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Title: VLT/X-Shooter spectroscopy of the afterglow of the Swift GRB 130606A Chemical abundances and reionisation at z similar to 6
Authors: Hartoog, O. E.
Malesani, D.
Fynbo, J. P. U.
Goto, T.
Kruehler, T.
Vreeswijk, P. M.
De Cia, A.
Xu, D.
Moller, P.
Covino, S.
D'Elia, V.
Flores, H.
Goldoni, P.
Hjorth, J.
Jakobsson, P.
Krogager, J-K.
Kaper, L.
Ledoux, C.
Levan, A. J.
Milvang-Jensen, B.
Sollerman, J.
Sparre, M.
Tagliaferri, G.
Tanvir, Nial
Postigo, A. D. U.
Vergani, S. D.
Wiersema, K.
Datson, J.
Salinas, R.
Mikkelsen, K.
Aghanim, N.
First Published: 20-Aug-2015
Publisher: EDP Sciences
Citation: Astronomy and Astrophysics, 2015, 580, A139 (15)
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 [Subscript: 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).
DOI Link: 10.1051/0004-6361/201425001
ISSN: 0004-6361
eISSN: 1432-0746
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Reproduced with permission from Astronomy & Astrophysics, © ESO. Deposited with reference to the publisher’s open access archiving policy, available at
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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