VLT/X-Shooter spectroscopy of the afterglow of the Swift GRB 130606A^{⋆,⋆⋆,⋆⋆⋆}

Chemical abundances and reionisation at z ~ 6

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, PO Box 94249 1090 GE Amsterdam, The Netherlands
e-mail: olgahartoog@gmail.com
² Dark Cosmology Centre, Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Cøpenhagen O, Denmark
³ Institute of Astronomy and Department of Physics, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, 30013 Hsinchu, Taiwan, R.O.C
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, 19001 Casilla, Santiago 19, Chile
⁵ Department of Particle Physics and Astrophysics, Faculty of Physics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁶ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching bei München, Germany
⁷ INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁸ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁹ ASI-Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
¹⁰ Laboratoire GEPI, Observatoire de Paris, CNRS-UMR8111, Univ. Paris-Diderot, 5 place Jules Janssen, 92195 Meudon, France
¹¹ APC, Astroparticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
¹² Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavk, Iceland
¹³ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
¹⁴ The Oskar Klein Centre, Department of Astronomy, AlbaNova, 106 91 Stockholm, Sweden
¹⁵ University of Leicester, Department of Physics and Astronomy, University Road, Leicester LE1 7RH, UK
¹⁶ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
¹⁷ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
¹⁸ Finnish Centre for Astronomy withESO, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁹ Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
²⁰ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern, 0315 Oslo, Norway
²¹ Institut d’Astrophysique Spatiale, CNRS (UMR 8617) Université Paris-Sud 11, Bâtiment 121, Orsay, France

Received: 16 September 2014
Accepted: 15 June 2015

Abstract

Context. 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) afterglows. Furthermore, spectroscopy of a GRB afterglow provides insight in its host galaxy, which is often too dim and distant to study otherwise.

Aims. For the Swift GRB 130606A at z = 5.913 we have obtained a high S/N spectrum covering the full optical and near-IR wavelength region at intermediate spectral resolution with VLT/X-Shooter. We aim to measure the degree of ionisation of the intergalactic medium (IGM) between z = 5.02−5.84 and to study the chemical abundance pattern and dust content of its host galaxy.

Methods. We estimated the UV continuum of the GRB afterglow using a power-law extrapolation, then measured the flux decrement due to absorption at Lyα,β, and γ wavelength regions. Furthermore, we fitted the shape of the red damping wing of Lyα. The hydrogen and metal absorption lines formed in the host galaxy were fitted with Voigt profiles to obtain column densities. We investigated whether ionisation corrections needed to be applied.

Results. Our measurements of the Lyα-forest optical depth are consistent with previous measurements of QSOs, but have a much smaller uncertainty. The analysis of the red damping wing yields a neutral fraction x_{H i}< 0.05 (3σ). We obtain column density measurements of H, Al, Si, and Fe; for C, O, S and Ni we obtain limits. The ionisation due to the GRB is estimated to be negligible (corrections <0.03 dex), but larger corrections may apply due to the pre-existing radiation field (up to 0.4 dex based on sub-DLA studies). Assuming that [ Si/Fe ] = +0.79 ± 0.13 is due to dust depletion, the dust-to-metal ratio is similar to the Galactic value.

Conclusions. 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 IGM in the early Universe, but because of internal scatter we need a larger statistical sample to draw robust conclusions. The high [Si/Fe] in the host can be due to dust depletion, α-element enhancement, or a combination of both. The very high value of [ Al/Fe ] = 2.40 ± 0.78 might be due to a proton capture process and is probably connected to the stellar population history. We estimate the host metallicity to be −1.7 < [ M/H ] < −0.9 (2%−13% of solar).