The X-ray binary Her X-1 consists of an accreting neutron star and the optical component HZ Her. The 35-day X-ray superorbital variability of this system is known since its discovery in 1972 by the Uhuru satellite and is believed to be caused by forced precession of a warped accretion disk tilted to the orbital plane. We argue that the observed features of the 35-day optical variability of HZ Her can be explained by free precession of the neutron star with a period close to that of the forced disk. The model parameters include a) the X-ray luminosity of the neutron star; b) the optical flux from the accretion disk; c) the tilt of the inner and outer edges of the accretion disk. A possible synchronization mechanism based on the coupling between the neutron star free precession and the dynamical action of non-stationary gas streams is discussed.

中文翻译：

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IMXB HZ Her/Her X-1 B 和 V 光曲线的 35 天超轨道周期建模

X 射线双星 Her X-1 由一颗吸积中子星和光学组件 HZ Her 组成。该系统的 35 天 X 射线超轨道变异性自 1972 年被 Uhuru 卫星发现以来就为人所知，据信是由倾斜到轨道平面的翘曲吸积盘的强制进动引起的。我们认为，观察到的 HZ Her 35 天光学变化的特征可以用中子星的自由进动来解释，其周期接近受迫盘的周期。模型参数包括 a) 中子星的 X 射线光度；b) 来自吸积盘的光通量；c) 吸积盘内外边缘的倾斜。讨论了一种基于中子星自由进动与非平稳气流动力学作用耦合的可能同步机制。