Theoretical Interpretation of Kilohertz QPO from LMXBs

Physics

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The discovery of kilohertz quasi-periodic brightness oscillations in several neutron-star low-mass X-ray binaries has given us a remarkable new probe of accretion near neutron stars and, very probably, the properties of matter at high densities. I will discuss the currently most promising model of these QPOs, the sonic-point model. In the sonic-point model, the frequency of the higher frequency QPO in a pair is the Keplerian frequency at the point in the disk flow where the inward radial velocity increases steeply with decreasing radius and typically becomes supersonic, and the frequency of the lower frequency QPO peak is the beat frequency between the sonic-point Keplerian frequency and the stellar spin frequency or its overtones. This model explains naturally the most important features of the kilohertz QPOs observed in the atoll sources, including their frequencies, large amplitudes, and high coherence, the frequent appearance of a pair of QPOs with similar FWHM, and the dependence of the QPO amplitude on photon energy. It is also consistent with the physical picture constructed previously from observations of the X-ray spectra and lower-frequency variability of weak-field accreting neutron stars in low-mass binary systems. If the sonic-point model is correct, one can derive upper limits to the mass and radius of the neutron star and, for an assumed equation of state, estimates of the mass and radius of the source from measurements of the kilohertz QPOs. In principle, one can constrain the properties of dense matter by ruling out equations of state, particularly if QPOs with frequencies greater than 1300 Hz are observed.

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