Power density spectra of gamma-ray bursts

Power density spectra (PDSs) of long gamma-ray bursts (GRBs) provide useful information on GRBs, indicating their self-similar temporal structure. The best power-law PDSs are displayed by the longest bursts (T-90 > 100 s) in which the range of self-similar timescales covers more than 2 decades. Shor ter bursts have apparent PDS slopes more strongly affected by statistical f luctuations. The underlying power law can then be reproduced with high accu racy by averaging the PDSs for a large sample of bursts. This power law has a slope alpha approximate to -5/3 and a sharp break at similar to 1 Hz. Th e power-law PDS provides a new sensitive tool for studies of GRBs. In parti cular, we calculate the PDSs of bright bursts in separate energy channels. The PDS flattens in the hard channel (hv > 300 keV) and steepens in the sof t channel (hv < 50 keV), while the PDS of bolometric light curves approxima tely follows the - 5/3 law. We then study dim bursts and compare them to th e bright ones. We find a strong correlation between the burst brightness an d the PDS slope. This correlation shows that the bursts are far from being standard candles and dim bursts should be intrinsically weak. The time dila tion of dim bursts is probably related to physical processes occurring in t he burst rather than to a cosmological redshift.