Sidebands to the lower kilohertz quasi-periodic oscillation in 4U 1636-53

In this paper we report on further observations of the third and fourth kilohertz quasi-periodic oscillations (QPOs) in the power spectrum of the low-mass X-ray binary (LMXB) 4U 1636-53. These kilohertz QPOs are sidebands to the lower kilohertz QPO. The upper sideband has a frequency 55.5 +/- 1.7Hz larger than that of the contemporaneously measured lower kilohertz QPO. Such a sideband has now been measured at a significance >6σ in the power spectra of three neutron-star LMXBs (4U 1636-53, 1728-34 and 1608-52). We also confirm the presence of a sideband at a frequency ~55Hz less than the frequency of the lower kilohertz QPO. The lower sideband is detected at a 3.5σ level only when the lower kilohertz QPO frequency is between 800 and 850Hz. In that frequency interval, the sidebands are consistent with being symmetric around the lower kilohertz QPO frequency. The upper limit to the rms amplitude of the lower sideband is significantly lower than that of the upper sideband for lower kilohertz QPO frequencies >850Hz. Symmetric sidebands are unique to 4U 1636-53. This might be explained by the fact that lower kilohertz QPO frequencies as high as 800-850Hz are rare for 4U 1728-34 and 1608-52. Finally, we also measured a low-frequency QPO at a frequency of ~43Hz when the lower kilohertz QPO frequency is between 700 and 850Hz. A similar low-frequency QPO is present in the power spectra of the other two systems for which a sideband has been observed. We briefly discuss the possibility that the sideband is caused by Lense-Thirring precession.