ar X iv : 0 80 9 . 01 89 v 1 [ as tr o - ph ] 1 S ep 2 00 8 Temporal variability of GRB early X - ray afterglows and GRB 080319 B prompt emission

We performed the first systematic search for the minimum variability time scale between 0.3 and 10 keV studying the 28 brightest early (< 3000 s) afterglows detected by Swift-XRT up to March 2008. We adopt the power spectrum analysis in the time domain: unlike the Fourier spectrum, this is suitable to study the rms variations at different time-scales. We find that early XRT afterglows show variability in excess of the Poissonian noise level on time-scales as short as ∼ 1 s (rest frame value), with the shortest tmin associated with the highest energy band. The γ−ray prompt emission of GRB 080319B shows a characteristic average variability time-scale < tvar >∼ 1s; this parameter undergoes a remarkable evolution during the prompt emission (BAT observation).