title: Early optical polarization of a gamma-ray burst afterglow.
abstract: We report the optical polarization of a gamma-ray burst (GRB) afterglow, obtained 203 seconds after the initial burst of gamma-rays from GRB 060418, using a ring polarimeter on the robotic Liverpool Telescope. Our robust (2sigma) upper limit on the percentage of polarization, less than 8%, coincides with the fireball deceleration time at the onset of the afterglow. The combination of the rate of decay of the optical brightness and the low polarization at this critical time constrains standard models of GRB ejecta, ruling out the presence of a large-scale ordered magnetic field in the emitting region.
authors.affiliation: Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK. cgm@astro.livjm.ac.uk
authors.forename: Carole G
authors.initials: CG
authors.lastname: Mundell
authors.forename: Iain A
authors.initials: IA
authors.lastname: Steele
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Smith
authors.forename: Shiho
authors.initials: S
authors.lastname: Kobayashi
authors.forename: Andrea
authors.initials: A
authors.lastname: Melandri
authors.forename: Cristiano
authors.initials: C
authors.lastname: Guidorzi
authors.forename: Andreja
authors.initials: A
authors.lastname: Gomboc
authors.forename: Chris J
authors.initials: CJ
authors.lastname: Mottram
authors.forename: David
authors.initials: D
authors.lastname: Clarke
authors.forename: Alessandro
authors.initials: A
authors.lastname: Monfardini
authors.forename: David
authors.initials: D
authors.lastname: Carter
authors.forename: David
authors.initials: D
authors.lastname: Bersier