The Distance and Interstellar Sight Line to GX 339−4

The distance to the black hole binary GX339-4 remains a topic of debate. Weexamine high-resolution optical spectra of the NaD lines resolving the velocitystructure along the line of sight. We find this to be complex, with at leastnine components, mostly blue-shifted, spanning a velocity range of nearly200km/s. The presence of components with a large blue-shift rules out a nearbylocation and requires that the binary be located at or beyond the tangentpoint, implying a lower limit to the distance of ~6kpc. The presence of asignificant red-shifted component at +30km/s is even more intriguing as GX339-4also has a slightly positive systemic velocity, suggesting that the source, andthis cloud, could be on the far side of the Galaxy, where the radial velocitiesdue to Galactic rotation become positive again. If this is the case, we requirea distance of ~15kpc. This is less secure than the 6kpc lower limit however. Wediscuss the implications of these possible distances for the outburst andquiescent luminosities, and the nature of the companion star, and argue that alarge distance is consistent with these characteristics. In particular, itwould explain the non-detection of the companion star during the fainteststates.Comment: Accepted for publication in the Astrophysical Journa