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Discovery of a New, Third Kilohertz Quasi-periodic Oscillation in 4U 1608–52, 4U 1728–34, and 4U 1636–53: Sidebands to the Lower Kilohertz Quasi-periodic Oscillation?

We report the discovery of a third kilohertz quasi-periodic oscillation (kHz QPO) in the power spectra of the low-mass X-ray binaries 4U 1608-52 (6.3 σ), 4U 1728-34 (6.0 σ), and 4U 1636-53 (3.7 σ) which is present simultaneously with the previously known kHz QPO pair. The new kHz QPO is found at a frequency that is 52.8 ± 0.9, 64 ± 2, and 58.4 ± 1.9 Hz higher than the frequency of the lower kHz QPO in 4U 1608-52, 4U 1728-34, and 4U 1636-53, respectively. The difference between the frequency of the new kHz QPO and the lower kHz QPO increased in 4U 1608-52 from 49.6 ± 1.4 to 53.9 ± 0.5 Hz when the frequency of the lower kHz QPO increased from 672 to 806 Hz. Simultaneously the difference between the frequency of the new kHz QPO and the upper kHz QPO increased by ~60 Hz, suggesting that the new kHz QPO is unrelated to the upper kHz QPO. In 4U 1636-53 a fourth, weaker, kHz QPO is simultaneously detected (3 σ) at the same frequency separation below the lower kHz QPO, suggesting that the new kHz QPOs are sidebands to the lower kHz QPO. We discuss the nature of this new kHz QPO and its implications on the models for the kHz QPOs.