the temporal slope and $\beta$ is the spectral slope. Hereafter we assume a standard $\Lambda$CDM cosmology with $H_0$ = 70.4 km s$^{-1}$ Mpc$^{-1}$, $\Omega_M$ = 0.27 and $\Omega_{\Lambda}$ = 0.73 [@Jarosik10].\

![Estimated slit throughputs (top panel) of the two 300V spectra due to the wavelength dependence of the PSF and the misalignment of the slit relative to the parallactic angle at the high airmass of these observations (2.3 at epoch 1 and 1.8 at epoch 2). In the bottom panel the ratio between the two slit throughputs (solid line) is overplotted on the ratio between the spectra at the two epochs (dotted line).[]{data-label="fig1"}](fig1.eps){width="85mm"}

Observations and data reduction
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*Swift* detection
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The *Swift* Burst Alert Telescope (BAT) triggered on on February 10th, 2008 at $T_0=$ 07:50:06 UT [@Grupe08]. The duration spanning 90 per cent of the GRB emission (15–350 keV) was $45\pm11$ s and its 15–150 keV fluence was $1.8\times\,10^{-6}$ erg cm$^{-2}$. The time integrated BAT spectrum is best fit by a simple power law with photon index $\Gamma=1.77\pm0.12$ [@Ukwatta08]. An X-ray afterglow was observed with the *Swift* X-Ray Telescope (XRT), starting 161 s (240 s) after the trigger in windowed timing (photon counting) mode. An optical afterglow was detected 