Variability of Gamma-Ray Bursts

Variability properties of Gamma-Ray Bursts were investigated using power spectrum of light curves of events recorded in BATSE and PHEBUS experiments. The averaging power spectra of more than 200 Ksec of BATSE data allowed us to obtain statistically robust results on flux variability on wide frequency scale (0.01 - 500 Hz) and for four energy bands. Bursts recorded in PHEBUS experiment were used for extension analysis for higher energy band. Significant variability was detected in the averaged power spectrum up to 100 Hz. The power density spectrum shows a complex structure which can be approximated in high frequencies by smoothed broken power law. Parameters of the approximation depend on both burst intensity and energy band. The overall dependencies may be explained only by cosmological evolution of GRB sources. We also investigate variability of early and late phases of GRB light curves. Within statistical errors the variability properties of both phases are identical. Based on the obtained results we discuss restrictions on model of GRB emission.