A natural explanation for periodic X-ray outbursts in Be/X-ray binaries

By A. Okazaki, I. Univ., Japan., I. O. Astronomy, UK., O. Strasbourg, France.. 2001

247 citations recorded by the source corpus.

When applied to Be/X-ray binaries, the viscous decretion disc model, which can successfully account for most properties of Be stars, naturally predicts the truncation of the circumstellar disc. The distance at which the circumstellar disc is truncated depends mainly on the orbital parameters and the viscosity. In systems with low eccentricity, the disc is expected to be truncated at the 3:1 resonance radius, for which the gap between the disc outer radius and the critical lobe radius of the Be star is so wide that, under normal conditions, the neutron star cannot accrete enough gas at periastron passage to show periodic X-ray outbursts (type I outbursts). These systems will display only occasional giant X-ray outbursts (type II outbursts). On the other hand, in systems with high orbital eccentricity, the disc truncation occurs at a much higher resonance radius, which is very close to or slightly beyond the critical lobe radius at periastron unless the viscosity is very low. In these systems, disc truncation cannot be ecient, allowing the neutron star to capture gas from the disc at every periastron passage and display type I outbursts regularly. In contrast to the rather robust results for systems with low eccentricity and high eccentricity, the result for systems with moderate eccentricity depends on rather subtle details. Systems in which the disc is truncated in the vicinity of the critical lobe will regularly display type I outbursts, whereas those with the disc signicantly smaller than the critical lobe will show only type II outbursts under normal conditions and temporary type I outbursts when the disc is strongly disturbed. In Be/X-ray binaries, material will be accreted via the rst Lagrangian point with low velocities relative to the neutron star and carrying high angular momentum. This may result in the temporary formation of accretion discs during type I outbursts, something that seems to be conrmed by observations.

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Citations in the corpus

Viscous effects on the interaction between the coplanar decretion disc and the neutron star in Be/X-ray binaries (2002, methodology): “Following their formulation, Negueruela & Okazaki (2001) and Okazaki & Negueruela (2001a) showed that the Be disc in Be/X-ray binaries is truncated via the resonant interaction with the neutron star as long as αSS ≪ 1.”
High-mass X-ray binaries in the Milky Way (2013, background): “…to a source proximity it is extremely bright in the optical and infrared wavebands (mV,B ≃ 6), that is allowing to investigate and modeling the physical properties and behavior of Be-systems at different time scales (see, e.g., Roche et al., 1993; Clark et al., 2001b; Okazaki and Negueruela, 2001).”
THE ROTATION RATES OF MASSIVE STARS: THE ROLE OF BINARY INTERACTION THROUGH TIDES, MASS TRANSFER, AND MERGERS (2012, background): “…loss by stellar wind or tides is not efficient enough, it is likely that a near-Keplerian disk is formed when the thermal motion of gas particles at the equator becomes sufficient to overcome the escape speed, such that they are launched into orbit around the star (Okazaki & Negueruela 2001).”
Be/X-ray binaries (2011, methodology): “Zhang et al. (2004) extended the application of the viscous decretion disc model of Okazaki & Negueruela (2001) to compact companions of arbitrary mass and showed that the most effective Be disc truncation would occur in relatively narrow systems.”
Compact Object Modeling with the StarTrack Population Synthesis Code (2005, background): “However, most Be XRBs show periodic outbursts and are called transient Be XRBs. Transient Be XRBs exhibit two different types of outbursts (e.g., Bildsten et al. 1997; Okazaki & Negueruela 2001; Hayasaki & Okazaki 2005; Baykal et al. 2005): 1.”