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

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  ± 0.12 reddened by an SMC-like extinction law with (a modest)  ± 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  ± 0.004. Based on the damped Lyα absorption line in the OA spectrum we measure the  column density to be  ± 0.2. This confirms the trend that GRBs tend to be located behind very large  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 , has  ± 0.001, ruling it out as the host of GRB 030429. The small impact parameter of this nearby galaxy, which is responsible for  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.