Probing GRB environments with time variability: ULTRASPEC fast imaging
  of GRB 080210
  We present high time resolution (1.09 s) photometry of GRB 080210 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 \Deltat > 9.2 min (2 per cent on \Deltat > 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}opt = 0.71 \pm 0.01
and {\beta}X = 1.59 \pm 0.07, in disagreement with the fireball model,
suggesting a non-standard afterglow for GRB 080210. We find AV = 0.18 \pm 0.03
mag optical extinction due to SMC-like dust and an excess X-ray absorption of
log (NH/cm-2) = 21.58 +0.18 -0.26 assuming Solar abundances. The spectral
analysis reveals a damped Ly{\alpha} absorber (log (NH I /cm-2) = 21.90 \pm
0.10) with a low metallicity ([X/H] = -1.21 \pm 0.16), likely associated with
the interstellar medium of the GRB host galaxy (z = 2.641).
