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

TL;DR

This study refines the black hole mass in XTE J1859+226 using spectroscopic and photometric data, providing more precise binary parameters and supporting the relativistic precession model for QPOs.

Contribution

The paper presents the first simultaneous spectroscopic and photometric observations during quiescence, refining the black hole mass and binary parameters of XTE J1859+226.

Findings

Black hole mass refined to 7.8 ± 1.9 M☉

Orbital period constrained to 0.276 ± 0.003 days

Mass ratio determined as 0.07 ± 0.01

Abstract

We present two contiguous nights of simultaneous time-resolved GTC spectroscopy and WHT 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 $0.276 \pm 0.003$ d and the radial velocity semi-amplitude of the donor star to $K_2 = 550 \pm 59$ 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 $66.6 \pm 4.3$ deg, a refined $K_2 = 562 \pm 40$ km s$^{-1}$ and mass ratio $q = M_2/M_1 = 0.07 \pm 0.01$. From these values we obtained an updated black hole mass of $M_1 = 7.8 \pm 1.9$ M$_\odot$. An independent mass estimate based on X-ray timing agrees well with our value, which gives further support for the outburst QPO triplet being explained by the relativistic precession model. We also obtained a companion star mass $M_2 = 0.55 \pm 0.16$ M$_\odot$, which is consistent with its K5-K7 V spectral type.