Sidebands to the lower kilohertz QPO in 4U1636-53

Abstract

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) 4U1636-53. These kilohertz QPOs are sidebands to the lower kilohertz QPO. The upper sideband has a frequency 55.5+-1.7 Hz larger than that of the contemporaneously measured lower kilohertz QPO. Such a sideband has now been measured at a significance >6 sigma in the power spectra of three neutron star LMXBs (4U1636-53, 4U1728-34, and 4U1608-52). We also confirm the presence of a sideband at a frequency ~55 Hz less than the frequency of the lower kilohertz QPO. The lower sideband is detected at a 3.5 sigma level, only when the lower kilohertz QPO frequency is between 800 and 850 Hz. In that frequency interval the sidebands are consistent with being symmetric around the lower kHz QPO frequency. The upper limit to the rms amplitude of the lower sideband is significant lower than that of the upper sideband for lower kilohertz QPO frequencies >850 Hz. Symmetric sidebands are unique to 4U1636-53. This might be explained by the fact that lower kilohertz QPO frequencies as high as 800-850 Hz are rare for 4U1728-34 and 4U1608-52. Finally, we also measured a low frequency QPO at a frequency of ~43 Hz when the lower kilohertz QPO frequency is between 700-850 Hz. 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.

6 pages, 5 Figures, accepted for publication in MNRAS

References in corpus (6)

• Nuclear-Powered Millisecond Pulsars and the Maximum Spin Frequency of Neutron Stars
• Non-linear resonance in nearly geodesic motion in low-mass X-ray binaries
• Evidence for a Millisecond Pulsar in 4U 1636-53 During a Superburst
• Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
• Non-linear resonance in the accretion disk of a millisecond pulsar
• Resonance in Forced Oscillations of an Accretion Disk and Kilohertz Quasi Periodic Oscillations

Cited by in corpus (8)

• X-ray time variability across the atoll source states of 4U 1636--53
• Disc-Jet coupling in the LMXB 4U1636-53 from INTEGRAL
• Mass-angular-momentum relations implied by models of twin peak quasi-periodic oscillations
• Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636-53
• High-frequency variability in neutron-star low-mass X-ray binaries
• Accreting Neutron Stars in Low-Mass X-Ray Binary Systems
• Rotational parameters of strange stars in comparison with neutron stars
• Simultaneous observations of a pair of kilohertz QPOs and a plausible 1860 Hz QPO from an accreting neutron star system