title: Ten per cent polarized optical emission from GRB 090102.
abstract: The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRBs) remains unclear. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present. In an alternative model, jets are threaded with large-scale magnetic fields that originate at the central engine and that accelerate and collimate the material. To distinguish between the models the degree of polarization in early-time emission must be measured; however, previous claims of gamma-ray polarization have been controversial. Here we report that the early optical emission from GRB 090102 was polarized at 10 +/- 1 per cent, indicating the presence of large-scale fields originating in the expanding fireball. If the degree of polarization and its position angle were variable on timescales shorter than our 60-second exposure, then the peak polarization may have been larger than ten per cent.
authors.affiliation: Astrophysics Research Institute, Liverpool John Moores University, CH41 1LD, UK. ias@astro.livjm.ac.uk
authors.forename: I A
authors.initials: IA
authors.lastname: Steele
authors.forename: C G
authors.initials: CG
authors.lastname: Mundell
authors.forename: R J
authors.initials: RJ
authors.lastname: Smith
authors.forename: S
authors.initials: S
authors.lastname: Kobayashi
authors.forename: C
authors.initials: C
authors.lastname: Guidorzi