GRB 990510: linearly polarized radiation from a fireball

Models for gamma-ray burst afterglows envisage an hyper-relativistic fireba ll that is decelerated in the ambient medium around the explosion site. Thi s interaction produces a shock wave which amplifies the magnetic field and accelerates electrons to relativistic energies, setting the conditions for an efficient production of synchrotron photons. If produced in a region of large-scale ordered magnetic field, synchrotron radiation can be highly pol arized. The optical transient associated with GRB 990510 was observed simil ar to 18.5 hr after the event and linear polarization in the R band was mea sured at a level of 1.7 +/- 0.2%. This is the first detection of linear pol arization in the optical afterglow of a gamma-ray burst. We exclude that th is polarization is due to dust in the interstellar material, either in our Galaxy or in the host galaxy of the gamma-ray burst. These results provide important new evidence in favor of the synchrotron origin of the afterglow emission, and constrains the geometry of the fireball and/or magnetic field lines.