Fourier-phase analysis of the orbiting bright-spot model for active galactic nucleus variability

Fourier power spectra and phases of a signal from a large number of radiating soul ces orbiting around a black hole are investigated, It is assumed that the individual sources (bright spots) are located in an accretion disc and their lifetime exceeds the corresponding orbital period, This model is relevant for the short-time-scale X-ray variability of active galactic nuclei, Previous works on this subject were mostly concentrated on temporal characteristics and power spectra of observed light curves, In our present contribution, Fourier phases are brought into consideration and studied systematically for a broad range of input parameters, In particular, conditions for the phase coherence art: discussed, It is shown that one can discriminate between the two classes of models that are currently under consideration - orbital motion of a large number of sources versus short-lived independent flares - although parameters of the model are not completely arbitrary. It is also shown that predicted power spectra depend rather strongly on the spot distribution across the disc surface, We conclude that the orbital motion of the spots cannot be the only reason for the source fluctuations, but it certainly influences observational properties of the source intrinsic variability.