Discovery of orbital eccentricity and evidence for orbital period increase of SS433

Abstract

ABSTRACT The examination of long-term (1979–2020) photometric observations of SS433 enabled us to discover a non-zero orbital eccentricity of e = 0.05 ± 0.01. We have also found evidence for a secular increase in the orbital period at a rate of $\dot{P}_\mathrm{b}=(1.0\pm 0.3)\times 10^{-7}$ s s−1. The binary orbital period increase rate makes it possible to improve the estimate of the binary mass ratio q = MX/MV > 0.8, where MX and MV are the masses of the relativistic object and the optical star, respectively. For an optical star mass of 10 M⊙, the mass of the relativistic object (a black hole) is MX > 8 M⊙. A neutron star in SS433 is reliably excluded because in that case the orbital period should decrease, in contradiction to observations. The derived value of $\dot{P}_\mathrm{b}$ sets a lower limit on the mass-loss rate in the Jeans mode from the binary system ≳7 × 10−6 M⊙ yr−1. The discovered orbital ellipticity of SS433 is consistent with the model of the slaved accretion disc tracing the precession of the misaligned optical star’s rotational axis.