Letter to the Editor

The reanalysis of spectra of GRB 080913 to estimate the neutral fraction of the IGM at a redshift of 6.7

¹ Astrophysics Group, Imperial College London, Blackett Laboratory, Prince Consort Road, London, SW7 2AZ, UK e-mail: m.patel06@imperial.ac.uk
² Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 21000 Copenhagen, Denmark

Received: 15 December 2009
Accepted: 22 February 2010

Abstract

Aims. We reanalyse optical spectra of the z=6.7 gamma-ray burst GRB 080913, adding hitherto unpublished spectra, to reassess the measurement of the neutral fraction of the IGM at high redshifts.

Methods. 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.

Results. We find a single significant absorption line redward of the Lyα continuum break, which we identify with the ii + Siii λ 0.126 μm 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, /cm^-2)<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, , 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 at a probability of 90%.