Twin peak separation in sources with kilohertz quasi-periodic oscillations caused by orbital motion

The expected range of kilohertz quasi-periodic oscillations (kHz QPOs) is studied regarding low-mass X-ray binaries with twin peaks in their Fourier spectra. The twin peaks are interpreted as a combination of epicyclic and precesion frequencies in a Schwarzschild metric. If the pairs are caused by general relativistic orbital motion of gaseous clumps around a neutron star, then the moderate size of the clumps and the eccentricity of their orbits can account for the nonconstant peak separations reported in Scorpius X-l and 4U 1608 - 52. The neutron star's mass comes out in the range of 1.6-1.9 M.. Variation in the orbital parameters of the clumps translates into an expected range in the plot of pair separation versus frequency, typically showing a decreasing slope.