Journal Article

Shape, shear and flexion – II. Quantifying the flexion formalism for extended sources with the ray-bundle method*

C. J. Fluke and P. D. Lasky

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 3, pages 1616-1628
Published in print September 2011 | ISSN: 0035-8711
Published online September 2011 | e-ISSN: 1365-2966 | DOI:
Shape, shear and flexion – II. Quantifying the flexion formalism for extended sources with the ray-bundle method*

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Flexion-based weak gravitational lensing analysis is proving to be a useful adjunct to traditional shear-based techniques. As flexion arises from gradients across an image, analytic and numerical techniques are required to investigate flexion predictions for extended image/source pairs. Using the Schwarzschild lens model, we demonstrate that the ray-bundle method for gravitational lensing can be used to accurately recover second flexion, and is consistent with recovery of zero first flexion. Using lens plane to source plane bundle propagation, we find that second flexion can be recovered with an error no worse than 1 per cent for bundle radii smaller than Δθ= 0.01θE and lens plane impact pararameters greater than θE+Δθ, where θE is the angular Einstein radius. Using source plane to lens plane bundle propagation, we demonstrate the existence of a preferred flexion zone. For images at radii closer to the lens than the inner boundary of this zone, indicative of the true strong lensing regime, the flexion formalism should be used with caution (errors greater than 5 per cent for extended image/source pairs). We also define a shear-zone boundary, beyond which image shapes are essentially indistinguishable from ellipses (1 per cent error in ellipticity). While suggestive that a traditional weak lensing analysis is satisfactory beyond this boundary, a potentially detectable non-zero flexion signal remains.

Keywords: gravitational lensing: strong; gravitational lensing: weak; methods: numerical; galaxies: haloes; dark matter

Journal Article.  7813 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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