Minimum variability time-scales of long and short GRBs

ABSTRACT We have investigated the variability of a sample of long and short Fermi/GBM Gammaray bursts (GRBs) using a fast wavelet technique to determine the smallest timescales. The results indicate different variability time scales for long and short burstsin the source frame and that variabilities on the order of a few milliseconds are notuncommon. The data also indicate an intriguing relation between the variability scaleand the burst duration.Key words: Gamma-ray bursts 1 INTRODUCTIONThe prompt emission from Gamma-ray Bursts (GRBs)shows very complicated time profiles that hitherto elude asatisfactory explanation. Fenimore & Ramirez-Ruiz (2000)reported a correlation between variability of GRBs and thepeak isotropic luminosity. The existence of the variability-luminosity correlation suggests that the prompt emissionlight curve is embedded with temporal information relatedto the microphysics of GRBs. Several models have been pro-posed to explain the observed temporal variability of GRBlightcurves. Leading models such as the internal shock model(reference) and the photospheric model (reference) link therapid variability directly to the activity of the central en-gine. Others invoke relativistic outflow mechanisms to sug-gest that local turbulence amplified through Lorentz boost-ing leads to causally disconnected regions which in turnact as independent centers for the observed prompt emis-sion. In more recent models, both Morsony et al. (2010)and Zhang & Yan (2011) argue that the temporal variabil-ity may show two different scales depending on the physicalmechanisms generating the prompt emission.In order to further our understanding of the promptemission phase of GRBs and to explicitly test some of thekey ingredients in the various models it is clearly importantto extract the variability for both short and long gamma-ray bursts in a robust and unbiased manner. It is also clear