Program: GN-2006A-Q-23

We have recently detected an absorption line component in the spectrum of the microquasar SS~433. We believe this component originates in the mass donor star in this unusual X-ray binary. 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. A preliminary radial velocity curve from this data combined with radial velocity amplitudes for the disk/compact object gives mass estimates for both components. The resulting mass for the compact companion is $2.9\pm 0.7 M_\odot$, making it likely a black hole, though the possibility of a neutron star cannot be ruled out. Further observations of SS~433 in the same spectral range will confirm whether this absorption line component is indeed from the mass donor. Observations from quadrature through eclipse will also greatly reduce the errors on our component masses and may allow us to confirm the nature of the compact object and the evolutionary stage of SS~433. This is a resubmission of an accepted proposal from semester 2004B which was not observed due to the tight scheduling constraints.