Discovery of microsecond time lags in kilohertz QPOs

B. A. Vaughan, M. Van Der Klis, M. Medez, J. Van Paradijs, R. A.D. Wljnands, W. H.G. Lewin, F. K. Lamb, D. Psaltis, E. Kuulkers, T. Oosterbroek

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


Using the Rossi X-Ray Timing Explorer, we have measured ~27 /us time delays in 830 Hz quasi-periodic oscillations (QPOs) between 4-6 and 11-17 keV in 4U 1608-52, with high-energy photons lagging low-energy photons, and found upper limits to the time delays of 45 jus between 2-6.5 and 6.5-67 keV in ~730 Hz QPOs in 4U 0614+091 and 30 /is between 8.7-12.4 and 12.4-67 keV in ∼870 Hz QPOs in 4U 1636-53. We also find that the cross-coherence function between QPOs at different energies is greater than 0.85 with 95% confidence in 4U 1608-52 and 4U 1636-53. If Compton upscattering of low-energy X-rays in a region with an optical depth of a few is responsible for the delays, then the Compton upscattering region is between a few kilometers and a few tens of kilometers in size.

Original languageEnglish (US)
Pages (from-to)L115-L118
JournalAstrophysical Journal
Issue number2 PART II
StatePublished - 1997


  • Accretion, accretion disks
  • X-rays: Stars

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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