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Twin peak high frequency quasi-periodic oscillations (HF QPOs) observed inthe power spectra of Low Mass X-ray Binaries (LMXBs), with either a black holeor a neutron star, have central frequencies that are typical of the orbitalmotion time-scale close to the compact object. Thus, twin HF QPOs might carrythe fingerprint of physical effects in a strongly curved space-time. We studythe azimuth phase \phi(t) for orbital motion in the Schwarzschild metric andcalculate the power spectra to check whether they display the features seen inthe observed ones. We show that the timing of \phi(t) on non-closed orbits canaccount for the observed twin peak HF QPOs. The uppermost couple of peaks infrequency has the lower peak that corresponds to the azimuthal frequency\nu_{\phi}, the upper one to \nu_{\phi}+\nu_r. The azimuth phase temporalbehavior \phi(t) on a slightly eccentric orbit in the Schwarzschild metric isdescribed by a linear function of slope \nu_{\phi} plus an oscillating term atthe relativistic radial frequency \nu_r. We deduce that the twin peak HF QPOsmight originate from a frequency modulated (FM) signal driven by the kinematicsof orbital motion in a curved space-time.

Twin peak quasi-periodic oscillations and the kinematics of orbital motion in a curved space-time