Implications of the absence of eclipsing in SS 433

We investigate the implications of the observed absence of photometric and X-ray eclipsing in SS 433 and find that the most plausible explanation is that the binary system has an unusually large mass ratio, $$q\gtrsim 14$$. A large q model is inconsistent with an orbital motion interpretation of the observed radial velocity (semi-amplitude $$K\approx75\enspace \text{km}\enspace\text{s}^{-1}$$ of the ‘stationary’ lines. Consequently, we interpret these observations in terms of source motion within a hot spot located on the disc, as is often observed in the shorter period cataclysmic variables. A parameter, α, is defined which measures the relative contribution of the hot spot and disc to the observed line spectrum. If $$\alpha\ll1$$ then the hot spot dominates and the compact object mass $$M_1\sim0.1\enspace M_\odot$$. If $$\alpha\gtrsim1$$ the disc dominates and $$M_1\gtrsim1.0M_\odot$$. Since the compact object in SS 433 is evidently the product of a supernova explosion, the most likely candidate is a neutron star of mass $$M_1\sim1.0M_\odot$$, in which case the companion star mass $$M_2\lesssim0.07M_\odot$$ and the system may be similar in mass to the shorter period cataclysmic variable WZ Sge. In this scenario the low-mass companion star is the remnant of the once massive star which must have existed for the system to have survived the supernova explosion. We speculate that the companion star has been reduced to its present low mass by a runaway masstransfer process driven by angular momentum loss from the system, as suggested for WZ Sge.