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 line-of-sight 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 line-of-sight flexion, which adds four new complex parameters to the lens model. Contrary to convergence and shear, the line-of-sight flexion cannot be projected onto the main lens plane. For a ΛCDM cosmology, we predict the typical line-of-sight flexion to be on the order of 10^-3 arcsec^-1 on galactic scales. Neglecting its effect in lens modelling is found to bias the recovery of other parameters; in particular, the line-of-sight shear can be biased up to 2σ. Accounting for the line-of-sight flexion in our minimal framework restores accuracy, at the cost of degrading precision. With current imaging capabilities, the line-of-sight 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.

中文翻译：

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强透镜效应的弱透镜效应：超出潮汐状态


强透镜图像的分析通常涉及外部会聚和剪切，旨在模拟主透镜顶部沿视线的扰动效果。这种视线扰动的描述假设相应的引力场可以在潮汐状态下处理。更进一步会引入额外的效果，称为弯曲，迄今为止，这种效果在强透镜效应中一直被忽视。在这项工作中，我们构建了一个视线弯曲的最小模型，它为镜头模型添加了四个新的复杂参数。与会聚和剪切相反，视线弯曲不能投影到主透镜平面上。对于 ΛCDM 宇宙学，我们预测典型的视线弯曲在银河尺度上约为 10 ^-3 arcsec ^-1 。发现忽略其在透镜建模中的影响会导致其他参数的恢复出现偏差；特别是，视线剪切可偏置达 2σ。在我们的最小框架中考虑视线弯曲可以恢复准确性，但代价是降低精度。以目前的成像能力，在单个强透镜图像上不太可能测量视线弯曲；因此，它必须被视为一个令人讨厌的参数，而不是其本身的可观察参数。