

that is not degenerate with the ellipticity of the main lens, and which could

thus be extracted from strong-lensing images. This result supports and improves

the recent proposal that Einstein rings might be powerful probes of cosmic

shear. As a second application, we investigate the distortions of

strong-lensing critical curves under line-of-sight effects, and more

particularly their correlations across the sky. We find that such correlations

may be used to probe, not only the large-scale structure of the Universe, but

also the dark-matter halo profiles of strong lenses. This last possibility

would be a key asset to improve the accuracy of the measurement of the

Hubble-Lema\^itre constant via time-delay cosmography.

arXiv: http://arxiv.org/abs/http://arxiv.org/abs/2104.08883v5