Volume 447, Number 1, February III 2006
|Page(s)||145 - 156|
|Published online||27 January 2006|
Low-resolution VLT spectroscopy of GRBs 991216, 011211 and 021211
European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile e-mail: email@example.com
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
Accepted: 6 October 2005
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 and , where the highest redshift most likely reflects the distance to the host galaxy. A third system may be detected at . 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 06 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 Lyα absorption line in the host, to which we fit a neutral hydrogen column density of log N() = , which indicates that it is a damped Lyα system. Using a curve-of-growth analysis, we estimate the Si, Fe and Al metallicity at the GRB 011211 redshift to be [Si/H] = , , and [Al/H] = . For GRB 021211, we detect a single emission line in a spectrum obtained tens of days after the burst, which we identify as  λ3727 at . 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
© ESO, 2006
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