The Not-so-massive Black Hole in the Microquasar GRS1915+105

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 Very Large Telescope. 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 yr permit us to determine a new binary ephemeris including a refined orbital period of P = 33.85 ± 0.16 days. The donor star radial velocity curves deliver a significantly improved determination of the donor semi-amplitude which is both accurate (K ₂ = 126 ± 1 km s^-1) and robust against choice of donor star template and spectral features used. We furthermore constrain the donor star's rotational broadening to vsin i = 21 ± 4 km s^-1, delivering a binary mass ratio of q = 0.042 ± 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 ± 0.6 M _⊙ is inferred, paired with an evolved mass donor of M ₂ = 0.47 ± 0.27 M _⊙. 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. 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.