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

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) AV = 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.