  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 about 1 s (rest frame value), with the shortest t_{min}
associated with the highest energy band. The gamma-ray prompt emission of
GRB080319B shows a characteristic average variability time-scale t_{var} of
about 1s; this parameter undergoes a remarkable evolution during the prompt
emission (BAT observation).
