Discovery of a second kilohertz quasi-periodic oscillation peak in 4U 1608-52

M. Méndez, M. Van Der Klis, J. Van Paradijs, W. H.G. Lewin, B. A. Vaughan, E. Kuulkers, W. Zhang, F. K. Lamb, D. Psaltis

Research output: Contribution to journalArticlepeer-review

92 Scopus citations


Using a new technique to improve the sensitivity to weak quasi-periodic oscillations (QPOs), we discovered a new QPO peak at about 1100 Hz in the March 1996 outburst observations of 4U 1608-52, simultaneous with the ∼600-900 Hz peak previously reported from these data. The frequency separation between the upper and the lower QPO peaks varied significantly from 232.7 ± 11.5 Hz on March 3 to 293.1 ± 6.6 Hz on March 6. This is the first case of a variable kilohertz peak separation in an atoll source. We discuss to what extent this result could be accommodated in beat-frequency models such as proposed for the kilohertz QPOs. We measured the rms fractional amplitude of both QPOs as a function of energy, and we found that the relation is steeper for the lower than for the upper frequency peak. This is the first source where such a difference between the energy spectrum of the two kilohertz QPOs could be measured.

Original languageEnglish (US)
Pages (from-to)L65-L69
JournalAstrophysical Journal
Issue number1 PART II
StatePublished - 1998


  • Accretion, accretion disks
  • Stars: individual (4U 1608-52)
  • Stars: neutron
  • X-rays: stars

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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