The mysterious optical afterglow spectrum of GRB 140506A at z = 0.889^⋆,⋆⋆

¹ Dark Cosmology Centre, Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
e-mail: jfynbo@dark-cosmology.dk
² European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
³ Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, 440 West Brooks Street, Norman, OK 73019, USA
⁴ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁵ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Caólica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
⁶ Millennium Institute of Astrophysics, Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
⁷ Institut d’Astrophysique de Paris, CNRS-UPMC, UMR 7095, 98bis Bd Arago, 75014 Paris, France
⁸ Astronomical Institute Anton Pannekoek, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, the Netherlands
⁹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85748 Garching, Germany
¹⁰ Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
¹¹ INAF–Brera Astronomical Observatory, via Bianchi 46, 23807 Merate (LC), Italy
¹² INAF–Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
¹³ ASI-Science Data Center, via del Politecnico snc, 00133 Rome, Italy
¹⁴ Laboratoire Galaxies Étoiles Physique et Instrumentation, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹⁵ APC, Astroparticule et Cosmologie, Univ. Paris Diderot, CNRS/IN2P3, CEA/Irfu, Obs. de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
¹⁶ Laboratoire GEPI, Observatoire de Paris, CNRS-UMR 8111, Univ. Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
¹⁷ Faculty of Mathematics and Physics, University of Ljubljana, Jadranska ulica 19, 1000 Ljubljana, Slovenia
¹⁸ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
¹⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
²⁰ European Southern Observatory, Karl-Schwarzschildstrasse 2, 85748 Garching, Germany
²¹ Instituto de Astrofísica de Andalucía, Glorieta de la Astronomía s/n, 18008 Granada, Spain
²² Scuola Normale Superiore di Pisa, Piazza dei Cavalieri 7, 56126 Pisa, Italy
²³ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK

Received: 1 August 2014
Accepted: 17 September 2014

Abstract

Context. Gamma-ray burst (GRB) afterglows probe sightlines to star-forming regions in distant star-forming galaxies. Here we present a study of the peculiar afterglow spectrum of the z = 0.889Swift GRB 140506A.

Aims. Our aim is to understand the origin of the very unusual properties of the absorption along the line of sight.

Methods. We analyse spectroscopic observations obtained with the X-shooter spectrograph mounted on the ESO/VLT at two epochs 8.8 h and 33 h after the burst, and with imaging from the GROND instrument. We also present imaging and spectroscopy of the host galaxy obtained with the Magellan telescope.

Results. The underlying afterglow appears to be a typical afterglow of a long-duration GRB. However, the material along the line of sight has imprinted very unusual features on the spectrum. First, there is a very broad and strong flux drop below 8000 Å (~4000 Å in the rest frame), which seems to be variable between the two spectroscopic epochs. We can reproduce the flux-drops both as a giant 2175 Å extinction bump and as an effect of multiple scattering on dust grains in a dense environment. Second, we detect absorption lines from excited H i and He i. We also detect molecular absorption from CH⁺.

Conclusions. We interpret the unusual properties of these spectra as reflecting the presence of three distinct regions along the line of sight: the excited He i absorption originates from an H ii-region, whereas the Balmer absorption must originate from an associated photodissociation region. The strong metal line and molecular absorption and the dust extinction must originate from a third, cooler region along the line of sight. The presence of at least three separate regions is reflected in the fact that the different absorption components have different velocities relative to the systemic redshift of the host galaxy.