The line-of-sight towards GRB 030429 at z : Probing the matter at stellar, galactic and intergalactic scales

Free access article

Issue		A&A Volume 427, Number 3, December I 2004


Page(s)		785 - 794
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20041233

A&A 427, 785-794 (2004)
DOI: 10.1051/0004-6361:20041233

The line-of-sight towards GRB 030429 at z $\mathsf{=2.66}$ : Probing the matter at stellar, galactic and intergalactic scales

P. Jakobsson^{1, 2}, J. Hjorth¹, J. P. U. Fynbo^{1, 3}, M. Weidinger^{3, 4}, J. Gorosabel^{5, 6}, C. Ledoux⁷, D. Watson¹, G. Björnsson², E. H. Gudmundsson², R. A. M. J. Wijers⁸, P. Møller⁴, K. Pedersen¹, J. Sollerman⁹, A. A. Henden¹⁰, B. L. Jensen¹, A. Gilmore¹¹, P. Kilmartin¹¹, A. Levan¹², J. M. Castro Cerón⁶, A. J. Castro-Tirado⁵, A. Fruchter⁶, C. Kouveliotou¹³, N. Masetti¹⁴ and N. Tanvir¹⁵

¹ Niels Bohr Institute, Astronomical Observatory, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
e-mail: pallja@astro.ku.dk
² Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík, Iceland
³ Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, 8000 Århus C, Denmark
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748, Garching bei München, Germany
⁵ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁸ Astronomical Institute, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁹ Stockholm Observatory, Department of Astronomy, AlbaNova, 106 91 Stockholm, Sweden
¹⁰ USRA/USNO Flagstaff Station, PO Box 1149, Flagstaff, AZ 86002, USA
¹¹ Mt John Observatory, Department of Physics and Astronomy, University of Canterbury, Christchurch 8020, New Zealand
¹² Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
¹³ NASA MSFC, SD-50 Huntsville, AL 35812, USA
¹⁴ Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, CNR, Via Gobetti 101, 40129 Bologna, Italy
¹⁵ Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK

(Received 5 May 2004 / Accepted 20 July 2004)

Abstract
We report the discovery of the optical afterglow (OA) of the long-duration gamma-ray burst GRB 030429, and present a comprehensive optical/near-infrared dataset used to probe the matter at different distance scales, i.e. in the burst environment, in the host galaxy and in an intervening absorber. A break in the afterglow light curve is seen approximately 1 day from the onset of the burst. The light curve displays a significant deviation from a simple broken power-law with a bright 1.5 mag bump with a duration of 2-3 days. The optical/near-infrared spectral energy distribution is best fit with a power-law with index $\beta = -0.36$ $\pm$ 0.12 reddened by an SMC-like extinction law with (a modest) A_V = 0.34 $\pm$ 0.04. In addition, we present deep spectroscopic observations obtained with the Very Large Telescope. The redshift measured via metal absorption lines in the OA is z = 2.658 $\pm$ 0.004. Based on the damped Ly $\alpha$ absorption line in the OA spectrum we measure the $\ion{H}{i}$ column density to be $\log N(\ion{H}{i}) = 21.6$ $\pm$ 0.2. This confirms the trend that GRBs tend to be located behind very large $\ion{H}{i}$ column densities. The resulting dust-to-gas ratio is consistent with that found in the SMC, indicating a low metallicity and/or a low dust-to-metal ratio in the burst environment. We find that a neighbouring galaxy, at a separation of only $1\farcs2$ , has z = 0.841 $\pm$ 0.001, ruling it out as the host of GRB 030429. The small impact parameter of this nearby galaxy, which is responsible for $\ion{Mg}{ii}$ absorption in the OA spectrum, is in contrast to previous identifications of most QSO absorption-selected galaxy counterparts. Finally, we demonstrate that the OA was not affected by strong gravitational lensing via the nearby galaxy.

Key words: gamma rays: bursts -- galaxies: distances and redshifts -- galaxies: high-redshift -- ISM: dust, extinction -- quasars: absorption lines

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