Average power density spectrum of long GRBs detected with BeppoSAX/GRBM and with Fermi/GBM

From past experiments the average power density spectrum (PDS) of gamma-ray bursts (GRBs) with unknown redshift was found to be modelled from 0.01 to 1 Hz with a power law, f(-alpha), with alpha broadly consistent with 5/3. Recent analysis of the Swift/Burst Alert Telescope (BAT) catalogue showed analogous results in the 15-150 keV band. We carried out the same analysis on the bright GRBs detected by BeppoSAX/Gamma-ray Burst Monitor (GRBM) and Fermi/Gamma-ray Burst Monitor (GBM). The BeppoSAX/GRBM data, in the energy range 40-700 keV and with 7.8 and 0.5 ms time resolutions, allowed us to explore for the first time the average PDS at very high frequencies (up to 1 kHz) and reveal a break around 1-2 Hz, previously found in Compton Gamma Ray Observatory/Burst and Transient Source Experiment data. The Fermi/GBM data, in the energy band 8-1000 keV, allowed us to explore for the first time the average PDS within a broad energy range. Our results confirm and extend the energy dependence of the PDS slope, according to which harder photons have shallower PDS.