GRB 130606A within a sub-DLA at redshift 5.91

Context. Events such as GRB 130606A at z = 5.91, offer an exciting new window into pre-galactic metal enrichment in these very high redshifthost galaxies. Aims. We study the environment and host galaxy of GRB 130606A, a high-z event, in the context of a high redshiftpopulation of GRBs. Methods. We have obtained multiwavelength observations from radio to gamma-ray, concentrating particularly on the X-ray evolution as well as the optical photometric and spectroscopic data analysis. Results. With an initial Lorentz bulk factor in the range 0∼ 65-220, the X-ray afterglowevolution can be explained by a timedependent photoionization of the local circumburst medium, within a compact and dense environment. The host galaxy is a sub-DLA (log N (H I) = 19.85± 0.15), with a metallicity content in the range from∼1/7 to∼1/60 of solar. Highly ionized species (N V and Si IV) are also detected. Conclusions. This is the second highest redshiftburst with a measured GRB-DLA metallicity and only the third GRB absorber with sub-DLA HI column density. GRB ’ lighthouses’ therefore o ffer enormous potential as backlightingsources to probe the ionization and metal enrichment state of the IGM at very high redshiftsfor the chemical signature of the first generation of stars.