Abstract

The merger of two dense stellar remnants including at least one neutron star is predicted to produce gravitational waves (GWs) and short-duration gamma ray bursts1,2. In the process, neutron-rich material is ejected from the system and heavy elements are synthesized by r-process nucleosynthesis1,3. The radioactive decay of these heavy elements produces additional transient radiation termed kilonova or macronova4,5,6,7,8,9,10. We report the detection of linear optical polarization, P = (0.50 ± 0.07)%, 1.46 days after detection of the GWs from GW 170817—a double neutron star merger associated with an optical macronova counterpart and a short gamma ray burst11,12,13,14. The optical emission from a macronova is expected to be characterized by a blue, rapidly decaying component and a red, more slowly evolving component due to material rich in heavy elements—the lanthanides15. The polarization measurement was made when the macronova was still in its blue phase, during which there was an important contribution from a lanthanide-free outflow. The low degree of polarization is consistent with intrinsically unpolarized emission scattered by galactic dust, suggesting a symmetric geometry of the emitting region and low inclination of the merger system. Stringent upper limits to the polarization degree from 2.45–9.48 days post-burst are consistent with the lanthanides-rich macronova interpretation.

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Change history

  • Correction 30 October 2017

    In the version of this Letter originally published, in the third paragraph of the text Kyutoku  et al. were not correctly cited and the sentence should have read: “As pointed out by Kyutoku at al.28, in the case of high optical depth to electron scattering (~1) and assuming spectral lines do not significantly depolarize the global emission, the linear polarization observed from the equatorial plane could be as high as a few per cent.” Also, in the Author contributions section, the final sentence should have read: “C.G.M. contributed to the writing of the paper.”

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Acknowledgements

This study was based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under European Southern Observatory programme 099.D-0116. We thank the European Southern Observatory—Paranal staff for carrying out excellent observations under difficult conditions during a hectic period. We also acknowledge partial funding from Agenzia Spaziale Italiana-Istituto Nazionale di Astrofisica grant I/004/11/3. K.W., A.B.H., R.L.C.S. and N.R.T. acknowledge funding from the Science and Technology Facilities Council. J.H. was supported by a VILLUM FONDEN Investigator grant (project number 16599). Y.Z.F. was supported by the National Natural Science Foundation of China under grant 11525313. C.G.M. acknowledges support from the UK Science and Technology Facilities Council. K.T. was supported by Japan Society for the Promotion of Science grant 15H05437 and a Japan Science and Technology Consortia grant. J.M. acknowledges the National Natural Science Foundation of China 11673062 and the Major Program of the Chinese Academy of Sciences(KJZD-EW-M06).

Author information

Affiliations

  1. Istituto Nazionale di Astrofisica / Brera Astronomical Observatory, via Bianchi 46, 23807, Merate (LC), Italy

    • S. Covino
    • , A. Melandri
    • , P. D’Avanzo
    • , M. G. Bernardini
    • , S. Campana
    • , W. Gao
    •  & G. Tagliaferri
  2. Department of Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

    • K. Wiersema
    • , A. B. Higgins
    • , N. R. Tanvir
    •  & R. L. C. Starling
  3. Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210008, China

    • Y. Z. Fan
    •  & Z. P. Jin
  4. Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, 980-8578, Japan

    • K. Toma
  5. Astronomical Institute, Tohoku University, Sendai, 980-8578, Japan

    • K. Toma
  6. Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK

    • C. G. Mundell
  7. Istituto Nazionale di Astrofisica / Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna, Via Gobetti 101, 40129, Bologna, Italy

    • E. Palazzi
    • , E. Pian
    •  & A. Rossi
  8. Laboratoire Univers et Particules de Montpellier, Université de Montpellier, Centre National de la Recherche Scientifique / IN2P3, Montpellier, 34095, France

    • M. G. Bernardini
  9. Gran Sasso Science Institute, 67100, L’Aquila, Italy

    • M. Branchesi
  10. Istituto Nazionale di Fisica Nucleare / Laboratori Nazionali del Gran Sasso, 67100, L’Aquila, Italy

    • M. Branchesi
  11. Istituto Nazionale di Astrofisica / Osservatorio Astronomico di Roma, Via di Frascati, 33, 00078, Monteporzio Catone, Italy

    • E. Brocato
  12. Istituto Nazionale di Astrofisica / Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125, Firenze, Italy

    • S. di Serego Alighieri
  13. Commissariat à l’Énergie Atomique Saclay—Direction de la Recherche Fondamentale/Irfu/Dèpartement d’Astrophysique, 91191, Gif-sur-Yvette, France

    • D. Götz
  14. Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100, Copenhagen, Denmark

    • J. P. U. Fynbo
    • , J. Hjorth
    •  & D. Malesani
  15. Department of Physics and Institute of Theoretical Physics, Nanjing Normal University, Nanjing, 210046, China

    • W. Gao
  16. Centre for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270, Ajdovščina, Slovenia

    • A. Gomboc
  17. Space Telescope Science Institute, Baltimore, MD, 21218, USA

    • B. Gompertz
  18. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748, Garching, Germany

    • J. Greiner
  19. Anton Pannekoek Institute, University of Amsterdam, Science Park 904, 1098XH, Amsterdam, The Netherlands

    • L. Kaper
    •  & R. A. M. J. Wijers
  20. Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778, Tautenburg, Germany

    • S. Klose
  21. Astrophysics Research Institute, Liverpool John Moores University, ic2, Liverpool Science Park, 146 Brownlow Hill, Liverpool, L3 5RF, UK

    • S. Kobayashi
    •  & I. Steele
  22. Faculty of Mathematics and Physics, University of Ljubljana, Jadranska 19, 1000, Ljubljana, Slovenia

    • D. Kopac
  23. Department of Physics, University of Warwick, Coventry, CV4 7AL, UK

    • A. J. Levan
  24. Yunnan Observatories, Chinese Academy of Sciences, 650011, Kunming, Yunnan Province, China

    • J. Mao
  25. Istituto Nazionale di Astrofisica / Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133, Milano, Italy

    • R. Salvaterra
  26. Department of Astronomy, University of Maryland, College Park, MD, 20742, USA

    • E. Troja
  27. Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC, 20052, USA

    • C. Kouveliotou
    •  & A. J. van der Horst
  28. Astronomy, Physics, and Statistics Institute of Sciences, Washington, DC, 20052, USA

    • A. J. van der Horst

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Contributions

All authors contributed to the work presented in this paper. S.C. and K.W. coordinated the data acquisition, analysed the data and wrote the paper. A.B.H., A.M., P.D., E.P. and N.T. contributed to the data analysis. Y.F. and K.T. contributed to the theoretical discussion. C.G.M. contributed to the writing of the paper.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to S. Covino.

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DOI

https://doi.org/10.1038/s41550-017-0285-z