History
QPOs were first identified in white dwarf systems and then in neutron star systems.
At first the neutron star systems found to have QPOs were of a class (Z sources and atoll sources) not known to have pulsations. The spin periods of these neutron stars were unknown as a result. These neutron stars are thought to have relatively low magnetic fields so the gas does not fall mostly onto their magnetic poles, as in accreting pulsars. Because their magnetic fields are so low, the accretion disk can get very close to the neutron star before being disrupted by the magnetic field.
The spectral variability of these neutron stars was seen to correspond to changes in the QPOs. Typical QPO frequencies were found to be between about 1 and 60 hertz. The fastest oscillations were found in a spectral state called the Horizontal Branch, and were thought to be a result of the combined rotation of the matter in the disk and the rotation of the collapsed star (the "beat frequency model"). During the Normal Branch and Flaring Branch, the star was thought to approach its Eddington luminosity at which the force of the radiation could repel the accreting gas. This could give rise to a completely different kind of oscillation.
Observations starting in 1996 with the Rossi X-ray Timing Explorer could detect faster variability, and it was found that neutron stars and black holes emit X-rays that have QPOs with frequencies up to 1000 hertz or so. Often "twin peak" QPOs were found in which two oscillations of roughly the same power appeared at high amplitutes. These higher frequency QPOs may show behavior related to that of the lower frequency QPOs.
Read more about this topic: Quasi-periodic Oscillation
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