Strongly polarized optical afterglows of gamma-ray bursts

The optical afterglows of the gamma-ray bursts can be strongly polarized, i n principle up to tens of percents, if (1) the afterglow is synchrotron rad iation from an ultrarelativistic blast, (2) the blast is beamed during the afterglow phase, i.e., the shock propagates within a narrow jet, (3) we obs erve at the right time from the right viewing angle, and (4) magnetic field s parallel and perpendicular to the jet have different proper strengths.