Gamma-ray bursts (GRBs) are the strongest explosions in the Universe since the Big Bang. They are believed to be produced either in the formation of black holes at the end of massive star evolution1,2,3 or the merging of compact objects4. Spectral and timing properties of GRBs suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe4; however, the physical properties (especially the structure and magnetic topologies) of the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of GRBs should provide crucial information on the highly relativistic jets5. As a result, there have been many reports of GRB polarization measurements with diverse results (see ref. 6); however, many such measurements suffer from substantial uncertainties, most of which are systematic (ref. 7 and the references therein). After the first successful measurements by the Gamma-Ray Burst Polarimeter (GAP) and Compton Spectrometer and Imager (COSI) instruments8,9,10, here we report a statistically meaningful sample of precise polarization measurements, obtained with the dedicated GRB polarimeter POLAR onboard China’s Tiangong-2 space laboratory. Our results suggest that the gamma-ray emission is at most polarized at a level lower than some popular models have predicted, although our results also show intrapulse evolution of the polarization angle. This indicates that the low polarization degrees could be due to an evolving polarization angle during a GRB.
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All data that support the plots within this paper and other findings of this study are available from the POLAR Collaboration (email@example.com) upon reasonable request.
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We gratefully acknowledge financial support from the National Basic Research Program (973 Program) of China (grant number 2014CB845800); Joint Research Fund in Astronomy, under cooperative agreement between the National Natural Science Foundation of China and Chinese Academy of Sciences (grant number U1631242); National Natural Science Foundation of China (grant numbers 11503028 and 11403028); Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDB23040400); Swiss Space Office of the State Secretariat for Education, Research and Innovation (ESA PRODEX Programme); National Science Center of Poland (grant number 2015/17/N/ST9/03556); and Youth Innovation Promotion Association of the Chinese Academy of Sciences (grant number 2014009). We also thank J. M. Burgess of MPE, Garching, Germany, for providing the energy spectra for GRB 170114A.
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Nature Astronomy (2019)