Letter to the Editor

SS 433: Two robust determinations fix the mass ratio

University of Oxford, Department of Physics, Keble Road, Oxford, OX1 3RH UK
e-mail: michael.bowler@physics.ox.ac.uk

Received: 22 August 2018
Accepted: 24 October 2018

Abstract

Context. The unique Galactic microquasar SS 433 is an X-ray binary with a 13.08 day orbital period. Over some forty years, estimates of the mass of the compact object have spanned the range 1–30 solar masses; from neutron star to massive stellar black hole.

Aims. To fix the mass ratio q and hence the mass of the super-Eddington accretor.

Methods. We present a new and robust estimate of q derived from data on the circumbinary disk and compare it with a recent determination from the observed constancy of the binary period.

Results. These two robust analyses agree to a remarkable extent. They reinforce each other; q is ∼0.7 and the mass of the compact object 15 ± 2 M_⊙.

Conclusions. The mass is such as to identify the compact object as a massive stellar black hole.