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Originally published in Science Express on 15 March 2007
Science 30 March 2007:
Vol. 315. no. 5820, pp. 1822 - 1824
DOI: 10.1126/science.1138484
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Reports

Early Optical Polarization of a Gamma-Ray Burst Afterglow

Carole G. Mundell,1* Iain A. Steele,1 Robert J. Smith,1 Shiho Kobayashi,1 Andrea Melandri,1 Cristiano Guidorzi,1,2,3 Andreja Gomboc,4 Chris J. Mottram,1 David Clarke,5 Alessandro Monfardini,1,6 David Carter,1 David Bersier1

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.

1 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead, CH41 1LD, UK.
2 Dipartimento di Fisica, Universitá di Milano-Bicocca, Piazza delle Scienze 3, 20126 Milano, Italy.
3 Istituto Nazionale di Astrofisica-Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy.
4 Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia.
5 Department of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK.
6 CNRS, Institut Néel, 25 Avenue des Martyrs, 38042 Grenoble, France.

* To whom correspondence should be addressed. E-mail: cgm{at}astro.livjm.ac.uk

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Science. ISSN 0036-8075 (print), 1095-9203 (online)