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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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.
The authors declare no competing financial interests.
Extended data figures and tables
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.
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.
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.
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.
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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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