We present results of optical spectroscopic observations of the mass donor star in SS 433 with Subaru and Gemini , with an aim to best constrain the mass of the compact object .
Subaru/FOCAS observations were performed on 4 nights of October 6–8 and 10 , 2007 , covering the orbital phase of \phi = 0.96 - 0.26 .
We first calculate cross correlation function ( CCF ) of these spectra with that of the reference star HD 9233 in the wavelength range of 4740–4840 Å .
This region is selected to avoid ‘ ‘ strong ’ ’ absorption lines accompanied with contaminating emission components , which most probably originate from the surroundings of the donor star , such as the wind and gas stream .
The same analysis is applied to archive data of Gemini/GMOS taken at \phi = 0.84 - 0.30 by .
From the Subaru and Gemini CCF results , the amplitude of radial velocity curve of the donor star is determined to be 58.3 \pm 3.8 km s ^ { -1 } with a systemic velocity of 59.2 \pm 2.5 km s ^ { -1 } .
Together with the radial velocity curve of the compact object , we derive the mass of the donor star and compact object to be M _ { O } =12.4 \pm 1.9 { M _ { \odot } } and M _ { X } =4.3 \pm 0.6 { M _ { \odot } } , respectively .
We conclude , however , that these values should be taken as upper limits .
From the analysis of the averaged absorption line profiles of strong lines ( mostly ions ) and weak lines ( mostly neutrals ) observed with Subaru , we find evidence for heating effects from the compact object .
Using a simple model , we find that the true radial velocity amplitude of the donor star could be as low as 40 \pm 5 km s ^ { -1 } in order to produce the observed absorption-line profiles .
Taking into account the heating of the donor star may lower the derived masses to M _ { O } = 10.4 ^ { +2.3 } _ { -1.9 } { M _ { \odot } } and M _ { X } = 2.5 ^ { +0.7 } _ { -0.6 } { M _ { \odot } } .
Our final constraint , 1.9 { M _ { \odot } } \leq M _ { X } \leq 4.9 { M _ { \odot } } , indicates that the compact object in SS 433 is most likely a low mass black hole , although the possibility of a massive neutron star can not be firmly excluded .