  We study strong-gravity effects on modulation of radiation emerging from
accreting compact objects as a possible mechanism for flux modulation in QPOs.
We construct a toy model of an oscillating torus in the slender approximation
assuming thermal bremsstrahlung for the intrinsic emissivity of the medium and
we compute observed (predicted) radiation signal including contribution of
indirect (higher-order) images and caustics in the Schwarzschild spacetime. We
show that the simplest oscillation mode in an accretion flow, axisymmetric
up-and-down motion at the meridional epicyclic frequency, may be directly
observable when it occurs in the inner parts of accretion flow around black
holes. Together with the second oscillation mode, an in-and-out motion at the
radial epicyclic frequency, it may then be responsible for the high-frequency
modulations of the X-ray flux observed at two distinct frequencies (twin
HF-QPOs) in micro-quasars.
