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Circular polarization in the optical afterglow of GRB 121024A

Nature volume 509, pages 201–204 (2014)Cite this article

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Abstract

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 burst1,2,3,4,5. 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 engine5 (the power source of the GRB). Theoretical models predict low degrees of linear polarization and no circular polarization at late times6,7,8, 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 jets9,10,11.

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Figure 1: Linear polarization of the afterglow of GRB 121024A.
Figure 2: Optical circular polarization measurements of the afterglow of GRB 121024A.
Figure 3: Optical polarimetry of quasars and GRB afterglows compared.

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Acknowledgements

This work is based on observations made with ESO telescopes at the Paranal Observatory under programme 090.D-0789. We thank all ING staff for their support of ACAM ToO observations. K.W. thanks J. Hinton for discussions. K.W. was supported by STFC. K.T. was supported by a JSPS Research Fellowship for Young Scientists no. 231446. A.J.v.d.H., R.A.M.J.W. and A.R. were supported by the European Research Council via Advanced Investigator grant no. 247295. R.L.C.S. was supported by a Royal Society Fellowship. Y.F. was supported by the 973 Programme of China, under grant 2013CB837000. D.M.R. was supported by a Marie Curie Intra European Fellowship within the 7th European Community Framework Programme under contract no. IEF 274805. This work was supported by the Australian Research Council (grant DP120102393). The William Herschel telescope and its override programme are operated on the island of La Palma by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester, funded by the UK Space Agency.

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Authors and Affiliations

  1. Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

    K. Wiersema, R. L. C. Starling, N. R. Tanvir, P. T. O’Brien & R. Willingale

  2. INAF/Brera Astronomical Observatory, via Bianchi 46, I-23807 Merate, LC, Italy

    S. Covino, S. Campana, G. Tagliaferri & S. D. Vergani

  3. Department of Earth and Space Science, Osaka University, Toyonaka, 560-0043, Japan

    K. Toma

  4. Astronomical Institute, Tohoku University, Sendai, 980-8578, Japan

    K. Toma

  5. Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578, Japan

    K. Toma

  6. Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, 1090 SJ Amsterdam, PO Box 94248, The Netherlands

    A. J. van der Horst, M. Min, R. A. M. J. Wijers, A. Rowlinson & O. E. Hartoog

  7. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, D-85748 Garching, Germany ,

    K. Varela, J. Greiner, J. Elliott, F. Knust, P. Schady & V. Sudilovsky

  8. International Centre for Radio Astronomy Research, Curtin University, Perth, GPO Box U1987, 6845, Western Australia, Australia

    P. A. Curran

  9. Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Science, Nanjing, 210008, China

    Y. Fan & Z. P. Jin

  10. Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, DK 2100 Copenhagen, Denmark ,

    J. P. U. Fynbo & J. Hjorth

  11. Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, E-18008 Granada, Spain ,

    J. Gorosabel

  12. Departamento de Física Aplicada I, Unidad Asociada Grupo Ciencia Planetarias UPV/EHU-IAA/CSIC, ETS Ingeniería, Universidad del País Vasco UPV/EHU, Alameda de Urquijo s/n, E-48013 Bilbao, Spain,

    J. Gorosabel

  13. Ikerbasque, Basque Foundation for Science, Alameda de Urquijo 36-5, E-48008 Bilbao, Spain ,

    J. Gorosabel

  14. Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000 Ljubljana, Slovenia ,

    A. Gomboc

  15. AIM (UMR 7158 CEA/DSM-CNRS-Université Paris Diderot) Irfu/Service d’Astrophysique, Saclay, F-91191 Gif-sur-Yvette Cedex, France ,

    D. Götz

  16. Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, IC2 Building, 146 Brownlow Hill, Liverpool L3 5RF, UK ,

    S. Kobayashi & C. Mundell

  17. Space Science Office, ZP12, NASA/Marshall Space Flight Center, Huntsville, 35812, Alabama, USA

    C. Kouveliotou

  18. Scuola Normale Superiore, 7, I-56126 Pisa, Italy ,

    E. Pian

  19. INAF/IASF Bologna, via Gobetti 101, I-40129 Bologna, Italy ,

    E. Pian

  20. Instituto de Astrofísica de Canarias (IAC), La Laguna, E-38200, Tenerife, Spain

    D. M. Russell

  21. Departamento de Astrofísica, Universidad de La Laguna, La Laguna, E-38206, Tenerife, Spain

    D. M. Russell

  22. New York University Abu Dhabi, Abu Dhabi, PO Box 129188, United Arab Emirates

    D. M. Russell

  23. INAF/IASF Milano, via E. Bassini 15, 20133 Milano, Italy ,

    R. Salvaterra

  24. INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy ,

    S. di Serego Alighieri

  25. Isaac Newton Group of Telescopes, Apartado de Correos 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain ,

    C. Fariña & R. Karjalainen

  26. Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany ,

    S. Klose

  27. Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

    A. J. Levan

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  1. K. Wiersema
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Contributions

K.W. and S.C. jointly led the VLT observing time proposals and defined the observing strategy. K.W. acquired, reduced and analysed the VLT data and took primary responsibility for writing the text of the paper; S.C. performed an independent data analysis. K.T., A.J.v.d.H. and M.M. provided the theoretical interpretation of the observations. K.V. and J.G. analysed the GROND data. O.E.H. led the WHT observing time proposal. All authors contributed to refining the text of the paper, or assisted in obtaining parts of the presented data set.

Corresponding author

Correspondence to K. Wiersema.

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Extended data figures and tables

Extended Data Figure 1 Host galaxy and afterglow image.

Left, VLT FORS2 Rspecial-band acquisition image, with the afterglow indicated by broken lines. Right, detection of the host galaxy in the late-time WHT ACAM r-band imaging.

Extended Data Figure 2 Polarimetry mask and afterglow brightness.

A small section of a single FORS2 Rspecial-band polarimetric exposure (this is the −45° angle chip 1 frame of the cir4 set), illustrating the shape of the aperture mask and brightness of the afterglow (indicated by an arrow). Each part of the sky that falls into the open part of the rectangular mask is imaged twice, in perpendicular polarizations, the o and e beams, which is why the same objects each time appear in two non-overlapping strips.

Extended Data Figure 3 Optical and X-ray afterglow light curves.

a, Swift XRT X-ray light curve in the time span covered by GROND observations; b, the full XRT light curve. c, Full GROND light curves in all seven bands (key at top right). Overplotted in a and c is the best-fitting smoothly broken power law (Methods), with a host galaxy contribution to the optical data. Residuals of this fit are shown in d.

Extended Data Figure 4 X-ray/optical spectral energy distribution of the afterglow of GRB 121024A.

Shown is a spectral energy distribution using the seven GROND photometric bands and simultaneous Swift XRT X-ray data. The overplotted solid line is the best-fitting absorbed power law; the dashed line shows the best-fitting power law without the effects of reddening and X-ray absorption. The horizontal error bars on the optical and X-ray data show the filter throughput and spectral bin-size, respectively. Vertical error bars show 1σ uncertainties on the fluxes.

Extended Data Table 1 Linear polarimetry results
Full size table
Extended Data Table 2 Circular polarimetry results
Full size table
Extended Data Table 3 GROND optical and near-infrared photometry of the afterglow
Full size table

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Wiersema, K., Covino, S., Toma, K. et al. Circular polarization in the optical afterglow of GRB 121024A. Nature 509, 201–204 (2014). https://doi.org/10.1038/nature13237

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