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abstract: |
    We present high time resolution (1.09 s) photometry of obtained with ULTRASPEC mounted on the ESO/3.6-m telescope, starting 68.22 min after the burst and lasting for 26.45 min. The light curve is smooth on both short (down to 2.18 s) and long time scales, confirmed by a featureless power spectrum. On top of the fireball power-law decay, bumps and wiggles at different time scales can, in principle, be produced by density fluctuations in the circumburst medium, substructures in the jet or by refreshed shocks. Comparing our constraints with variability limits derived from kinematic arguments, we exclude under-density fluctuations producing flux dips larger than 1 per cent with time scales $\Delta t > 9.2$ min (2 per cent on $\Delta t > 2.3$ min for many fluctuating regions). In addition, we study the afterglow VLT/FORS2 spectrum, the optical-to-X-ray spectral energy distribution (SED) and the time decay. The SED is best fit with a broken power law with slopes $\beta_{\mathrm{opt}}=0.71\pm0.01$ and $\beta_{X}=1.59\pm0.07$, in disagreement with the fireball model, suggesting a non-standard afterglow for . We find $A_V=0.18\pm0.03$ mag optical extinction due to SMC-like dust and an excess X-ray absorption of log $(N_{\mathrm{H}}/$cm$^{-2})=21.58^{+0.18}_{-0.26}$ assuming Solar abundances. The spectral analysis reveals 