Nature, formation and evolution of High Mass X-ray Binaries

The aim of this review is to describe the nature, formation and evolution of the three kinds of high mass X-ray binary (HMXB) population: i. systems hosting Be stars (BeHMXBs), ii. systems accreting the stellar wind of supergiant stars (sgHMXBs), and iii. supergiant stars overflowing their Roche lobe. There are now many new observations, from the high-energy side (mainly from the INTEGRAL satellite), complemented by multi-wavelength observations (mainly in the optical, near and mid-infrared from ESO facilities), showing that a new population of supergiant HMXBs has been recently revealed. New observations also suggest the existence of evolutionary links between Be and stellar wind accreting supergiant X-ray binaries. I describe here the observational facts about the different categories of HMXBs, discuss the different models of accretion in these sources (e.g. transitory accretion disc versus clumpy winds), show the evidences of a link between different kinds of HMXBs, and finally compare observations with population synthesis models.

💡 Research Summary

The review by Sylvain Chaty provides a comprehensive synthesis of the nature, formation, and evolution of high‑mass X‑ray binaries (HMXBs), focusing on three principal subclasses: Be‑type systems (BeHMXBs), supergiant wind‑fed systems (sgHMXBs), and supergiant Roche‑lobe overflow systems. The paper begins with a historical overview of X‑ray astronomy, noting that roughly 114 HMXBs are known in the Milky Way, accounting for about 38 % of all Galactic X‑ray binaries. The author emphasizes that the mode of mass transfer—whether via a decretion disc around a rapidly rotating Be star, a dense radiatively driven wind from an OB supergiant, or direct Roche‑lobe overflow—determines the observable properties of each subclass.

BeHMXBs host a neutron star on a wide, eccentric orbit around a B0–B2e donor. The Be star’s circumstellar disc, truncated by tidal resonances, produces periodic Type I outbursts when the neutron star passes periastron, and occasional giant Type II outbursts when the disc expands dramatically. The disc’s size and density are traced by H α emission and infrared excess. Approximately 50 such systems are known in the Galaxy and over 35 in the Small Magellanic Cloud, where an unexpectedly high incidence is linked to recent star‑formation bursts triggered by tidal interactions.

Supergiant HMXBs are divided into wind‑fed and Roche‑lobe overflow groups. In wind‑fed sgHMXBs, the compact object accretes directly from a fast (600–900 km s⁻¹), dense stellar wind (ρ∝r⁻²). This leads to relatively steady X‑ray luminosities (10³⁵–10³⁶ erg s⁻¹) punctuated by rapid variability caused by wind clumping. Roche‑lobe overflow sgHMXBs, by contrast, channel material through the inner Lagrangian point, forming a persistent accretion disc and achieving luminosities up to 10³⁸ erg s⁻¹. The paper discusses the “common envelope” phase as a crucial evolutionary step that shrinks the orbit dramatically, allowing short‑period sgHMXBs to emerge.

A central analytical tool is the Corbet diagram, which plots neutron‑star spin period versus orbital period. BeHMXBs follow a tight P_spin ∝ P_orb² relation, reflecting efficient angular‑momentum transfer from the decretion disc. sgHMXBs show a much weaker correlation, consistent with the low angular‑momentum efficiency of wind accretion.

The INTEGRAL satellite, operating above 20 keV, has been pivotal in uncovering a previously hidden population of heavily absorbed sgHMXBs. While pre‑INTEGRAL catalogs listed only ~4 % of HMXBs as sgHMXBs, the INTEGRAL surveys now identify ~32 % of the HMXB sample as supergiant systems, a factor of eight increase. This includes the discovery of Supergiant Fast X‑ray Transients (SFXTs), whose brief, luminous flares are best explained by accretion of dense wind clumps.

Population‑synthesis comparisons suggest that BeHMXBs can evolve into sgHMXBs as the donor star leaves the main sequence, loses its disc, and develops a strong wind. The common‑envelope phase further produces short‑period sgHMXBs. While the overall observed distribution of HMXB subclasses broadly matches theoretical predictions, uncertainties remain regarding the timescales of the Be‑to‑supergiant transition and the detailed physics of clumpy wind accretion.

In conclusion, the integration of high‑energy observations from INTEGRAL with multi‑wavelength data has dramatically refined our picture of HMXB demographics, highlighted evolutionary links between Be and supergiant systems, and underscored the importance of wind structure and binary interaction in shaping the life cycles of massive X‑ray binaries.