  In this thesis a computational tool for modelling spectral features of X-ray
sources in a strong gravitational field is developed.
  Six transfer functions, the gravitational and Doppler shift, lensing,
emission angle, relative time delay, change of the polarization angle and
azimuthal emission angle, have been computed for light rays emitted from the
equatorial plane of the Kerr black hole and received by an observer at
infinity. The values of these functions for different angular momenta of the
black hole and inclination angles of the observer in Kerr ingoing coordinates
have been stored in the form of tables in a FITS file. Graphical
representations of the results are shown as an atlas of contour figures of
these functions.
  For the modelling of the spectra of an accretion disc, several general
relativistic codes have been developed. Some of them can be used inside a more
general spectral fitting package XSPEC for routine fitting of the data measured
by X-ray satellites. These are the models for the relativistic emission lines,
Compton reflection and for the general use as a relativistic extension of
existing models (convolution models). Other components of the tool have
extended features for studying non-stationary emission from accretion discs and
polarimetry.
  The newly developed codes have been employed in several applications: data
from XMM-Newton observations of Seyfert galaxy MCG-6-30-15 have been fitted,
emission from an X-ray illuminated spot orbiting near a black hole was
simulated, and polarization from an accretion disc illuminated by a primary
source located above the black hole was computed.
