The high-redshift gamma-ray burst GRB 140515A^⋆,⋆⋆

A comprehensive X-ray and optical study

¹ INAF–Osservatorio Astronomico Brera, via E. Bianchi 46, 23807 Merate ( LC), Italy
e-mail: andrea.melandri@brera.inaf.it
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ Unidad Asociada Grupo Ciencias Planetarias (UPV/EHU, IAA-CSIC), Departamento de Física Aplicada I, E.T.S. Ingeniería, Universidad del País Vasco (UPV/EHU), Alameda de Urquijo s/n, 48013 Bilbao, Spain
⁴ Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
⁵ Universitá degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁶ Racah Institute of Physics, The Hebrew University of Jerusalem, 91904 Jerusalem, Israel
⁷ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, 1000 Ljubljana, Slovenia
⁸ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁹ ASI – Science Data Center, via del Politecnico snc, 00133 Roma, Italy
¹⁰ INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone ( RM), Italy
¹¹ The Oskar Klein Center, Department of Astronomy, AlbaNova, Stockholm University, 10691 Stockholm, Sweden
¹² APC, U. Paris Diderot, CNRS/IN2P3, CEA/IRFU, Obs. Paris, Sorbonne Paris Cité, France
¹³ Particle Astrophysics Center, Fermi National Accelerator Laboratory, Batavia, IL 60510, USA
¹⁴ Department of Astronomy & Astrophysics, The University of Chicago, Chicago, IL 60637, USA
¹⁵ Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, IL 60637, USA
¹⁶ INAF – IASF Milano, via E. Bassini 15, 20133 Milano, Italy
¹⁷ GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, 5 place Jule Janssen, 92190 Meudon, France
¹⁸ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, 100012 Beijing, PR China

Received: 2 June 2015
Accepted: 13 July 2015

Abstract

High-redshift gamma-ray bursts (GRBs) offer several advantages when studying 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 this kind of distant events. We present the multi-wavelength analysis of the high-zSwift GRB GRB 140515A (z = 6.327). The best estimate of the neutral hydrogen fraction of the intergalactic medium towards the burst is x_HI ≤ 0.002. The spectral absorption lines detected for this event are the weakest lines ever observed in GRB afterglows, suggesting that GRB 140515A exploded in a very low-density environment. Its circum-burst medium is characterised by an average extinction (A_V ~ 0.1) that seems to be typical of z ≥ 6 events. The observed multi-band light curves are explained either with a very hard 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 in a Pop III (or in a Pop II star with a local environment enriched by Pop III) massive star is unlikely.