Stability of helium accretion discs in ultracompact binaries

Abstract

Stellar companions of accreting neutron stars in ultra compact X-ray binaries (UCXBs) are hydrogen-deficient. Their helium or C/O accretion discs are strongly X-ray irradiated. Both the chemical composition and irradiation determine the disc stability with respect to thermal and viscous perturbations. At shorter periods, UCXBs are persistent, whereas longer-period systems are mostly transient. To understand this behaviour one has to derive the stability criteria for X-ray irradiated hydrogen-poor accretion discs. We use a modified and updated version of the Dubus et al. code describing time-dependent irradiated accretion discs around compact objects. We obtained the relevant stability criteria and compared the results to observed properties of UCXBs. Although the general trend in the stability behaviour of UCXBs is consistent with the prediction of the disc instability model, in a few cases the inconsistency of theoretical predictions with the system observed properties is weak enough to be attributed to observational and/or theoretical uncertainties. Two systems might require the presence of some amount of hydrogen in the donor star.

Astronomy & Astrophysics, in press

References in corpus (4)

• Evolution of Low-Mass Helium Stars in Semidetached Binaries
• Dependence of the orbital modulation of X-rays from 4U 1820-303 on the accretion rate
• ADAFs, accretion discs and outbursts in compact binaries
• Discovery of the accretion-powered millisecond pulsar SWIFT J1756.9-2508 with a low-mass companion

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• Evolution of low-mass binaries with black-hole components
• The Formation, Evolution and Parameters of Short-Period Low-Mass X-Ray Binaries with Black-Hole Components