Interpretation of observations of the circumbinary disk of SS 433
Context. The Galactic microquasar SS433 possesses a circumbinary disk most clearly seen in the brilliant Balmer Hα emission line. The orbital speed of the glowing material is an important determinant of the mass of the binary system. The circumbinary disk may be fed through the L2 point and in turn may feed a very extended radio feature known as the ruff. Aims. We present (i) an analysis of spectroscopic optical data from Hα and He I spectral lines which reveal the circumbinary disk (ii) comparisons of the rather different signals, to better understand the disk and improve estimates of the rotational speed of the inner rim (iii) a simple model that naturally explains some apparently bizarre spectral variations with orbital phase. Methods. Published spectra, taken almost nightly over two orbital periods of the binary system, show Hα and He I lines. These were analysed as superpositions of Gaussian components and a simple model in terms of a circumbinary disk was constructed. The possible contributions to the signal of an outflow through the L2 point were considered. Results. The data can be understood in terms of a hot spot, generated in proximity to the compact object and rotating round the inner circumbinary disk with a period of 13 days. The glowing material fades with time, quite slowly for the Hα source but more rapidly for the He I spectral lines. The orbital speed of the inner rim is approximately 250 kms −1 . It may be that absorption lines attributed to the atmosphere of the companion are in fact formed in this circumbinary material.