Weak lensing of strong lensing: beyond the tidal regime
Abstract
The analysis of strong lensing images usually involves an external convergence and shear, which are meant to model the effect of perturbations along the line of sight, on top of the main lens. Such a description of lineofsight perturbations supposes that the corresponding gravitational fields can be treated in the tidal regime. Going one step further introduces additional effects, known as flexion, which have been hitherto neglected in strong lensing. In this work, we build a minimal model for the lineofsight flexion, which adds four new complex parameters to the lens model. Contrary to convergence and shear, the lineofsight flexion cannot be projected onto the main lens plane. For a $\Lambda$CDM cosmology, we predict the typical lineofsight flexion to be on the order of $10^{3} \mathrm{arcsec}^{1}$ on galactic scales. Neglecting its effect in lens modelling is found to bias the recovery of other parameters; in particular, the lineofsight shear can be biased up to $2\sigma$. Accounting for the lineofsight flexion in our minimal framework restores accuracy, at the the cost of degrading precision. With current imaging capabilities, the lineofsight flexion is unlikely to be measurable on individual strong lensing images; it must therefore be considered a nuisance parameter rather than an observable in its own right.
 Publication:

arXiv eprints
 Pub Date:
 May 2024
 DOI:
 10.48550/arXiv.2405.12091
 arXiv:
 arXiv:2405.12091
 Bibcode:
 2024arXiv240512091D
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 21 + 5 pages, 12 figures. v2: added a citation. v3: minor adjustments