The nature of the jets and the role of magnetic fields in gamma-ray bursts (GRBs) remains unclear1,2. In a baryon-dominated jet only weak, tangled fields generated in situ through shocks would be present3. In an alternative model, jets are threaded with large-scale magnetic fields that originate at the central engine and that accelerate and collimate the material4. 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 controversial5,6,7,8. 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.
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The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. C.G.M. acknowledges financial support from the Royal Society and Research Councils UK. We thank J. Marchant for preparing Fig. 4 for us.
Author Contributions I.A.S. designed and built the instrument, and did data reduction and analysis. C.G.M. developed the initial scientific justification for the instrument and interpreted results. R.J.S. did data reduction and analysis. S.K. did theoretical interpretation of results. C.G. coordinated observations and identified the afterglow.
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Steele, I., Mundell, C., Smith, R. et al. Ten per cent polarized optical emission from GRB 090102. Nature 462, 767–769 (2009). https://doi.org/10.1038/nature08590
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