An Intrinsic Degeneracy in a Simplified Semi-Analytic Two-Spot Model for Thermal X-Ray Pulse-Profiles

Abstract

We identify an intrinsic degeneracy in the simplified semi-analytic two-spot model for parameter inference in thermal X-ray pulse-profile modeling. Although the exact algebraic degeneracy exists in our simplified model without Doppler effects, we generate synthetic pulse-profiles and show that this degeneracy can still result in two peaks on the likelihood surface if Doppler effects, statistic errors, and the instrument response are considered. The analytic degeneracy we discovered may provide intuition for multi-modal posterior distributions discovered in previous works~\citep{Vinciguerra2023,Vinciguerra2024,Miller2019,Miller2021}. However, whether the effect of this degeneracy persists in real observational posteriors with background models and more complex spot geometries still requires further systematic study.