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

Abstract

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 well reproduced by a model in which X-rays from a compact object towards us are periodically obscured by a precessing ring at the outermost part of an accretion disk around the central object. A situation is considered in which matter from a companion star flows into a gravitational field of a compact star carrying a certain amount of specific angular momentum and first forms a geometrically thick ring-tube along the Keplerian circular orbit. For the model to well fit to the observations, it is necessary that the optical depth of the ring-tube for Compton scattering, $\tau \simeq 1 \sim 2$, the ring matter temperature, $T \simeq 10^{5} \sim 10^{6}$ K and the ionization parameter, $\xi \simeq 10^{2}$ erg cm s$^{-1}$ due to X-ray heating from the central X-ray source. From simple energetics- and perturbation-arguments, we find that a precession 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 is estimated to be $\sim 10^{6}$ s, and is shown to be comparable to the time for an 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 and tends to shrink to the tube center. The flow across the ring circulating flow should excite turbulent motions, and angular momenta of the matter would be effectively transferred across the tube. Finally, a steady flow should be established from the companion star through the accretion ring to the accretion disk towards the central compact star.