The bimodal distribution of donor stars in X-ray binaries

TL;DR

The paper builds a large, Gaia-backed census of X-ray binaries by deriving donor-star parameters for 288 systems from Gaia DR3 atmospheric data and MIST isochrones, enabling a data-driven view of donor populations. It reveals a statistically bimodal distribution in donor mass and temperature, with a valley near $3\,M_{\odot}$ or $11{,}000\,\mathrm{K}$, and attributes this to a parallel-tracks relationship between evolutionary stage and key stellar parameters. The two-track picture explains the scarcity of intermediate-mass donors and aligns with a clear dichotomy between HMXBs and LMXBs, including differences in mass ratios and accretion modes. The work also proposes an optical-search approach for HMXB candidates by targeting a sparse region in high-luminosity, large-radius MS-like parameter space and cross-matching with wide X-ray surveys, offering a practical route for discovering new systems.

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.

Figures (12)

• Figure 1: Comparison of donor star masses between this paper and references. Panel (1): Donor mass derived in this work versus values from the references. Panel (2): Histogram of the relative difference between our mass values and references values. The dashed line indicates the median value.
• Figure 2: The observed distribution of the donor stars in XRBs. Different colors represent different locations. Green represents MW, and red and orange represents LMC and SMC respectively. Black represent all XRBs.
• Figure 3: The occurrence ratio distribution of the donor stars in XRBs. The black lines represent the observed distribution of XRBs, while the gray lines represent the distribution of random background stars from the MW and the MCs. The blue lines are the ratio of the black lines to the gray lines, representing the occurrence ratio of XRBs after removing the influence of background stars distribution.
• Figure 4: Parameters relationship for the donor stars in XRBs. Different colors represent different types of XRBs from literature. Different colors represent different types of XRBs, that are originate from the original literature, as described in Section \ref{['catalogs']}. Different shapes represent different locations, LMC and SMC refer to the Large and Small Magellanic Clouds, respectively. Disk and Other denote the Galactic disk direction (-15 $<$ b $<$ 15) and other directions (halo), respectively. Two gray lines in panel 9 enclose the region of main-sequence stars.
• Figure 5: Same as Fig. \ref{['fig:relation']} but for period and other parameters.