Discovery of kilohertz quasi-periodic oscillations in the z source GX 5-1

摘要

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 and ～500 Hz on the left part of the horizontal branch to ～700 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 6.6/100 to 2.4/100, respectively.