Physics of the GRB 030328 afterglow and its environment

Aims.To investigate the physical nature of the afterglow emission. We report on the photometric, spectroscopic and polarimetric observations of the optical afterglow of Gamma-Ray Burst (GRB) 030328 detected by HETE-2. Methods.Photometric, spectroscopic and polarimetric monitoring of the optical afterglow. Results.Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices $\alpha_1 = 0.76$ $\pm$ 0.03, $\alpha_2 = 1.50$ $\pm$ 0.07 and a break at $t_{\rm b}$ = 0.48 $\pm$ 0.03 days after the GRB, provides the best fit of the optical afterglow decline. This shape is interpreted as due to collimated emission, for which we determine a jet opening angle $\theta_{\rm jet} \sim$ 3$\fdg$2. An achromatic bump starting around ~0.2 d after the GRB is possibly marginally detected in the optical light curves. Optical spectroscopy shows the presence of two rest-frame ultraviolet metal absorption systems at z = 1.5216 $\pm$ 0.0006 and at z = 1.295 $\pm$ 0.001, the former likely associated with the GRB host galaxy. Analysis of the absorption lines at z = 1.5216 suggests that the host of this GRB may be a Damped Lyman-$\alpha$ Absorber. The optical V-band afterglow appears polarized, with P = (2.4 $\pm$ 0.6)% and $\theta$ = $170^{\circ}$ $\pm$ $7^{\circ}$, suggesting an asymmetric blastwave expansion. An X-ray-to-optical spectral flux distribution of the GRB 030328 afterglow was obtained at 0.78 days after the GRB and fitted using a broken powerlaw, with an optical spectral slope $\beta_{\rm opt} = 0.47$ $\pm$ 0.15, and an X-ray slope $\beta_{\rm X} = 1.0$ $\pm$ 0.2. Conclusions.The discussion of the results in the context of the "fireball model" shows that the preferred scenario for this afterglow is collimated structured jet with fixed opening angle in a homogeneous medium.