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X-ray light curves of three X-ray pulsars, SMC X-1, LMC X-4 and Her X-1,folded with their respective super-orbital periods, are shown to be wellreproduced by a model in which X-rays from a compact object towards us areperiodically obscured by a precessing ring at the outermost part of anaccretion disk around the central object. A situation is considered in whichmatter from a companion star flows into a gravitational field of a compact starcarrying a certain amount of specific angular momentum and first forms ageometrically thick ring-tube along the Keplerian circular orbit. For the modelto well fit to the observations, it is necessary that the optical depth of thering-tube for Compton scattering, $\tau \simeq 1 \sim 2$, the ring mattertemperature, $T \simeq 10^{5} \sim 10^{6}$ K and the ionization parameter, $\xi\simeq 10^{2}$ erg cm s$^{-1}$ due to X-ray heating from the central X-raysource. From simple energetics- and perturbation-arguments, we find that aprecession of such a ring is rather stable and possible to be excited in the$T$ and $\xi$ ranges. The time during which matter accumulates in the ring isestimated to be $\sim 10^{6}$ s, and is shown to be comparable to the time foran accretion disk to extend from the ring. It is discussed that in the above$T$ and $\xi$ ranges, the ring-tube matter could become thermally unstable.Then, relatively high density regions in the ring-tube further cools down andtends to shrink to the tube center. The flow across the ring circulating flowshould excite turbulent motions, and angular momenta of the matter would beeffectively transferred across the tube. Finally, a steady flow should beestablished from the companion star through the accretion ring to the accretiondisk towards the central compact star.

publications of the astronomical society of japan

Reproductions of super-orbital X-ray light-curves with the precessing  accretion ring model and implications on accretion flows through accretion  rings