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Probing gamma-ray burst environments with time variability: ULTRASPEC fast imaging of GRB 080210

Authors


  • Based on observations collected with the ULTRASPEC visitor instrument built by a consortium from the Universities of Sheffield, Warwick, the UK Astronomy Technology Centre and ESO, mounted at the ESO/3.6-m telescope on La Silla, Chile, and on target-of-opportunity observations collected in service mode under programme ID 080.D-0526, PI Vreeswijk, with the FOcal Reducer/low dispersion Spectrograph 2 (FORS2) installed at the Cassegrain focus of the Very Large Telescope (VLT), Unit 1, Antu, operated by the European Southern Observatory (ESO) on Cerro Paranal in Chile.

E-mail: annalisa@raunvis.hi.is

ABSTRACT

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, by 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 Δt > 9.2 min (2 per cent on Δt > 2.3 min for many fluctuating regions). In addition, we study the VLT/FORS2 afterglow spectrum, the optical-to-X-ray spectral energy distribution (SED) and the time decay. The SED is best fitted with a broken power law with slopes βopt= 0.71 ± 0.01 and βX= 1.59 ± 0.07, in disagreement with the fireball model, suggesting a non-standard afterglow for GRB 080210. We find AV= 0.18 ± 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 Lyα absorber (loginline imagecm−2) = 21.90 ± 0.10) with a low metallicity ([X/H]=−1.21 ± 0.16), likely associated with the interstellar medium of the GRB host galaxy (z= 2.641).

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