Issue A&A Volume 447, Number 1, February III 2006 Page(s) 145 - 156 Section Extragalactic astronomy DOI 10.1051/0004-6361:20053795

A&A 447, 145-156 (2006)
DOI: 10.1051/0004-6361:20053795

Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211

P. M. Vreeswijk^{1, 2}, A. Smette^{1, 3, 4}, A. S. Fruchter⁵, E. Palazzi⁶, E. Rol^{2, 7}, R. A. M. J. Wijers^{2, 8}, C. Kouveliotou⁹, L. Kaper², E. Pian^{6, 10}, N. Masetti⁶, F. Frontera^{11, 6}, J. Hjorth¹², J. Gorosabel¹³, L. Piro¹⁴, J. P. U. Fynbo¹², P. Jakobsson¹², D. Watson¹², P. T. O'Brien⁷ and C. Ledoux<sup>1

1</sup>  European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
    e-mail: pvreeswi@eso.org
²  Astronomical Institute "Anton Pannekoek", University of Amsterdam & Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³  Institut d'Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, 4000 Liège, Belgium
⁴  Chercheur qualifié, FNRS, Belgium
⁵  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁶  INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy
⁷  Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
⁸  Department of Physics and Astronomy, SUNY Stony Brook, NY 11794-3800, USA
⁹  National Space Science Technology Center, NASA/MSFC, XD-12, 320 Sparkman Drive, Huntsville, AL 35805, USA
¹⁰  INAF - Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
¹¹  Dipartimento di Fisica, Università di Ferrara, via Paradiso 12, 44100 Ferrara, Italy
¹²  Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹³  Instituto de Astrofísica de Andalucía, CSIC, Apartado 3004, 18080 Granada, Spain
¹⁴  INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy

(Received 8 July 2005 / Accepted 6 October 2005)

Abstract
We present low-resolution VLT spectroscopy of the afterglow of the gamma-ray bursts (GRBs) 991216, 011211 and 021211. Our spectrum of GRB 991216 is the only optical spectrum for this afterglow. It shows two probable absorption systems at z=0.80 and z=1.02, where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at z=0.77. HST imaging of the field, obtained 4 months after the burst, has resulted in the detection of two amorphous regions of emission, one at the projected afterglow position, and the other 0 6 away. The spectrum shows a depression in flux in between 4000 Å and 5500 Å. This could be the result of a 2175 Å-type extinction feature in the host of GRB 991216, but at a rather red wavelength of 2360 Å. If this interpretation is correct, it is the first time the extinction feature is seen in a GRB afterglow spectrum. It is centered at a wavelength similar to that of the ultra-violet (UV) bumps inferred from observations of a few UV-strong, hydrogen-poor stars in the Galaxy. All significant absorption lines (except for one) detected in the spectrum of GRB 011211 are identified with lines originating in a single absorption system at , the redshift of the GRB 011211 host galaxy. We also detect the Ly absorption line in the host, to which we fit a neutral hydrogen column density of log N( ) = , which indicates that it is a damped Ly system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB 011211 redshift to be [Si/H] = -0.9^+0.6_-0.4, , and [Al/H] = -1.0^+0.5_-0.3. For GRB 021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as [ ] 3727 at z=1.006. The corresponding unobscured [ ] star-formation rate is 1.4 M yr^-1.

Key words: gamma rays: bursts -- galaxies: abundances -- galaxies: distances and redshifts -- galaxies: quasars: absorption lines

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