2003ApJ...595L..25M

We investigate the circular polarization (CP) from gamma-ray burst (GRB) afterglows. We show that a tangled magnetic field cannot generate CP without an ordered field, because there is always an oppositely directed field, so that no handedness exists. This implies that the observation of CP could be a useful probe of an ordered field, which carries valuable information on the GRB central engine. By solving the transfer equation of polarized radiation, we find that the CP reaches 1% at radio frequencies and 0.01% at optical for the forward shock and 10%-1% at radio and 0.1%-0.01% at optical for the reverse shock.