1999ApJ...519..779S

As part of our multiwavelength campaign of observations of GX 339-4 in 1996, we present our Keck spectroscopy performed on May 12 UT. At this time, neither the All-Sky Monitor (ASM) on the Rossi X-Ray Timing Explorer (RXTE) nor the Burst and Transient Source Experiment (BATSE) on the Compton Gamma-Ray Observatory (CGRO) detected the source. The optical emission was still dominated by the accretion disk, with V~17 mag. The dominant emission line is Hα, and for the first time we are able to resolve a double-peaked profile. The peak separation Δv=370±40 km.s^–1. Double-peaked Hα emission lines have been seen in the quiescent optical counterparts of many black hole X-ray novae. However, we find that the peak separation is significantly smaller in GX 339-4, implying that the optical emission comes from a larger radius than in the novae. The Hα emission line may be more akin to the one in Cyg X-1, where it is very difficult to determine whether the line is intrinsically double peaked, because absorption and emission lines from the companion star dominate.