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

TL;DR

This study refines the black hole and donor star parameters in GRS1915+105 using near-infrared spectroscopy, revealing a typical black hole mass and demonstrating the effectiveness of infrared methods for dynamical mass measurements.

Contribution

It provides the first robust infrared dynamical measurement of the black hole mass in GRS1915+105, improving previous estimates and validating infrared spectroscopy for such studies.

Findings

Black hole mass of 10.1 +/- 0.6 solar masses

Refined orbital period of 33.85 +/- 0.16 days

Donor star mass of 0.47 +/- 0.27 solar masses

Abstract

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 +/- 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 +/- 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 vsini=21 +/-4 km/s, delivering a binary mass ratio of q=0.042 +/- 0.024. If we combine these new constraints with distance and inclination estimates derived from modelling the radio 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 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.