Abstract Recent observational results indicate that quasi-periodic intensity oscillations (QPOs) in luminous X-ray sources are of two or three distinct types. These types are defined by the spectral state of the source (whether on the horizontal, normal, or flaring branches of an X-ray color-color diagram) and by the properties of the QPOs themselves. The sources exhibiting these QPOs are thought to be low-mass binary'systems in which a neutron star is accreting matter from its companion via an accretion disk. Models proposed so far can be classified according to whether the magnetic field of the neutron star is or is not important, and whether the quasi-periodic intensity oscillations are produced by luminosity oscillations or beaming. The strong red noise that accompanies horizontal branch oscillations is expected in many models. The weakness of the red noise accompanying normal branch oscillations poses more or less of a difficulty for all models so far proposed. The hard lags observed in horizontal branch oscillations may be to Comptonization by a central corona. If so, these oscillations are probably caused by oscillations in the luminosity of the X-ray star. The existence of a central corona would also help to explain why periodic oscillations at the spin frequency of the neutron star are so weak in these sources. Obscuration by rotating or oscillating disk matter has several attractive features as an explanation of horizontal branch oscillations but is difficult to reconcile with the Comptonization interpretation of the hard lags and places severe restrictions on the angles of inclination of QPO systems and on the dynamics of the inner disk. At present, the beat-frequency model appears to be the most attractive model for horizontal branch oscillations. It provides a quantitative explanation of the frequencies, frequency-intensity correlations, and other properties of such oscillations. Calculations of the amplitudes of periodic oscillations at the spin frequency of the neutron star give amplitudes of a fraction of 1%, consistent with current observational limits. Detection of such oscillations remains the most important test of the model. An accretion disk instability may be the most promising explanation of normal branch oscillations.
Models of QPOs in luminous low-mass X-ray binaries
Advances in Space Research ; 8 , 2-3 ; 421-447
1988-01-01
27 pages
Article (Journal)
Electronic Resource
English
Models of QPOs in luminous low-mass X-ray binaries
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