# Fold Lens Flux Anomalies: A Geometric Approach

## TL;DR

This paper introduces a geometric method to predict flux ratios in fold lens systems, helping identify substructure in gravitational lenses by comparing observed fluxes to smooth potential models.

## Contribution

A novel geometric approach for constraining flux ratios in fold lens systems, reducing reliance on complex simulations and aiding substructure detection.

## Key findings

- The method accurately estimates flux ratios in 11 lens systems.
- Deviations from predicted ratios indicate presence of substructure.
- Long-wavelength observations align with smooth potential predictions.

## Abstract

We develop a new approach for studying flux anomalies in quadruply-imaged fold lens systems. We show that in the absence of substructure, microlensing, or differential absorption, the expected flux ratios of a fold pair can be tightly constrained using only geometric arguments. We apply this technique to 11 known quadruple lens systems in the radio and infrared, and compare our estimates to the Monte Carlo based results of Keeton, Gaudi, and Petters. We show that a robust estimate for a flux ratio from a smoothly varying potential can be found, and at long wavelengths those lenses deviating from from this ratio almost certainly contain significant substructure.