Discovery of Kilohertz Quasi-periodic Oscillations in the Z Source GX 5−1

We discovered two simultaneous kHz quasi-periodic oscillations (QPOs) in thebright low-mass X-ray binary and Z source GX 5-1 with the Rossi X-ray TimingExplorer. In the X-ray color-color and hardness-intensity diagram a clear Ztrack is traced out, which shifted between observations. The frequencies of thetwo kHz QPOs increased from ~215 Hz and ~500 Hz on the left part of thehorizontal branch to ~700 Hz and ~890 Hz, respectively, on the upper part ofthe normal branch. With increasing frequency the FWHM and rms amplitude (8.6-60keV) of the higher-frequency kHz QPO decreased from 300 to 30 Hz, and from 6.6to 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 onthe Z track. The kHz QPO separation was consistent with being constant at298+/-11 Hz. Simultaneously with the kHz QPOs horizontal branch oscillations(HBOs) were detected with frequencies between 18 and 56 Hz.Comment: 11 Pages, 4 figures (one color figure). Accepted for publication in the Astrophysical Journal Letter