2006A&A...455..423M

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. Photometric, spectroscopic and polarimetric monitoring of the optical afterglow. Photometry, collected at 7 different telescopes, shows that a smoothly broken powerlaw decay, with indices α₁=0.76±0.03, α₂=1.50±0.07 and a break at t_b=0.48±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 θ_jet∼3.2°. An achromatic bump starting around ∼0.2d 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±0.0006 and at z=1.295±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-α Absorber. The optical V-band afterglow appears polarized, with P=(2.4±0.6)% and θ=170°±7°, 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 β_opt=0.47±0.15, and an X-ray slope β_X=1.0±0.2. 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.