The search for the optical counterpart to gravitational wave (GW) 17081711 quickly allowed the discovery of the possible counterpart, named AT2017gfo (also known as SSS17a), in the outskirts of the elliptical galaxy NGC4993 at about 40 Mpc14. Spectroscopic observations showed that this source was highly unusual and probably associated with the GW event and a short gamma ray burst (GRB)16. Only a few possible detections of macronova emission have been reported in the literature and in all of these studies the temporal and spectral evolution of the afterglows of on-axis short GRBs was analysed17,18,19,20. GRB 170817 A is intrinsically the weakest short GRB detected so far, which may be a regular short GRB but viewed from an off-axis orientation. The off-axis scenario is also helpful in reconciling the probability of GW–GRB association for this event21,22. Therefore SSS17a offers a unique opportunity to study a macronova emission plausibly not polluted by the GRB emission. The combination of a likely off-axis jet and the potential ability of Laser Interferometer Gravitational-Wave Observatory/Virgo data to constrain the merger geometry and orientation gave great urgency to a polarimetric measurement of the symmetry and orientation of the optical emission region(s) post-merger. Measuring the degree of polarization of the electromagnetic emission provides unique constraints on the geometry of the system and the properties of any incipient magnetic fields23,24.
Our linear polarimetry campaign consisted of a set of five observations carried out with the European Southern Observatory Very Large Telescope equipped with the FOcal Reducer and low dispersion Spectrograph (FORS2; http://www.eso.org/sci/facilities/paranal/instruments/fors.html) starting on 18 August 2017 and spanning almost ten days. After that, the transient was too faint for a reliable polarimetric analysis. Details of the observing setup, data reduction and analysis are reported in the Methods. The derived degree of linear polarization, position angle and optical brightness after instrumental corrections had been applied are given in Table 1. A complete observation log, including the dates of the observations, exposure times, filters and seeings, is reported in Table 2. The Stokes parameters for optical transient and nearby field stars for the first four epochs are shown in Fig. 1. Over the duration of our campaign, the transient showed a degree of linear polarization and a position angle fully consistent with that shown by stars in the field whose polarization is induced by dust in our Galaxy. This implies that the macronova emission is essentially unpolarized at a level driven by the photometric uncertainties and the spread of polarization shown by field stars (that is, 0.4–0.5%).