Discovery of Microsecond Time Lags in Kilohertz QPOs

Using the Rossi X-Ray Timing Explorer, we have measured ~27 μs 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 μs between 2-6.5 and 6.5-67 keV in ~730 Hz QPOs in 4U 0614+091 and 30 μs 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.

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