Interpretation of observations of the circumbinary disk of SS 433
  Context. The Galactic microquasar SS 433 is possessed of a circumbinary disk
most clearly seen in the brilliant Balmer H alpha 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. To present
an analysis of spectroscopic optical data from H alpha and He I spectral lines
which reveal the circumbinary disk. To use comparisons of the rather different
signals to better understand the disk and improve estimates of the rotational
speed of the inner rim. To present a simple model which naturally explains some
apparently bizarre spectral variations with orbital phase. Methods. Published
spectra, taken almost nightly over two orbital periods of the binary system,
are analysed. H alpha and He I lines are analysed as superpositions of Gaussian
components and a simple model constructed. Results. The data are understood in
terms of a hot spot, generated by proximity of the compact object, rotating
round the inner circumbinary disk with a period of 13 days. The glowing
material fades with time, quite slowly for the H alpha source but more rapidly
for the He I spectral lines. The orbital speed of the inner rim is
approximately 250 km/s. Conclusions. The mass of the binary system must exceed
40 solar masses and the compact object must be a rather massive stellar black
hole. The corollary is that the orbital speed of the companion must exceed 130
km/s.
