Newly formed black holes of stellar mass launch collimated outflows (jets) of ionized matter that approach the speed of light. These outflows power prompt, brief and intense flashes of γ-rays known as γ-ray bursts (GRBs), followed by longer-lived afterglow radiation that is detected across the electromagnetic spectrum. Measuring the polarization of the observed GRB radiation provides a direct probe of the magnetic fields in the collimated jets. Rapid-response polarimetric observations of newly discovered bursts have probed the initial afterglow phase1,2,3, and show that, minutes after the prompt emission has ended, the degree of linear polarization can be as high as 30 per cent—consistent with the idea that a stable, globally ordered magnetic field permeates the jet at large distances from the central source3. By contrast, optical4,5,6 and γ-ray7,8,9 observations during the prompt phase have led to discordant and often controversial10,11,12 results, and no definitive conclusions have been reached regarding the origin of the prompt radiation or the configuration of the magnetic field. Here we report the detection of substantial (8.3 ± 0.8 per cent from our most conservative simulation), variable linear polarization of a prompt optical flash that accompanied the extremely energetic and long-lived prompt γ-ray emission from GRB 160625B. Our measurements probe the structure of the magnetic field at an early stage of the jet, closer to its central black hole, and show that the prompt phase is produced via fast-cooling synchrotron radiation in a large-scale magnetic field that is advected from the black hole and distorted by dissipation processes within the jet.
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E.T. thanks L. Piro and K. Murase for comments. We thank the RATIR (Reionization And Transients InfraRed) project team and the staff of the Observatorio Astronómico Nacional on Sierra San Pedro Mártir, and acknowledge the contribution of L. Georgiev and J.S. Bloom to the development of this observatory. RATIR is a collaboration between the University of California, the Universidad Nacional Autonóma de México, the NASA Goddard Space Flight Center and Arizona State University, and benefits from the loan of an H2RG detector and hardware and software support from Teledyne Scientific and Imaging. RATIR, the automation of the Harold L. Johnson Telescope of the Observatorio Astronómico Nacional on Sierra San Pedro Mártir, and the operation of both are funded through NASA grants NNX09AH71G, NNX09AT02G, NNX10AI27G and NNX12AE66G, CONACyT grants INFR-2009-01-122785 and CB-2008-101958, UNAM PAPIIT grant IN113810, and UC MEXUS-CONACyT grant CN 09-283. The MASTER project is supported in part by the Development Program of Lomonosov Moscow State University, Moscow Union OPTICA, Russian Science Foundation grant 16-12-00085. This work was supported in part by NASA Fermi grants NNH14ZDA001N and NNH15ZDA001N. 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.
Reviewer Information Nature thanks D. Giannios and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Different emission components shape the temporal evolution of GRB 160625B. On timescales of seconds to minutes after the explosion, we observe bright prompt (solid lines) and reverse-shock (dotted lines) components. On timescales of hours to weeks after the burst, emission from the forward shock (dashed lines) becomes the dominant component from X-rays down to radio energies. After about 14 days, the afterglow emission falls off at all wavelengths. This phenomenon, known as jet-break, is caused by the beamed geometry of the outflow. Error bars denote 1σ limits; upper limits are 3σ. Times are given with reference to the LAT trigger time T0. FS, forward shock; RS, reverse shock; a subscript ‘v’ refers to frequency; u, V, r, i, z, y, J and H denote specific optical filters.
For each of the four polarization epochs, we simulated and examined a large number of data sets with similar photometric properties and no intrinsic afterglow polarization. a, Results of 105 simulations for the first epoch (95–115 seconds). b, As for a, but for the second epoch (144–174 seconds). c, Results of 106 simulations for the third epoch (186–226 seconds). d, As for c, but for the fourth epoch (300–360 seconds). The observed values are shown by vertical arrows. The probability of obtaining by chance a polarization measurement as high as the observed value is also reported. Πmin, minimum polarization value.
a, γ-ray light curves in the soft (50–300 keV) energy band. b, γ-ray light curves in the hard (5–40 MeV) energy band. Optical data (blue circles) have been arbitrarily rescaled. The squared points (in the background) show the γ-ray light curves re-binned by adopting the same time intervals as for the optical observations. Times are given with reference to the LAT trigger time T0. The horizontal bars show the time interval (5 s) over which the observation was taken.
The afterglow evolution can be described by the combination of forward-shock (dashed lines) and reverse-shock (dotted lines) emission. The best-fit model is shown with solid lines. The peak flux of the forward-shock component is about 0.4 mJy, much lower than the optical flux measured at T < T0 + 350 seconds. This shows that the forward-shock emission is negligible during the prompt phase. Error bars denote 1σ limits; upper limits are 3σ.
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Troja, E., Lipunov, V., Mundell, C. et al. Significant and variable linear polarization during the prompt optical flash of GRB 160625B. Nature 547, 425–427 (2017). https://doi.org/10.1038/nature23289
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