Circular polarization in the optical afterglow of GRB 121024A.
Gamma-ray bursts (GRBs) are most probably powered by collimated relativistic outflows (jets) from accreting black holes at cosmological distances. Bright afterglows are produced when the outflow collides with the ambient medium. Afterglow polarization directly probes the magnetic properties of the jet when measured minutes after the burst, and it probes the geometric properties of the jet and the ambient medium when measured hours to days after the burst. High values of optical polarization detected minutes after the burst of GRB 120308A indicate the presence of large-scale ordered magnetic fields originating from the central engine (the power source of the GRB). Theoretical models predict low degrees of linear polarization and no circular polarization at late times, when the energy in the original ejecta is quickly transferred to the ambient medium and propagates farther into the medium as a blast wave. Here we report the detection of circularly polarized light in the afterglow of GRB 121024A, measured 0.15 days after the burst. We show that the circular polarization is intrinsic to the afterglow and unlikely to be produced by dust scattering or plasma propagation effects. A possible explanation is to invoke anisotropic (rather than the commonly assumed isotropic) electron pitch-angle distributions, and we suggest that new models are required to produce the complex microphysics of realistic shocks in relativistic jets.
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK.
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Wiersema
INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807 Merate (LC), Italy.
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Covino
1] Department of Earth and Space Science, Osaka University, Toyonaka 560-0043, Japan [2] Astronomical Institute, Tohoku University, Sendai 980-8578, Japan [3] Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai 980-8578, Japan.
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Toma
Astronomical Institute 'Anton Pannekoek', University of Amsterdam, PO Box 94248, 1090 SJ Amsterdam, The Netherlands.
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van der Horst
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
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Varela
Astronomical Institute 'Anton Pannekoek', University of Amsterdam, PO Box 94248, 1090 SJ Amsterdam, The Netherlands.
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Min
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
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Greiner
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK.
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Starling
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK.
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Tanvir
Astronomical Institute 'Anton Pannekoek', University of Amsterdam, PO Box 94248, 1090 SJ Amsterdam, The Netherlands.
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Wijers
INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807 Merate (LC), Italy.
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Campana
International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.
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Curran
Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Science, Nanjing 210008, China.
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Fan
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK 2100 Copenhagen, Denmark.
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Fynbo
1] Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, E-18008 Granada, Spain [2] Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, Departamento de Física Aplicada I, ETS Ingeniería, Universidad del País Vasco UPV/EHU, Alameda de Urquijo s/n, E-48013 Bilbao, Spain [3] Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, E-48008 Bilbao, Spain.
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Gorosabel
Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia.
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Gomboc
AIM (UMR 7158 CEA/DSM-CNRS-Université Paris Diderot) Irfu/Service d'Astrophysique, Saclay, F-91191 Gif-sur-Yvette Cedex, France.
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Götz
Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK 2100 Copenhagen, Denmark.
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Hjorth
Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Science, Nanjing 210008, China.
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Jin
Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, IC2 Building, 146 Brownlow Hill, Liverpool L3 5RF, UK.
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Kobayashi
Space Science Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, Alabama 35812, USA.
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Kouveliotou
Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, IC2 Building, 146 Brownlow Hill, Liverpool L3 5RF, UK.
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Mundell
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK.
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O'Brien
1] Scuola Normale Superiore, 7, I-56126 Pisa, Italy [2] INAF/IASF Bologna, via Gobetti 101, I-40129 Bologna, Italy.
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Pian
Astronomical Institute 'Anton Pannekoek', University of Amsterdam, PO Box 94248, 1090 SJ Amsterdam, The Netherlands.
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Rowlinson
1] Instituto de Astrofísica de Canarias (IAC), E-38200 La Laguna, Tenerife, Spain [2] Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain [3] New York University Abu Dhabi, PO Box 129188, Abu Dhabi, United Arab Emirates.
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Russell
INAF/IASF Milano, via E. Bassini 15, 20133 Milano, Italy.
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Salvaterra
INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy.
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di Serego Alighieri
INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807 Merate (LC), Italy.
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Tagliaferri
INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807 Merate (LC), Italy.
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Vergani
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
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Elliott
Isaac Newton Group of Telescopes, Apartado de Correos 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain.
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Fariña
Astronomical Institute 'Anton Pannekoek', University of Amsterdam, PO Box 94248, 1090 SJ Amsterdam, The Netherlands.
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Hartoog
Isaac Newton Group of Telescopes, Apartado de Correos 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain.
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Karjalainen
Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany.
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Klose
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
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Knust
Department of Physics, University of Warwick, Coventry CV4 7AL, UK.
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Levan
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
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Schady
Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany.
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Sudilovsky
Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK.
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Willingale