of wavelength for the two 300V FORS2 spectra. The bottom panel shows the ratio between the throughputs compared with the ratio between the two spectra. The agreement indicates that the slit throughputs have been reasonably well calculated.\
The response curve correction was performed using observations of the standard star LTT3864. The flux calibration was rescaled using $R$-band VLT/FORS2 images. $R$-band photometry was secured with VLT/FORS2, before and after our spectroscopic observation. Calibration was carried out by observing the Landolt standard fields SA100 and Rubin149, which allowed us to obtain a photometric accuracy of 0.02 mag. The GRB observations were carried out at large airmass (1.7–2.5), but so was also one of the two standard fields, which allowed a reliable extinction coefficient to be computed, in agreement with the value tabulated in the ESO web page. The photometric conditions were excellent according to the Paranal night logs.\

Data analysis
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ULTRASPEC light curve {#sec3p1}
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![The power spectrum of the ULTRASPEC light curve in fractional $\textrm{rms}^2$ units, after correcting for the decay. The minimum period detectable, given the time resolution, is 2.18 s, while the longest time scale monitored is 1587 s. The peaks in the power spectrum are generated by 