We present a new dynamical study of the black hole X-ray transient GRS1915+105 making use of near-infrared spectroscopy obtained with X-shooter at the VLT .
We detect a large number of donor star absorption features across a wide range of wavelengths spanning the H and K bands .
Our 24 epochs covering a baseline of over 1 year permit us to determine a new binary ephemeris including a refined orbital period of P = 33.85 \pm 0.16 d. The donor star radial velocity curves deliver a significantly improved determination of the donor semi-amplitude which is both accurate ( K _ { 2 } = 126 \pm 1 km/s ) and robust against choice of donor star template and spectral features used .
We furthermore constrain the donor star ’ s rotational broadening to v \sin { i } = 21 \pm 4 km/s , delivering a binary mass ratio of q = 0.042 \pm 0.024 .
If we combine these new constraints with distance and inclination estimates derived from modeling the radio emission , a black hole mass of M _ { BH } = 10.1 \pm 0.6 M _ { \odot } is inferred , paired with an evolved mass donor of M _ { 2 } = 0.47 \pm 0.27 M _ { \odot } .
Our analysis suggests a more typical black hole mass for GRS1915+105 rather than the unusually high values derived in the pioneering dynamical study by Greiner et al .
( 2001 ) .
Our data demonstrate that high-resolution infrared spectroscopy of obscured accreting binaries can deliver dynamical mass determinations with a precision on par with optical studies .