A refined dynamical mass for the black hole in the X-ray transient XTE J1859+226

We present two contiguous nights of simultaneous time-resolved Gran Telescopio Canarias spectroscopy and William Herschel Telescope photometry of the black hole X-ray transient XTE J1859+226, obtained in 2017 July during quiescence. Cross-correlation of the individual spectra against a late K-type spectral template enabled us to constrain the orbital period to 0.276 +/- 0.003 d and the radial velocity semi-amplitude of the donor star to K-2 = 550 +/- 59 km s(-1). An ellipsoidal modulation is detected in the photometric r- and i-band light curves, although it is strongly contaminated by flickering activity. By exploiting correlations between the properties of the double-peaked H alpha emission-line profile and the binary parameters, we derived an orbital inclination of 66.6 +/- 4.3 deg, a refined K-2 = 562 +/- 40 km s(-1) and mass ratio q = M-2/M-1 = 0.07 +/- 0.01. From these values, we obtained an updated black hole mass of M-1 = 7.8 +/- 1.9 M-circle dot. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst quasi-periodic oscillation triplet being explained by the relativistic precession model. We also obtained a companion star mass M-2 = 0.55 +/- 0.16 M-circle dot, which is consistent with its K5-K7 V spectral type.