  Spectral lines from a source orbiting around a compact object are studied.
Time variations of observed frequency and count rate due to motion of the
source and gravitational lensing are considered. Gravitational field of the
central object is described by the Kerr metric. It is shown that: (i)
simultaneous temporal and frequency resolution enables us to restrict
parameters of the model (inclination angle, position of the source, angular
momentum of the black hole); (ii) techniques of image restoration, familiar
from other fields of astronomy, can be applied to study inner regions of active
galactic nuclei. This contribution is relevant for extremely variable X-ray
sources with high parameter of efficiency, such as Seyfert 1 galaxy PHL 1092
observed by ROSAT.
