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 12CO and 13CO 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 ν sin i = 26±3 km s-1 from the 12CO/13CO 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 = Md/Mx = 0.058 ± 0.033. This result, combined with (P, K, i) = (33.5 d, 140 km s-1, 66°) gives a more refined mass estimate for the black hole, Mx = 14.0 ±4.4 Mo, than previously estimated, using an inclination of i = 66° ± 2° (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 Md = 0.81 ± 0.53 M o, 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.