Discovery of microsecond time lags in kilohertz QPO

Using the Rossi X-ray Timing Explorer we have measured ∼27μs time delays in 830Hz quasiperiodic oscillations (QPO) between 4–6keV and 11–17keV in 4U 1608–52, with high-energy photons lagging low energy photons, and found upper limits to the time delays of 45μs between 2–6.5keV and 6.5–67keV in ∼730Hz QPO in 4U 0614+091 and 30μs between 8.7–12.4keV and 12.4–67keV in ∼870Hz QPO in 4U 1636–53. We also find that the cross coherence function between QPO at different energies is > 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. Subject headings: accretion, accretion disks — stars: neutron — X-rays: stars — stars: individual (4U 1608–52, 4U 0614+614, 4U 1636–53) Also Facultad de Ciencias Astronómicas y Geof́ısicas, Universidad Nacional de La Plata, Paseo del Bosque S/N, 1900 La Plata, Argentina Also Department of Physics, University of Alabama at Huntsville, Huntsville, AL 35899