High-redshift gamma-ray bursts have several advantages for the study of the distant universe , providing unique information about the structure and properties of the galaxies in which they exploded .
Spectroscopic identification with large ground-based telescopes has improved our knowledge of the class of such distant events .
We present the multi-wavelength analysis of the high- z Swift gamma-ray burst GRB 140515A ( z = 6.327 ) .
The best estimate of the neutral hydrogen fraction of the intergalactic medium ( IGM ) towards the burst is x _ { HI } \leq 0.002 .
The spectral absorption lines detected for this event are the weakest lines ever observed in gamma-ray burst afterglows , suggesting that GRB 140515A exploded in a very low density environment .
Its circum-burst medium is characterised by an average extinction ( A _ { V } \sim 0.1 ) that seems to be typical of z \geq 6 events .
The observed multi-band light curves are explained either with a very flat injected spectrum ( p = 1.7 ) or with a multi-component emission ( p = 2.1 ) .
In the second case a long-lasting central engine activity is needed in order to explain the late time X-ray emission .
The possible origin of GRB 140515A from a Pop III ( or from a Pop II stars with local environment enriched by Pop III ) massive star is unlikely .