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

D. Steeghs, J.E. McClintock, S.G. Parsons, M.J. Reid, S. Littlefair, V.S. Dhillon

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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.
Original languageEnglish
Article numberArt No. 185
Number of pages7
JournalThe Astrophysical Journal
Issue number2
Publication statusPublished - 10 May 2013
Externally publishedYes


  • binaries: close
  • techniques: radial velocities
  • X-rays: binaries

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