The rotational broadening and the mass of the donor star of GRS 1915+105

The binary parameters of the microquasar GRS 1915 + 105 have been determined by the detection of Doppler-shifted (CO)-C-12 and (CO)-C-13 lines in its K-band spectrum (Greiner et al. 2001b). Here, we present further analysis of the same K-band VLT spectra and we derive a rotational broadening of the donor star of nu sin i = 26 +/- 3 km s(-1) from the (CO)-C-12/(CO)-C-13 lines. Assuming that the K-type star is tidally locked to the black hole and is filling its Roche-lobe surface, then the implied mass ratio is q = M-d/M-x = 0.058 +/- 0.033. This result, combined with (P; K; i) = (33.5 d, 140 km s(-1), 66degrees) gives a more refined mass estimate for the black hole, M-x = 14.0 +/- 4.4 M-., than previously estimated, using an inclination of i = 66degrees +/- 2degrees (Fender et al. 1999) as derived from the orientation of the radio jets and a more accurate distance. The mass for the early K-type giant star is M-d = 0.81 +/- 0.53 M-., consistent with a more evolved stripped-giant donor star in GRS 1915 + 105 than, for example, the donor star of the prototype black-hole X-ray transient, V404 Cyg which has the longest binary period after GRS 1915 + 105.