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# The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo

## Abstract

The gravitational-wave event GW 170817 was generated by the coalescence of two neutron stars and produced an electromagnetic transient, labelled AT 2017gfo, that was the target of a massive observational campaign. Polarimetry is a powerful diagnostic tool for probing the geometry and emission processes of unresolved sources, and the observed linear polarization for this event was consistent with being mostly induced by intervening dust, suggesting that the intrinsic emission was weakly polarized (P < 0.4–0.5%). Here we present a detailed analysis of the linear polarization expected from a merging neutron-star binary system by means of 3D Monte Carlo radiative transfer simulations assuming a range of possible configurations, wavelengths, epochs and viewing angles. We find that polarization originates from the non-homogeneous opacity distribution within the ejecta and can reach levels of 1% at early times (one to two days after the merger) and in the optical R band. Smaller polarization signals are expected at later epochs and different wavelengths. From the viewing-angle dependence of the polarimetric signal, we constrain the observer orientation of AT 2017gfo to within about 65° from the polar direction. The detection of non-zero polarization in future events will unambiguously reveal the presence of a lanthanide-free ejecta component and unveil its spatial and angular distribution.

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• ### Kilonovae

Living Reviews in Relativity Open Access 16 December 2019

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## Acknowledgements

M.B. acknowledges support from the Swedish Research Council (Vetenskapsrå det) and the Swedish National Space Board. S.C. acknowledges support from ASI grant I/004/11/3 and partial financial support by the GRAWITA collaboration. K.K. is supported by the Japanese Society for the Promotion of Science (JSPS) Kakenhi Grant-in-Aid for Scientific Research (grant numbers JP16H06342, JP17H01131 and JP18H04595). J.R.M. is supported through a Royal Society University Research Fellowship. K.T. is supported by JSPS Kakenhi grant numbers 15H05437 and 18H01245, and also by a JST grant 'Building of Consortia for the Development of Human Resources in Science and Technology'. J.B. is supported by a University of Sheffield PhD studentship.

## Author information

Authors

### Contributions

All authors contributed to the work presented in this paper. M.B. carried out the model simulations and analysis and led the writing of the manuscript. S.C. provided the polarimetric data of AT 2017gfo and helped with the writing. K.K. and M.T. provided theoretical insights on hydrodynamical models, carried out radiative transfer calculations to estimate opacities and helped with the writing of the manuscript.

### Corresponding authors

Correspondence to M. Bulla or S. Covino.

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### Competing interests

The authors declare no competing interests.

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Bulla, M., Covino, S., Kyutoku, K. et al. The origin of polarization in kilonovae and the case of the gravitational-wave counterpart AT 2017gfo. Nat Astron 3, 99–106 (2019). https://doi.org/10.1038/s41550-018-0593-y

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• DOI: https://doi.org/10.1038/s41550-018-0593-y

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