On the perturbative picture and the Chang-Refsdal lens approximation for planetary microlensing

Under the perturbative picture of planetary microlensing, the planet is considered to act as a uniform-shear Chang-Refsdal lens on one of the two images produced by the host star that comes close to the angular Einstein radius of the planet, leaving the other image unaffected. However, this uniform-shear approximation is only valid for isolated planetary caustics and breaks down in the resonant regime. Recently, the planetary-caustic degeneracy arising from the above formalism is found to generalize to the regime of central and resonant caustics, indicating that the perturbative picture and Chang-Refsdal lens approximation may have been underexplored in the past. Here, I introduce a new variable-shear Chang-Refsdal lens approximation, which not only supports central and resonant caustics, but also enables full magnification maps to be calculated analytically. Moreover, I introduce the generalized perturbative picture, which relaxes the required proximity between the planet and the image being perturbed in the previous work. Specifically, the planet always perturbs the image in the same half of the lens plane as the planet itself, leaving the other image largely unaffected. It is demonstrated how this new framework results in the offset degeneracy as a consequence of physical symmetry. The generalized perturbative picture also points to an approach to solve the two-body lens equation semi-analytically. The analytic and semi-analytic microlensing solutions associated with this work may allow for substantially faster light-curve calculations and modelling of observed events. A Python implementation is provided.