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

¹ 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 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.