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arXiv · 2510.14524

The bimodal distribution of donor stars in X-ray binaries

Jia Zhang·Guo-Bao Zhang·Li-Ying Zhu·Sheng-Bang Qian·Xiao Zhou·Er-Gang Zhao
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Abstract

The classification of X-ray binaries into high- and low-mass types has historically lacked a unified, data-driven quantitative criterion, and large-scale statistical studies of the donor star population have been limited. In this work, we address this gap by compiling data for 3,964 XRBs and deriving a plentiful set of physical parameters (mass, radius, age, and evolutionary stage) for a sub-sample of 288 donor stars using Gaia DR3 spectral data and stellar evolution models. We find a statistically bimodal distribution in the donor star parameters, which is characterized by a valley at approximately 3 $M_{\odot}$ or 11,000 K. We uncover the physical mechanism behind this bimodality: a previously unreported ``parallel tracks'' phenomenon observed in the relationship between the donor's evolutionary stage and its fundamental parameters, such as luminosity and radius. These two tracks represent distinct main-sequence populations, and the valley between them corresponds to the sparsely populated pre- and post-main-sequence evolutionary phases.

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Jia Zhang, Guo-Bao Zhang, Li-Ying Zhu, Sheng-Bang Qian, Xiao Zhou, Er-Gang Zhao. 2025-10-16. The bimodal distribution of donor stars in X-ray binaries. https://doi.org/10.1088/1538-3873%2Fae0369

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