The bimodal distribution of donor stars in X-ray binaries
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 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.
Cite
@article{arxiv.2510.14524,
title = {The bimodal distribution of donor stars in X-ray binaries},
author = {Jia Zhang and Guo-Bao Zhang and Li-Ying Zhu and Sheng-Bang Qian and Xiao Zhou and Er-Gang Zhao},
journal= {arXiv preprint arXiv:2510.14524},
year = {2025}
}