  In the context of high-frequency quasi-periodic oscillation (HF QPOs) we
further explore the appearance of an observable signal generated by hot spots
moving along quasi-elliptic trajectories close to the innermost stable circular
orbit in the Schwarzschild spacetime. The aim of our investigation is to reveal
whether observable characteristics of the Fourier power-spectral density can
help us to distinguish between the two competing models, namely, the idea of
bright spots orbiting on the surface of an accretion torus versus the scenario
of intrinsic oscillations of the torus itself. We take the capabilities of the
present observatories (represented by the Rossi X-ray Timing Explorer, RXTE)
into account, and we also consider the proposed future instruments (represented
here by the Large Observatory for X-ray Timing, LOFT).
