2010A&A...512L...3P

We reanalyse optical spectra of the z=6.7 gamma-ray burst GRB 080913, adding hitherto unpublished spectra, to reassess the measurement of the neutral fraction of the IGM at high redshifts. In the data reduction, we take particular care to minimise systematic errors in the sky subtraction, which are evident in the published spectrum, and compromise our analysis. The final combined spectrum has a higher signal-to-noise ration (S/N) than the previously published spectrum by a factor of 1.3. We find a single significant absorption line redward of the Lyα continuum break, which we identify with the SII+SiII λ0.126µm blend, at z=6.733. The sharp spectral break at Lyα implies a comparatively low total column density of neutral hydrogen along the line of sight, log(N_HI/cm²)<20. We model the absorption with a host-galaxy DLA, surrounded by an ionised region of unknown size r, within the IGM of neutral fraction, x_HI. Despite knowing the source redshift, and the improved S/N of the spectrum, when fitting only over wavelengths redward of Lyα, no useful constraints on x_HI can be obtained. We consider the possibility of including the ionised region, blueward of Lyα, when constraining the fit. For the optimistic assumption that the ionised region is transparent, τ_GP≪1, we find that the region is of small size r<2 proper Mpc, and we obtain an upper limit to the neutral fraction of the IGM at z=6.7 of x_HI<0.73 at a probability of 90%.