@article{7200aaa1a5ad40b4a404190e9a5eba85,
title = "Discovery of microsecond time lags in kilohertz QPOs",
abstract = "Using the Rossi X-Ray Timing Explorer, we have measured \textasciitilde{}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 \textasciitilde{}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.",
keywords = "Accretion, accretion disks, X-rays: Stars",
author = "Vaughan, \{B. A.\} and \{Van Der Klis\}, M. and M. Medez and \{Van Paradijs\}, J. and Wljnands, \{R. A.D.\} and Lewin, \{W. H.G.\} and Lamb, \{F. K.\} and D. Psaltis and E. Kuulkers and T. Oosterbroek",
note = "Funding Information: The authors acknowledge Michael Nowak and Lev Titarchuk for useful comments and suggestions. B. A. V. acknowledges support from the United States National Aeronautics and Space Administration (NASA) under grants NAG 5-3340 and NAG 5-3293. J. v. P. acknowledges support from NASA under grant NAG 5-3269. T. O. acknowledges a European Space Agency (ESA) research fellowship. F. K. L. and D. P. acknowledge NSF grants AST-9315133 and AST-9618524 and NASA grant NAG 5-2925. M. M. is a fellow of the Consejo Nacional de Investiga-ciones Cient{\'i}ficas y T{\'e}cnicas de la Rep{\'u}blica Argentina. W. H. G. L. acknowledges support from NASA. This work was supported in part by the Netherlands Organization for Scientific Research (NWO) under grant PGS 78-277 and by the Netherlands Foundation for Research in Astronomy (ASTRON) under grant 781-76-017.",
year = "1997",
doi = "10.1086/310740",
language = "English (US)",
volume = "483",
pages = "L115--L118",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "American Astronomical Society",
number = "2 PART II",
}