GX 339-4 - Black hole or neutron star X-ray binary?

Optical spectra of the highly variable X-ray source GX 339-4 show an emission-line spectrum characteristic of an accretion disk. Velocity variations suggest the orbital period lies between 0.2 and 2 days. Periods in this range limit the mass of the compact star to <2.5 M_sun;. The distance estimated from the interstellar lines and the color is ≡4 kpc. At this distance the velocity expected from galactic rotation differs by over 100 km s-1 from the observed systemic velocity.