Title: Spectroscopic observations of the mass donor star in SS 433
Author(s): K. Kubota, Y. Ueda (Kyoto U.), S. Fabrika, E. Barsukova (SAO)
Abstract: The microquasar SS 433 is a target of great interest in
modern astronomy as a unique Galactic source that shows steady
relativisitic jets. Although it has been studied for about 30 years
since its discovery, the compact object has not been firmly identified
yet. The best way to solve this problem is to determine its mass
function by directly measuring the Doppler shifts of the companion
star due to the binary motion. We observed SS 433 using the FOCAS
instrument on Subaru on 2007 October 5-9, and obtained good quarity
spectra over a one-third of the orbital period. We cross-correlate
these spectra with that of the standerd star HD 9233 in spectral
regions where only "weak" absorption lines are present to avoid
emission components not originating from the surface of the companion
star. We made the same analysis using archive data of Gemini. From the
Subaru and Gemni results, the amplitude of radial velocity curve of
the companion star is found to be 60.5 +/- 4.0 km/s with a gamma
velocity of 53.8 +/- 2.7 km/s. Combining our results with the radial
velocity of the compact star previously studied, we determine the mass
of the companion star and the compact star to be M_o=12.6(+4.2/-3.4)
M_sun and M_x=4.5(+1.3/-1.1) M_sun, respectively. Our results
establish that the compact star of the SS 433 is a black hole.

Abstract should be within about 250 words.