X-ray Binary Evolution Across Cosmic Time

High redshift galaxies permit the study of the formation and evolution of X-ray binary populations on cosmological timescales, probing a wide range of metallicities and star-formation rates. In this paper, we present results from a large scale population synthesis study that models the X-ray binary populations from the first galaxies of the universe until today. We use as input to our modeling the Millennium II Cosmological Simulation and the updated semi-analytic galaxy catalog by Guo et al. (2011) to self-consistently account for the star formation history and metallicity evolution of the universe. Our modeling, which is constrained by the observed X-ray properties of local galaxies, gives predictions about the global scaling of emission from X-ray binary populations with properties such as star-formation rate and stellar mass, and the evolution of these relations with redshift. Our simulations show that the X-ray luminosity density (X-ray luminosity per unit volume) from X-ray binaries in our Universe today is dominated by low-mass X-ray binaries, and it is only at z>2.5 that high-mass X-ray binaries become dominant. We also find that there is a delay of 1.1 Gyr between the peak of X-ray emissivity from low-mass Xray binaries (at z2.1) and the peak of star-formation rate density (at z3.1). The peak of the X-ray luminosity from high-mass X-ray binaries (at z3.9), happens ~0.8 Gyr before the peak of the star-formation rate density, which is due to the metallicity evolution of the Universe.

💡 Research Summary

This paper presents a comprehensive, cosmological‑scale population synthesis study of X‑ray binary (XRB) evolution from the first galaxies to the present day. Using the Millennium II dark‑matter simulation together with the semi‑analytic galaxy catalog of Guo et al. (2011), the authors self‑consistently track the star‑formation histories (SFHs) and metallicity (Z) evolution of galaxies across cosmic time. These galaxy‑level histories serve as inputs for a binary‑population synthesis code (an extended version of StarTrack) that simulates the formation, evolution, and X‑ray output of both high‑mass X‑ray binaries (HMXBs) and low‑mass X‑ray binaries (LMXBs). Model parameters—such as the initial mass function, binary mass‑ratio distribution, orbital eccentricities, supernova kick velocities, and metallicity‑dependent wind mass‑loss rates—are calibrated against observed scaling relations in nearby galaxies (LX–SFR and LX–M*).

The simulations yield several key results. First, the present‑day X‑ray luminosity density (ρLX) is dominated (~70 %) by LMXBs, reflecting their long lifetimes and the accumulated contribution from old stellar populations. By contrast, at redshifts z > 2.5 the X‑ray output becomes HMXB‑dominated because low metallicity enhances the formation efficiency of massive compact objects and shortens the evolutionary timescale to the X‑ray phase. Second, the peak of the LMXB emissivity occurs at z ≈ 2.1 (cosmic age ≈ 3 Gyr), lagging the global star‑formation‑rate density (SFRD) peak at z ≈ 3.1 by roughly 1.1 Gyr. This delay mirrors the time required for low‑mass stars to evolve into compact objects and for binary interactions (e.g., common‑envelope evolution) to bring the system into an X‑ray bright configuration. Third, the HMXB emissivity peaks earlier, at z ≈ 3.9, about 0.8 Gyr before the SFRD maximum. The earlier HMXB peak is driven by the rapid evolution of massive stars and the strong metallicity dependence of wind mass loss: low‑Z environments produce heavier black holes and reduce orbital widening, thereby boosting HMXB formation efficiency.

The authors also demonstrate that the canonical linear scaling relations between X‑ray luminosity and SFR or stellar mass are not static. The LX/SFR ratio rises by a factor of ≳ 2 between z ≈ 2 and 4, while the LX/M* ratio reaches a maximum around z ≈ 1–2. These trends arise from the combined effects of metallicity evolution and the differing delay times of HMXBs and LMXBs. Consequently, the cosmic X‑ray background at high redshift is expected to be largely powered by metal‑poor HMXBs, a factor that may influence heating and ionization of the intergalactic medium during the epoch of reionization.

The paper discusses uncertainties, highlighting that the most influential parameters are the metallicity‑dependence of wind mass loss, the initial binary orbital properties, and the supernova kick distribution. The authors argue that forthcoming X‑ray observatories such as Athena and Lynx, together with JWST measurements of galaxy metallicities at z > 3, will be crucial for testing and refining these models.

In summary, this work bridges galaxy formation theory and X‑ray binary astrophysics on a cosmological scale, revealing that low‑mass X‑ray binaries dominate the present‑day X‑ray output while high‑mass systems take over at early times due to metallicity‑driven efficiency gains. The derived redshift‑dependent scaling relations and the quantified time lags between star formation and X‑ray emission provide essential inputs for models of galaxy evolution, the thermal history of the intergalactic medium, and the interpretation of future deep X‑ray surveys.