doi.eso.org

• Bulla, Mattia [Oskar Klein Centre, Department of Physics, Stockholm University, Sweden]
• Covino, Stefano [Istituto Nazionale di Astrofisica–Brera Astronomical Observatory, Merate, Italy]
• Patat, Ferdinando [European Southern Observatory (ESO)]
• Kyutoku, Koutarou [Theory Center, Institute of Particle and Nuclear Studies, KEK, Tsukuba, Japan; Department of Particle and Nuclear Physics, Sokendai, Tsukuba, Japan; Interdisciplinary Theoretical and Mathematical Sciences Program, RIKEN, Wako, Japan; Center for Gravitational Physics, Yukawa Institute for Theoretical Physics, Kyoto University, Japan]
• Maund, Justyn R. [Department of Physics and Astronomy, University of Sheffield, UK]
• Tanaka, Masaomi [Astronomical Institute, Tohoku University, Sendai, Japan; National Astronomical Observatory of Japan, National Institutes of Natural Sciences, Mitaka, Japan]
• Toma, Kenji [Astronomical Institute, Tohoku University, Sendai, Japan; Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan]
• Wiersema, Klaas [Department of Physics & Astronomy and Leicester Institute of Space & Earth Observation, University of Leicester, UK; University of Warwick, Coventry, UK]
• D’Avanzo, Paolo [Istituto Nazionale di Astrofisica–Brera Astronomical Observatory, Merate, Italy]
• Higgins, Adam B. [Department of Physics & Astronomy and Leicester Institute of Space & Earth Observation, University of Leicester, UK]
• Mundell, Carole G. [Department of Physics, University of Bath, UK]
• Palazzi, Eliana [INAF–Astrophysics and Space Science Observatory, Bologna, Italy]

We present the first results of a campaign aimed at characterising the linear polarisation signals and thus the geometry of binary neutron star mergers (i.e., kilonovae). We carried out the first polarimetric observations of a kilonova called AT 2017gfo, using the FOcal Reducer/low dispersion Spectrograph 2 (FORS2). We predicted for the first time the polarisation signatures expected from kilonovae and highlighted the best strategy to detect linear polarisation in future events. Our studies demonstrate how the detection of polarisation will constrain crucial parameters of these systems, such as the inclination and composition, distribution and extent of the different components of the ejecta.