The mass function of GX 339-4 from spectroscopic observations of its donor star

TL;DR

This study uses spectroscopic observations of GX 339-4's donor star to measure its radial velocity, rotational velocity, and mass function, providing new constraints on the black hole's mass and suggesting it may lie in the 'mass-gap' between neutron stars and black holes.

Contribution

First spectroscopic measurement of the donor star's properties in GX 339-4, leading to revised estimates of the black hole mass and implications for the mass-gap.

Findings

Mass function f(M) = 1.91 ± 0.08 M_sun, lower than previous estimates.

Black hole mass constrained between 2.3 and 9.5 M_sun.

Donor star is a K-type star contributing ~45-50% of J/H-band light.

Abstract

We obtained 16 VLT/X-shooter observations of GX 339-4 in quiescence in the period May - September 2016 and detected absorption lines from the donor star in its NIR spectrum. This allows us to measure the radial velocity curve and projected rotational velocity of the donor for the first time. We confirm the 1.76 day orbital period and we find that $K_2$ = $219 \pm 3$ km s$^{-1}$, $\gamma = 26 \pm 2$ km s$^{-1}$ and $v \sin i = 64 \pm 8$ km s$^{-1}$. From these values we compute a mass function $f(M) =1.91 \pm 0.08~M_{\odot}$, a factor $\sim 3$ lower than previously reported, and a mass ratio $q = 0.18 \pm 0.05$. We confirm the donor is a K-type star and estimate that it contributes $\sim 45-50\%$ of the light in the $J$- and H-band. We constrain the binary inclination to $37^\circ < i < 78^\circ$ and the black hole mass to $2.3~M_{\odot} < M_\mathrm{BH} < 9.5~M_{\odot}$. GX 339-4 may therefore be the first black hole to fall in the 'mass-gap' of $2-5~M_{\odot}$.