VLT/X-Shooter spectroscopy of the afterglow of the Swift GRB 130606A: Chemical abundances and reionisation at z ~ 6

The reionisation of the Universe is a process that is thought to have ended around z ~ 6, as inferred from spectroscopy of distant bright background sources, such as quasars (QSO) and gamma-ray burst (GRB) after glows. We aim to measure the degree of ionisation of the intergalactic medium (1GM) between z = 5.02-5.84 and to study the chemical abundance pattern and dust content of its host galaxy. We estimated the UV continuum of the GRB afterglow using a power-law extrapolation, then measured the flux decrement due to absorption at Ly[alpha], [beta], and [gamma] wavelength regions. Our measurements confirm that the Universe is already predominantly ionised over the redshift range probed in this work, but was slightly more neutral at z [>] 5.6. GRBs are useful probes of the ionisation state of the 1GM in the early Universe, but because of internal scatter we need a larger statistical sample to draw robust conclusions.