Binary-lens Microlensing Degeneracy: Impact on Planetary Sensitiity and Mass-ratio Function

Gravitational microlensing is a unique method for discovering cold planets across a broad mass range. Reliable statistics of the microlensing planets require accurate sensitivity estimates. However, the impact of the degeneracies in binary-lens single-source (2L1S) models that affect many actual planet detections is often omitted in sensitivity estimates, leading to potential self-inconsistency of the statistics studies. In this work, we evaluate the effect of the 2L1S degeneracies on planetary sensitivity by simulating a series of typical microlensing events and comprehensively replicating a realistic planet detection pipeline, including the anomaly identification, global 2L1S model search, and degenerate model comparison. We find that for a pure-survey statistical sample, the 2L1S degeneracies reduce the overall planetary sensitivity by 5%–10%, with the effect increasing at higher planet-host mass ratios. This bias leads to an underestimation of planet occurrence rates and a flattening of the inferred mass- ratio function slope. This effect will be critical for upcoming space-based microlensing surveys like the Roman or Earth 2.0 missions, which are expected to discover \(O(10^3)\) planets. We also discuss the computational challenges and propose potential approaches for future applications.

Recommended citation: Yuxin Shang et al 2025 AJ 170 339
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