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

本文用半解析方法和三维光滑粒子流体动力学数值模拟方法研究了Be/X射线双星中Be衰变盘与中子星星的相互作用。我们得到了以下结论。1. Be/X射线双星中的Be-星星盘在共振半径处被中子星星施加的负力矩截断。共振截断在小偏心率或中等偏心率以及轨道周期小于几十天的系统中特别有效。在这些系统中，从Be星星中衰变出来的物质积累起来，因此盘状物的密度比孤立的Be恒星周围的密度更快。在较高的轨道偏心率和/或较长的轨道周期的情况下，由于较弱的共振截断，中子星星的质量捕获率变得更高。这解释了显示I型爆发（周期性X射线爆发）的系统的轨道参数的趋势。在Be星盘中，由于单臂棒势的直接驱动，激发出偏心模。轨道偏心率越大，生长时间越短.与共面系统不同，在未对准系统中中子星星的质量俘获率有一个次峰。对于Be星盘和轨道平面之间的小倾角（i[小于或类似]30°），次峰比主峰弱得多，在质量捕获率曲线中形成驼峰。然而，对于大倾角（i[大于或类似]45°），它变得与主峰相当。5.从Be-星星盘转移来的物质在中子星星周围形成了一个依赖于时间的吸积盘。在Be星星的近星点通道处，盘体收缩，然后恢复其半径。中子星星的吸积速率远低于Be-星星盘的质量转移速率。

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.