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We present a new dynamical study of the black hole X-ray transientGRS1915+105 making use of near-infrared spectroscopy obtained with X-shooter atthe VLT. We detect a large number of donor star absorption features across awide range of wavelengths spanning the H and K bands. Our 24 epochs covering abaseline of over 1 year permit us to determine a new binary ephemeris includinga refined orbital period of P=33.85 +/- 0.16 d. The donor star radial velocitycurves deliver a significantly improved determination of the donorsemi-amplitude which is both accurate (K_2=126 +/- 1 km/s) and robust againstchoice of donor star template and spectral features used. We furthermoreconstrain the donor star's rotational broadening to vsini=21 +/-4 km/s,delivering a binary mass ratio of q=0.042 +/- 0.024. If we combine these newconstraints with distance and inclination estimates derived from modelling theradio emission, a black hole mass of M_BH=10.1 +/- 0.6 M_sun is inferred,paired with an evolved mass donor of M_2=0.47 +/- 0.27 M_sun. Our analysissuggests a more typical black hole mass for GRS1915+105 rather than theunusually high values derived in the pioneering dynamical study by Greiner etal. (2001). Our data demonstrate that high-resolution infrared spectroscopy ofobscured accreting binaries can deliver dynamical mass determinations with aprecision on par with optical studies.

The not-so-massive black hole in the microquasar GRS1915+105