Power density spectra of modes of orbital motion in strongly curved space–time: obtaining the observable signal

Abstract

High frequency quasi-periodic oscillations (HF QPOs) appear in the X-ray\nvariability of several accreting low-mass binaries. In a series of works it was\nsuggested that these QPOs may have connection to inhomogeneities orbiting close\nto an inner edge of the accretion disc. In this paper we explore the appearance\nof an observable signal generated by small radiating circular hot spots moving\nalong quasi-elliptic trajectories close to the innermost stable circular orbit\nin the Schwarzschild spacetime. Our consideration takes into account the\ncapabilities of observatories that have been operating in the past two decades\nrepresented by the Rossi X-ray Timing Explorer (RXTE) and the proposed future\ninstruments represented by the Large Observatory for X-ray Timing (LOFT). For\nthese purposes we choose such model parameters that lead to lightcurves\ncomparable to those observed in Galactic black hole sources, in particular the\nmicroquasar GRS 1915+105. We find that when a weak signal corresponding to the\nhot-spot Keplerian frequency is around the limits of the RXTE detectability,\nthe LOFT observations can clearly reveal its first and second harmonics.\nMoreover, in some specific situations the radial epicyclic frequency of the\nspot can be detected as well. Finally, we also compare the signal produced by\nthe spots to the signal produced by axisymmetric epicyclic disc-oscillation\nmodes and discuss the key differences that could be identified via the proposed\nfuture technology. We conclude that the ability to recognize the harmonic\ncontent of the signal can help to distinguish between the different proposed\nphysical models.\n