1 INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
2 ASI-Science Data Center, via del Politecnico snc, 00133 Rome, Italy
3 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
4 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
5 INAF-Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
6 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
7 European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001 Santiago 19, Chile
8 INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
9 Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, E.T.S. Ingenieria, Universidad del Pais Vasco UPV/EHU, Alameda de Urquijo s/n, 48013 Bilbao, Spain
10 Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, 48008 Bilbao, Spain
11 INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
12 Max Planck Institute for Extraterrestrial Physics, 85748 Garching bei München, Germany
13 Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
14 School of Physics and Astronomy University of Southampton, Southampton, Hampshire, SO17 1BJ, United Kingdom
15 University of Oxford, Department of Physics, Astrophysics, Keble Road, Oxford, OX1 3RH, UK
16 Scuola Normale Superiore, Piazza dei Cavalieri 7, 56126 Pisa, Italy
17 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
18 GEPI, Observatoire de Paris, CNRS, Univ. Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
19 Physics Department, University of Calabria, 87036 Rende, Italy
Received: 15 November 2013
Accepted: 17 February 2014
Aims. We present a study of the environment of the Swift long gamma-ray burst GRB 120327A at z ≈ 2.8 through optical spectroscopy of its afterglow.
Methods. We analyzed medium-resolution, multi-epoch spectroscopic observations (R ~ 7000−12 000, corresponding to ~15−23 km s-1, S/N = 15−30 and wavelength range 3000−25 000 Å) of the optical afterglow of GRB 120327A, taken with X-shooter at the VLT 2.13 and 27.65 hr after the GRB trigger.
Results. The first epoch spectrum shows that the ISM in the GRB host galaxy at z = 2.8145 is extremely rich in absorption features, with three components contributing to the line profiles. The hydrogen column density associated with GRB 120327A has log NH/cm-2 = 22.01 ± 0.09, and the metallicity of the host galaxy is in the range [X/H] = −1.3 to −1.1. In addition to the ground state lines, we detect absorption features associated with excited states of C ii, O i, Si ii, Fe ii, and Ni ii, which we used to derive information on the distance between the host absorbing gas and the site of the GRB explosion. The variability of the Fe iiλ2396 excited line between the two epochs proves that these features are excited by the GRB UV flux. Moreover, the distance of component I is found to be dI = 200+100-60 pc, while component II is located closer to the GRB, at dII = 100+40-30 pc. These values are among the lowest found in GRBs. Component III does not show excited transitions, so it should be located farther away from the GRB. The presence of H2 molecules is firmly established, with a molecular fraction f in the range f = 4 × 10-7–10-4. This particularly low value can be attributed to the small dust content. This represents the third positive detection of molecules in a GRB environment.
Key words: ISM: abundances / dust, extinction / ISM: molecules / galaxies: ISM
Based on observations collected at the European Southern Observatory, ESO, the VLT/Kueyen telescope, Paranal, Chile, proposal code: 088.A-0051.
The reduced spectrum is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (220.127.116.11) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A38
© ESO, 2014