Evidence for a black hole in the X-ray transient XTE J1859+226

We present the results of time-resolved optical photometry and spectroscopy of the X-ray transient XTE J1859+226 (V406Vul). Photometric observations taken during 2000 and 2008 reveal the presence of the secondary star's ellipsoidal modulation. Further photometry obtained in 2010 shows the system similar or equal to 1 mag brighter than its quiescence level and the ellipsoidal modulation diluted by strong flaring activity. Spectroscopic data obtained with the 10.4-m GTC in 2010 reveal radial velocity variations of similar to 500 km s(-1) over 3 h. A simultaneous fit to the photometry and spectroscopy using sinusoids to represent the secondary star's ellipsoidal and radial velocity variations, yields an orbital period of 6.58 +/- 0.05 h and a secondary star's radial velocity semi-amplitude of K-2 = 541 +/- 70 km s(-1). The implied mass function is f (M) = 4.5 +/- 0.6M(circle dot), significantly lower than previously reported but consistent with the presence of a black hole in XTE J1859+226. The lack of eclipses sets an upper limit to the inclination of 70 degrees which yields a lower limit to the black hole mass of 5.42M(circle dot).