SS 433: O I lines and the circumbinary disk

Abstract

Context. The Galactic microquasar SS 433 may be the only local ULX. The stationary Balmer Hα and He I lines have two horned structures, interpreted as emission from a circumbinary disk orbiting a system of total mass approximately 40 M ⊙ . The implication is that the compact object is a rather massive black hole. Aims. To place on record the results of observations of two O I emission lines in the spectra of SS 433; observations that support not only the presence of a circumbinary disk orbiting the system but also indicate its stability during flare episodes. Methods. Stationary Hα and He I spectra were analysed some years ago and are in excellent agreement with radiation from a circumbinary disk. All the evidence so far presented was adduced during periods in which SS 433 was quiescent; there has been little concerning the existence or state of the circumbinary disk during the violent episodes associated with radio flares. The behaviour of O I emission lines before and during such an outburst is addressed. Results. The 8446 Å O I line has a two horned structure and the two components of the split run railroad straight over some eighty days, in the latter half of which there were violent changes in Hα and He I emission, culminating in a radio flare. Conclusions. O I 7772 Å is mostly formed in a dense equatorial wind but O I 8446 Å is fluoresced in the circumbinary disk. That disk is not perturbed sensibly by the violent behaviour in the vicinity of the compact object. O I 8446 Å acts as a tracer for the stable circumbinary disk orbiting at over 200 km s −1 , far outside the binary system.