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We present results of optical spectroscopic observations of the mass donorstar in SS 433 with Subaru and Gemini, with an aim to best constrain the massof the compact object. Subaru/FOCAS observations were performed on October 6-8and 10, 2007, covering the orbital phase of phi=0.96-0.26. We first calculatecross correlation function of these spectra with that of the reference star HD9233 in the wavelength range of 4740-4840 Angstrom. This region is selected toavoid 'strong' absorption lines accompanied with contaminating emissioncomponents. The same analysis is applied to archive data of Gemini/GMOS takenat phi=0.84-0.30 by Hillwig & Gies (2008). From the Subaru and Gemini CCFresults, the amplitude of radial velocity curve of the donor star is determinedto be 58.3+/-3.8 km s-1 with a systemic velocity of 59.2+/-2.5 km s-1. Togetherwith the radial velocity curve of the compact object, we derive the mass of thedonor star and compact object to be M_O=12.4+/-1.9 M_sun and M_X=4.3+/-0.6M_sun, respectively. We conclude, however, that these values should be taken asupper limits. From the analysis of the averaged absorption line profiles ofstrong lines and weak lines observed with Subaru, we find evidence for heatingeffects from the compact object. Using a simple model, we find that the trueradial velocity amplitude of the donor star could be as low as 40+/-5 km s-1 inorder to produce the observed absorption-line profiles. Taking into account theheating of the donor star may lower the derived masses to M_O=10.4 +2.3/-1.9M_sun and M_X=2.5 +0.7/-0.6 M_sun. Our final constraint, 1.9 M_sun< M_X <4.9M_sun, indicates that the compact object in SS 433 is most likely a low massblack hole, although the possibility of a massive neutron star cannot be firmlyexcluded.

Subaru and Gemini Observations of SS 433: New Constraint on the Mass of the Compact Object