Inhibition of Accretion by the Stellar Wind in Misaligned Be/X-ray Binaries

Abstract

Be/X-ray binaries (BeXRBs) constitute a major subclass of high-mass X-ray binaries. They show intermittent X-ray activity with $L_X > 10^{36} {\rm erg s}^{-1}$, while remaining quiescent most of the time with $L_X < 10^{34} {\rm erg s}^{-1}$. BeXRBs generally have eccentric orbits as a result of supernova kicks when neutron stars were born. In these systems, the same kicks are also likely to make the binary orbital axis misaligned with the spin axis of the Be star. In such systems, when the neutron star captures gas from the equatorial disk of the Be star, the resulting accretion disk is in general tilted to both the Be disk plane and to the binary orbital plane. This raises an interesting possibility that in misaligned BeXRBs, the polar wind of the Be star collides with the accretion disk and significantly affects its structure by the large ram pressure. In this paper, we study the effects of the stellar wind on the accretion dynamics in misaligned BeXRBs. Using analytical wind and disk models, we first compare the wind's ram pressure with the gas pressures of the accretion flow to derive a condition for the stellar wind to strongly suppress accretion, and then apply the condition to a sample of BeXRBs whose relevant parameters are well determined or constrained. We find that wind-driven inhibition is a plausible mechanism for suppressing accretion in systems with slowly rotating neutron stars in wide orbits, where the classical propeller mechanism is expected to be inefficient. The effect is particularly important if the accretion flow is hot and low-density, or after the accretion rate has declined from the outburst level.