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 K₂ = 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 α emission-line profile and the binary parameters, we derived an orbital inclination of 66.6 ± 4.3 deg, a refined K₂ = 562 ± 40 km s^-1 and mass ratio q = M₂/M₁ = 0.07 ± 0.01. From these values, we obtained an updated black hole mass of M₁ = 7.8 ± 1.9 M_⊙. 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 M₂ = 0.55 +/- 0.16 M_⊙, which is consistent with its K5-K7 V spectral type.