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

TL;DR

This study provides evidence for a black hole in XTE J1859+226 through optical photometry and spectroscopy, revealing orbital parameters and mass estimates consistent with a black hole presence.

Contribution

First detailed optical time-resolved photometry and spectroscopy of XTE J1859+226 establishing orbital parameters and supporting black hole identification.

Findings

Orbital period of 6.58 hours determined.

Radial velocity semi-amplitude of 541 km/s measured.

Black hole mass lower limit of 5.42 solar masses established.

Abstract

We present the results of time-resolved optical photometry and spectroscopy of the X-ray transient XTE J1859+226 (V406 Vul). Photometric observations taken during 2000 and 2008 reveals the presence of the secondary star's ellipsoidal modulation. Further photometry obtained in 2010 shows the system ~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 reveals radial velocity variations of ~500 km/s 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. The implied mass function is f(M)=4.5+-0.6 Msun, 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 5.42 Msun.