The Geometric Crisis in Cygnus~X-3: Limitations of Wind-Fed Accretion and the Case for Roche-Lobe Overflow

Abstract

Cygnus~X-3 is a Galactic X-ray binary with a 4.8-hr orbital period operating in the ultraluminous regime. Although the system is viewed at relatively low inclination ($i\approx28^\circ$), it exhibits a deep orbital modulation. Recent IXPE observations show strong linear polarization orthogonal to the radio jet, indicating that the X-ray emission is dominated by reflection from the inner walls of a supercritical outflow funnel. We propose a Hybrid Roche-lobe overflow (RLOF) scenario in which a massive Wolf-Rayet donor effectively fills its Roche lobe with a focused wind driving a super-Eddington accretion stream. Using a numerical synthesis of the folded light curve, we show that the modulation is reproduced when the central funnel is periodically occulted by a vertically extended, shock-heated Turbulent Wall formed by stream impact on the outer disk rim. This produces a phase lag ($\Delta\phi\approx0.11$) between X-ray minimum and binary conjunction, with extended attenuation by the WR wind defining a broader Suppression Region. This geometry explains the enhanced iron-line equivalent width during X-ray minimum via a coronagraphic effect. The large radial-velocity amplitude of FeXXVI measured by XRISM ($K_{\rm obs}\approx430$ km s$^{-1}$) and its zero-crossing at $\phi_X=0.0$ arise naturally in the stream-impact region rather than from orbital motion of the compact object. Finally, we show that the observed secular orbital expansion ($\dot P>0$) follows directly from highly non-conservative mass transfer with inner-disk mass loss, indicating that Cygnus~X-3 is a stable, long-lived system in a supercritical accretion regime.