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 \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 \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 A _ { V } = 0.18 \pm 0.03 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 a damped Ly \alpha absorber ( log ( N _ { \mathrm { H { \sc 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 ) .