Discovery of KiloHertz Quasi-Periodic Oscillations in the Z source GX 5-1

We discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in the bright low-mass X-ray binary and Z source GX 5-1 with the Rossi X-ray Timing Explorer. In the X-ray color-color and hardness-intensity diagram a clear Z track is traced out, which shifted between observations. The frequencies of the two kHz QPOs increased from ~215 Hz and ~500 Hz on the left part of the horizontal branch to ~700 Hz and ~890 Hz, respectively, on the upper part of the normal branch. With increasing frequency the FWHM and rms amplitude (8.6-60 keV) of the higher-frequency kHz QPO decreased from 300 to 30 Hz, and from 6.6 to 2.4%, respectively. The FWHM and amplitude of the lower-frequency kHz QPO (50-100 Hz and 3-4% rms) did not correlate with the position of the source on the Z track. The kHz QPO separation was consistent with being constant at 298+/-11 Hz. Simultaneously with the kHz QPOs horizontal branch oscillations (HBOs) were detected with frequencies between 18 and 56 Hz.