The detection of linear polarization in the afterglow of GRB 990510 and its theoretical implications

We present the recent discovery of linear polarization of the optical afterglow of GRB 990510. Effects that could introduce spurious polarization are discussed, showing that they do not apply to the case of GRB 990510, which is then intrinsically polarized. It will be shown that this observation constrains the emission mechanism of the afterglow radiation, the geometry of the fireball and degree of order of the magnetic field. We then present the theoretical interpretations of this observation with particular emphasis on the possibility of observing polarization in beamed fireballs.