Spectroscopic observations of the mass donor star in SS 433

Abstract

We have recently detected an absorption line component in the spectrum of the microquasar SS 433. We believe this component originates from the mass donor star. The absorption line component has radial velocities opposite that of the disk, and the strength of the lines relative to the continuum varies as expected for a supergiant mass donor star in the system. The spectrum resembles that of a mid-A supergiant star. Recent observations by other groups have led to conflicting results about the nature of the absorption spectrum. Here we present results from new blue spectra of SS 433 that were made at the orbital and precessional phases when the mass donor is best seen in the foreground above the plane of the obscuring disk. The orbital phase variations of the absorption line velocities and strengths are consistent with line formation in the photosphere of an A-supergiant companion. We present new estimates of the masses of both stars: Mo = 12:5±3:3 M⊙ and Mx = 4:4±0:8 M⊙.