A Ubiquitous Unifying Degeneracy in Two-Body Microlensing Systems

TL;DR

This paper introduces the offset degeneracy in two-body microlensing systems, revealing a universal symmetry that unifies existing degeneracies, resolves inconsistencies, and enhances understanding of microlensing phenomena.

Contribution

It uncovers the offset degeneracy, a new symmetry in microlensing, and demonstrates its ubiquity and unifying role across different degeneracy types, improving interpretation of microlensing events.

Findings

Offset degeneracy is common in planetary microlensing events.

It unifies close-wide and inner-outer degeneracies.

Parameters in degenerate configurations follow a simple relation.

Abstract

While gravitational microlensing by planetary systems provides unique vistas on the properties of exoplanets, observations of a given 2-body microlensing event can often be interpreted with multiple distinct physical configurations. Such ambiguities are typically attributed to the close-wide and inner-outer types of degeneracies that arise from transformation invariances and symmetries of microlensing caustics. However, there remain unexplained inconsistencies between aforementioned theories and observations. Here, leveraging a fast machine learning inference framework, we present the discovery of the offset degeneracy, which concerns a magnification-matching behaviour on the lens-axis and is formulated independent of caustics. This offset degeneracy unifies the close-wide and inner-outer degeneracies, generalises to resonant topologies, and upon reanalysis, not only appears ubiquitous in previously published planetary events with 2-fold degenerate solutions, but also resolves prior inconsistencies. Our analysis demonstrates that degenerate caustics do not strictly result in degenerate magnifications and that the commonly invoked close-wide degeneracy essentially never arises in actual events. Moreover, it is shown that parameters in offset degenerate configurations are related by a simple expression. This suggests the existence of a deeper symmetry in the equations governing 2-body lenses than previously recognised.