Optical and NIR monitoring of the GRB 020405 afterglow

Optical and near-infrared (NIR) observations of GRB020405 started about 1 day after the GRB and extended over ∼70 days. Pho-tometry shows that the early decay is consistent with a single power law of index α = 1.54±0.06 in all bands. The late epoch light curves, sampled with HST and VLT, exhibit a plateau or slight rebrightening around 10-20 days after the GRB. This bump can be modeled with a SN2002ap template underlying the afterglow. Alternatively, the late-epoch data can also be fitted using a power law with index steeper (α ′ = 1.85±0.15) than that of the early decay phase, in agreement with a late shell collision interpretation. Spectroscopy indicates that the GRB is at z = 0.691 and that the host galaxy complex is angularly close to a system of at least two galaxies at z = 0.472. R-band polarimetry shows that the afterglow is polarized, with P = 1.5±0.4 % and polarization angle θ = 172 • ±8 • .