INTEGRAL observations of SS433: system's parameters and nutation of supercritical accretion disc

Abstract

Based on multiyear INTEGRAL observations of SS433 in 2003-2011, a composite IBIS/ISGRI 18-60 keV orbital light curve is constructed around zero precessional phases ψ_pr = 0 at the maximum accretion disc opening angle. It shows a peculiar shape with significant excess near the orbital phase φ_orb = 0.25, which is not seen in the softer 2-10 keV energy band. The 40-60 keV orbital light curve demonstrates two almost equal humps at phases ∼0.25 and ∼0.75, most likely due to nutation effects of the accretion disc. The nutational variability of SS433 in 15-50 keV with a period of ≃6.290 d is independently found from analysis of Swift/Burst Alert Telescope data. The change of the off-eclipse 18-60 keV X-ray flux with the precessional phase shows a double-wave form with strong primary maximum at ψ_pr = 0 and weak but significant secondary maximum at ψ_pr = 0.6. A weak variability of the 18-60 keV flux in the middle of the orbital eclipse correlated with the disc precessional phase is also observed. The joint analysis of the broad-band 18-60 keV orbital and precessional light curves confirms the presence of a hot extended corona in the central parts of the supercritical accretion disc and constrains the binary mass ratio in SS433 in the range 0.5 ≳ q ≳ 0.3, suggesting the black hole nature of the compact object.