Identification of the Mass Donor Star’s Spectrum in SS 433

We present spectroscopy of the microquasar SS 433 obtained near primaryeclipse and disk precessional phase Psi = 0.0, when the accretion disk isexpected to be most ``face-on''. The likelihood of observing the spectrum ofthe mass donor is maximized at this combination of orbital and precessionalphases since the donor is in the foreground and above the extended diskbelieved to be present in the system. The spectra were obtained over fourdifferent runs centered on these special phases. The blue spectra show clearevidence of absorption features consistent with a classification of A3-7 I. Thebehavior of the observed lines indicates an origin in the mass donor. Theobserved radial velocity variations are in anti-phase to the disk, theabsorption lines strengthen at mid-eclipse when the donor star is expected tocontribute its maximum percentage of the total flux, and the line widths areconsistent with lines created in an A supergiant photosphere. We discuss andcast doubt on the possibility that these lines represent a shell spectrumrather than the mass donor itself. We re-evaluate the mass ratio of the systemand derive masses of 10.9 +/- 3.1 Msun and 2.9 +/- 0.7 Msun for the mass donorand compact object plus disk, respectively. We suggest that the compact objectis a low mass black hole. In addition, we review the behavior of the observed emission lines from boththe disk/wind and high velocity jets.Comment: submitted to ApJ, 24 pages, 7 figure