Highly polarized light from stable ordered magnetic fields in GRB 120308A

After the initial burst of prompt gamma rays that defines a Gamma Ray Burst (GRB), expanding ejecta collide with the circumburst medium and begin to decelerate at the onset of the afterglow, where – models predict – a forward shock travels outwards through the circumburst medium and a reverse shock propagates backwards into the oncoming flow. Light from the reverse shock should be highly polarized if the jet magnetic field is globally ordered and advected from the central engine, with a position angle predicted to remain stable in magnetized baryonic jet models or vary randomly with time if the magnetic field is produced locally by plasma or MHD instabilities. Optical linear polarization degrees P ~ 10% have previously been detected in the early afterglow, but the lack of temporal measurements prevented definitive tests of competing jet models. Hours to days after the GRB, polarization levels are low (P < 4%), when emission from the shocked ambient medium dominates15–17. Here we report the detection of P = 28 ± 4% in the immediate afterglow of Swift GRB 120308A, 240 sec after the trigger, decreasing to P = 16 +5 −4% over the subsequent 10 minutes. The polarization position angle remains stable, changing by no more than 15 degrees over this time, with a possible trend suggesting gradual rotation and ruling out plasma or MHD instabilities. Instead, the polarization properties of GRB 120308A show that GRBs contain magnetized baryonic jets with large-scale uniform magnetic fields that can survive for significant time after the initial explosion.