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

Rudy Wijnands, Mariano Méndez, Michiel Der Van Klis, Dimitrios Psaltis, Erik Kuulkers, Frederick K. Lamb

Research output: Contribution to journalArticlepeer-review

45 Scopus citations


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% 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 were detected with frequencies between 18 and 56 Hz.

Original languageEnglish (US)
Pages (from-to)L35-L38
JournalAstrophysical Journal
Issue number1 PART II
StatePublished - 1998


  • Accretion, accretion disks
  • Stars: Individual (gx 5-1)
  • Stars: Neutron
  • X-rays: Stars

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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