The Mass Function of GX 339-4 from Spectroscopic Observations of Its Donor Star

We obtained 16 VLT/X-shooter observations of GX 339-4 in quiescence during the period 2016 May-September 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 +/- 3 km s(-1), gamma = 26 +/- 2 km s(-1), and nu sin i = 64 +/- 8 km s(-1). From these values we compute a mass function f(M) = 1.91 +/- 0.08 M-circle dot, a factor similar to 3 lower than previously reported, and a mass ratio q = 0.18 +/- 0.05. We confirm the donor is a K-type star and estimate that it contributes similar to 4%-50% of the light in the J- and H-bands. We constrain the binary inclination to 37 degrees < i < 78 degrees and the black hole (BH) mass to 2.3 M-circle dot < M-BH < 9.5 M-circle dot. GX 339-4 may therefore be the first BH to fall in the mass-gap of 2-5M(circle dot).