The high-redshift gamma-ray burst GRB140515A

Abstract

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 GRB140515A ($z=6.327$). The best estimate of the neutral hydrogen fraction of the intergalactic medium (IGM) towards the burst is $x_{HI}\le 0.002$. The spectral absorption lines detected for this event are the weakest lines ever observed in gamma-ray burst afterglows, suggesting that GRB140515A exploded in a very low density environment. Its circum-burst medium is characterised by an average extinction (A${}_{\mathrm{V}}\sim 0.1$) that seems to be typical of $z\ge 6$ 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 GRB140515A from a Pop III (or from a Pop II stars with local environment enriched by Pop III) massive star is unlikely.

10 pages, 8 figures, 5 tables, submitted to Astronomy & Astrophysics

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