The strongly polarized afterglow of GRB 020405

We report polarization measurements and photometry for the optical afterglow of the gamma-ray burst GRB 020405. We measured a highly significant 9.9% polarization (in V band) 1.3 days after the burst and argue that it is intrinsic to the GRB. The light-curve decay is well fitted by a t(-1.72) power law; we do not see any evidence for a break between 1.24 and 4.3 days after the burst. We discuss these measurements in the light of several models of GRB afterglows.