random noise, see Section \[sec3p1\] for more details. The lack of a preferred frequency confirms the smoothness of the light curve.[]{data-label="fig3"}](fig3.eps){width="85mm"}

The ULTRASPEC light curve is plotted in Fig. \[fig2\]. We Fourier transformed the ULTRASPEC light curve in order to investigate the variability and possible periodicities over all time scales. We exclude the flux variation due to the afterglow natural evolution by first correcting the light curve with the power-law fit to the decay. Figure \[fig3\] shows the power spectra of the unbinned light curve. Thanks to the fast sampling of ULTRASPEC, we can monitor the power spectrum down to 2.18 s time scales. The power spectrum of the light curve shows a peak at 2.8 s, likely due to random noise. Indeed, the peak height is not significant (3 per cent of the flux, corresponding to log $(\Delta F/F)= -1.57$, (log $\Delta t/t)=-3.2$ at 81.4 min), well below the instrument detection limit, see Sec. \[sec4p2\]. We performed a Monte Carlo type generation of 100,000 light curves with random variability equal to the measured GRB variability and found peaks of this height and greater to occur in 6 per cent of cases. In addition, the phase-folded light curve does not 