Origin of the X-Ray Activity in Be/X-Ray Binaries

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We have studied the interaction between the Be decretion disk and the neutron star in Be/X-ray binaries, by a semi-analytical method and by numerical simulations using a three dimensional Smoothed Particle Hydrodynamics code. We have obtained the following conclusions.1. The Be-star disk in Be/X-ray binaries is truncated at a resonant radius by the negative torque exerted by the neutrons star. The resonant truncation is particularly effective in systems with small or moderate eccentricity and orbital period shorter than several tens of days. In these systems, the material decreted from the Be star accumulates, so that the disk becomes denser more rapidly than if around an isolated Be star.2. The mass-capture rate by the neutron star becomes higher due to weaker resonant truncation in the case of a higher orbital eccentricity and/or a longer orbital period. This explains the trend in the orbital parameters of systems which have shown Type I outbursts (periodic X-ray outbursts).3. An eccentric mode is excited in the Be-star disk through direct driving due to a one-armed bar potential of the binary. The growth time is shorter for a higher orbital eccentricity.4. The mass-capture rate by the neutron star in misaligned systems has a secondary peak, unlike in coplanar systems. For small inclination angles (i【less than or similar】30°) between the Be-star disk and the orbital plane, the secondary peak is much weaker than the primary peak, making a hump in the mass-capture rate profile. However, for large inclination angles (i【greater than or similar】45°), it becomes comparable to the primary peak.5. The material transferred from the Be-star disk forms a time-dependent accretion disk around the neutron star. The disk shrinks at the periastron passage of the Be star and restores its radius afterwards. The accretion rate on to the neutron star is much lower than that of the mass-transfer rate from the Be-star disk.