kHz Quasiperiodic Oscillations in Low-Mass X-Ray Binaries as Probes of General Relativity in the Strong-Field Regime

We consider the interpretation of a pair of kHz quasiperiodic oscillations (QPOs) in the Fourier spectra of two low-mass x-ray binaries, Sco X-1 and 4U1608-52, hosting an old accreting neutron star. The observed frequency difference of these QPOs decreases as their frequency increases, contrary to simple beat frequency models. We show that the behavior of these QPOs is instead well matched in terms of the fundamental frequencies for test particle motion in the gravitational field of the neutron star, for reasonable star masses, and nearly independent of the star spin. These results are not reproduced through the post--Newtonian approximation of general relativity. kHz QPOs from x-ray binaries likely provide an accurate laboratory for strong-field general relativity.