Multipole Formulae for Gravitational Lensing Shear and Flexion

TL;DR

This paper extends gravitational lensing shear and flexion equations into multipole expansions, revealing how different multipoles relate to mass distributions and discussing implications for lens modeling and E/B mode separation.

Contribution

It generalizes monopole aperture-mass shear formulae to all multipoles and flexions, clarifies the role of internal and external mass moments, and discusses practical implications for lens modeling.

Findings

m>=0 multipoles depend only on internal mass moments

m<0 multipoles depend only on external mass moments

E/B separation is limited to monopole moments in lensing data

Abstract

The gravitational lensing equations for convergence, potential, shear, and flexion are simple in polar coordinates and separate under a multipole expansion once the shear and flexion spinors are rotated into a ``tangential'' basis. We use this to investigate whether the useful monopole aperture-mass shear formulae generalize to all multipoles and to flexions. We re-derive the result of Schneider and Bartelmann that the shear multipole m at radius R is completely determined by the mass multipole at R, plus specific moments Q^m_in and Q^m_out of the mass multipoles internal and external, respectively, to R. The m>=0 multipoles are independent of Q_out. But in contrast to the monopole, the m<0 multipoles are independent of Q_in. These internal and external mass moments can be determined by shear (and/or flexion) data on the complementary portion of the plane, which has practical implications for lens modelling. We find that the ease of E/B separation in the monopole aperture moments does {\em not} generalize to m!=0: the internal monopole moment is the {\em only} non-local E/B discriminant available from lensing observations. We have also not found practical local E/B discriminants beyond the monopole, though they could exist. We show also that the use of weak-lensing data to constrain a constant shear term near a strong-lensing system is impractical without strong prior constraints on the neighboring mass distribution.