Following a BeppoSAX alert ( Piro 1999a ) and the discovery of the OT at SAAO ( Vreeswijk et al .
1999a ) , we observed GRB 990510 with the FORS instrument on ESO ’ s VLT Unit 1 ( ‘ Antu ’ ) .
The burst is unremarkable in gamma rays , but in optical is the first one to show good evidence for jet-like outflow ( Stanek et al .
1999 , Harrison et al .
1999 ) .
We report the detection of significant linear polarization in the afterglow : it is 1.6 \pm 0.2 % 0.86 days after trigger , and after 1.81 days is consistent with that same value , but much more uncertain .
The polarization angle is constant on a time scale of hours , and may be constant over one day .
We conclude that the polarization is intrinsic to the source and due to the synchrotron nature of the emission , and discuss the random and ordered field geometries that may be responsible for it .