Sonic‐Point Model of Kilohertz Quasi‐periodic Brightness Oscillations in Low‐Mass X‐Ray Binaries

Strong, coherent, quasi-periodic brightness oscillations (QPOs) withfrequencies ranging from about 300 Hz to 1200 Hz have been discovered with theRossi X-ray Timing Explorer in the X-ray emission from some fifteen neutronstars in low-mass binary systems. Two simultaneous kilohertz QPOs differing infrequency by 250 to 350 Hertz have been detected in twelve of the fifteensources. Here we propose a model for these QPOs. In this model the X-ray sourceis a neutron star with a surface magnetic field of 10^7 to 10^10 G and a spinfrequency of a few hundred Hertz, accreting gas via a Keplerian disk. Thefrequency of the higher-frequency QPO in a kilohertz QPO pair is the Keplerianfrequency at a radius near the sonic point at the inner edge of the Keplerianflow whereas the frequency of the lower-frequency QPO is approximately thedifference between the Keplerian frequency at a radius near the sonic point andthe stellar spin frequency. This model explains naturally many properties ofthe kilohertz QPOs, including their frequencies, amplitudes, and coherence. Weshow that if the frequency of the higher-frequency QPO in a pair is an orbitalfrequency, as in the sonic-point model, the frequencies of these QPOs placeinteresting upper bounds on the masses and radii of the neutron stars in thekilohertz QPO sources and provide new constraints on the equation of state ofmatter at high densities. Further observations of these QPOs may providecompelling evidence for the existence of a marginally stable orbit, confirminga key prediction of general relativity in the strong-field regime.Comment: 67 pages, including 15 figures and 5 tables; uses aas2pp4; final version to appear in the Astrophysical Journal on 1 December 199