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 (2-sigma) 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.

8 pages including 1 figure. Accepted for publication in Science; published online 15 March 2007; 10.1126/science.1138484

References in corpus (2)

• Gravitational Lens Time Delays: A Statistical Assessment of Lens Model Dependences and Implications for the Global Hubble Constant
• Evolution of the polarization of the optical afterglow of the gamma-ray burst GRB 030329

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