[Paper Review] Stellar wind accretion in high-mass X-ray binaries

I. Negueruela|arXiv (Cornell University)|Jul 16, 2009

Astrophysical Phenomena and Observations3 citations

TL;DR

This review examines wind accretion in high-mass X-ray binaries (HMXBs), focusing on the unresolved tension between observational evidence for accretion discs and computational models that disfavor their formation. Despite strong indications from neutron star spin evolution and X-ray variability, simulations suggest angular momentum transfer is inefficient, leaving the existence of stable discs in supergiant X-ray binaries (SGXBs) as a major open question in high-energy astrophysics.

ABSTRACT

Recent discoveries have confirmed the existence of a large population of X-ray sources fuelled by accretion from the stellar wind of an OB supergiant. Such systems are powerful laboratories to study many aspects of astrophysics. Over the last decades, the physics of accretion in these systems has been the subject of extensive research, mainly through numerical methods. In spite of this effort, large uncertainties remain in our understanding, reflecting the complexity of the physical situation. A crucial issue that remains open is the possible formation of accretion discs. Though the spin evolution of neutron stars in these systems suggests that angular momentum is, at least occasionally, accreted, and many observational facts seem to require the existence of discs, computational results do not favour this possibility. In this brief review, I will summarise some of the open questions in this area.

Motivation & Objective

• To assess the current state of understanding regarding accretion from stellar winds in high-mass X-ray binaries (HMXBs), particularly in systems with OB supergiant donors.
• To identify and analyze the key unresolved problem: whether stable accretion discs form during wind accretion, despite observational evidence suggesting otherwise.
• To evaluate the role of wind clumping, magnetic fields, and orbital dynamics in shaping accretion variability and X-ray emission in SGXBs.
• To compare SGXBs with other HMXB subtypes, such as Be/X-ray transients and γ-ray binaries, to identify common physical mechanisms and distinguishing features.
• To highlight the limitations of current models and the need for improved simulations and observations to resolve discrepancies between theory and observation.

Proposed method

• Utilizes the Bondi-Hoyle-Lyttleton accretion model to estimate accretion rates in wind-fed systems, based on the relative velocity and gravitational potential of the compact object.
• Applies numerical simulations to model the complex, supersonic, and clumpy nature of stellar winds from OB supergiants, focusing on angular momentum transport and disc formation.
• Analyzes observational data from X-ray pulsars, including pulse period variations and orbital modulations, to infer accretion geometry and spin evolution.
• Compares the behavior of SGXBs with Be/X-ray transients and γ-ray binaries to identify similarities and differences in accretion physics and wind structure.
• Reviews the role of wind clumping and magnetic fields in modulating accretion rates and producing stochastic X-ray variability.
• Evaluates the implications of recent discoveries—such as giant flares in Vela X-1 and phase-locked outbursts in IGR J16479–4514—for existing accretion models.

Experimental results

Research questions

• RQ1 Can stable accretion discs form in high-mass X-ray binaries with OB supergiant donors during wind accretion, despite simulations suggesting angular momentum transfer is inefficient?
• RQ2 Why do neutron star spin periods in SGXBs show evidence of spin-up, implying angular momentum accretion, when numerical models predict minimal net accretion of angular momentum?
• RQ3 What physical mechanisms explain the stochastic and burst-like X-ray variability observed in SGXBs, such as giant flares in Vela X-1 and off-states?
• RQ4 How do wind clumping and magnetic fields influence the accretion process in SGXBs, and to what extent do they explain the observed variability and low average luminosities?
• RQ5 Why do only a few HMXBs produce γ-rays, and what distinguishes γ-ray binaries like LS 5039 and LS I +61° 303 from non-γ-ray systems with similar characteristics?

Key findings

• Observational evidence from neutron star spin evolution in SGXBs strongly suggests that angular momentum is accreted, implying the possible existence of transient or unstable accretion discs.
• Despite this, numerical simulations of wind accretion do not favor the formation of stable, long-lived accretion discs due to inefficient angular momentum transport in supersonic, clumpy flows.
• Vela X-1 exhibits both giant flares (flux increases by ~10 over a few hours) and deep off-states (flux drops by orders of magnitude), indicating highly variable and stochastic accretion, likely driven by wind clumping and magnetic interactions.
• The discovery of phase-locked outbursts in IGR J16479–4514 with a 3.3-day orbital period challenges existing models of stochastic accretion, suggesting a more coherent or structured accretion mechanism.
• The γ-ray binary LS 5039, with an O6.5 V star in a close orbit, has a wind structure expected to differ significantly from that of Be stars, yet its high-energy emission remains unexplained by current models.
• The lack of γ-ray emission in SAX J0635+0533, despite its similarity to known γ-ray binaries, indicates that additional physical ingredients—beyond a fast-spinning pulsar and eccentric orbit—are required for γ-ray production.