SS433: A massive X-ray binary in an advanced evolutionary stage

INTEGRAL IBIS/ISGRI 18-60 keV observations of SS433 performed in 2003-2011 enabled for the first time the hard X-ray phase-resolved orbital and precessional light curves and spectra to be constructed. The spectra can be fitted by a power-law with photon index ≃ 3.8 and remain almost constant while the X-ray flux varies by a factor of a few. This suggests that the hard X-ray emission in SS433 is produced not in relativistic jets but in an extended quasi-isothermal hot 'corona' surrounding central parts of a supercritical accretion disc. Regular variations of the hard X-ray flux in SS433 exhibit, on top of the orbital and precessional variability, a nutational variability with a period of ~ 6.29 d. For the first time, a joint analysis of the broadband 18-60 keV orbital and precessional light curves was performed in the model that assumes a significant Roche lobe overfilling by the optical star, up to its filling the outer Lagrangian surface enabling mass loss through the outer Lagrangian L₂ point. From this modeling, the relativistic-to-optical component mass ratio q =M_x /M_v ≳ 0.4 ÷ 0.8 is estimated. An analysis of the observed long-term stability of the orbital period of SS433 with an account of the recent observations of SS433 by the VLTI GRAVITY interferometer enabled an independent mass ratio estimate q > 0.6. This estimate in combination with the radial velocity semi-amplitude for stationary He II emission, K_x = 168 ± 18^{km s - 1} (Hillwig et al., 2004) suggest the optical component mass in SS433 M_v > 12 M_⊙. Thus, the mass of the relativistic component in SS433 is M_x > 7 M_⊙, which is close to the mean mass of black holes in X-ray binaries ( ~ 8 M_⊙). The large binary mass ratio in SS433 allows us to understand why there is no common envelope in this binary at the secondary mass transfer evolutionary stage and the system remains semi-detached (van den Heuvel et al., 2017). We also discuss unsolved issues and outline prospects for further study of SS433.