2004A&A...414L..13H

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., 2001Natur.414..522G). Here, we present further analysis of the same K-band VLT spectra and we derive a rotational broadening of the donor star of υsin i=26±3km/s 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=(M_d)/(M_x)=0.058±0.033. This result, combined with (P,K,i)=(33.5d,140km/s,66°) gives a more refined mass estimate for the black hole, M_x=14.0±4.4M_☉, than previously estimated, using an inclination of i=66°±2° (Fender et al., 1999MNRAS.304..865F) 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±0.53M_☉, 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.