Models of QPOs in luminous low-mass X-ray binaries

Models of quasi-periodic oscillations (QPOs) in luminous X-ray sources and observational studies relevant to these models are reviewed. QPOs are found to belong to two or three distinct types defined by the spectral state of the source and the properties of the QPOs themselves. Models can be classified according to whether the magnetic field of the neutron star is or is not important and whether the QPOs are produced by luminosity oscillations or beaming. The weakness of the red noise component of the normal branch power spectrum and the discovery that the phase of horizontal branch oscillations and normal branch oscillations at higher energies lags that at lower energies are discussed. The implications of these observations for QPO models are presented. Several QPO models are assessed, including the boundary layer hot spot model, beat-frequency modulated accretion model, the beat-frequency modulated dissipation model, nonradial g-mode model, reflecting clumps model, and the oscillating obscuration model.