The unpolarized macronova associated with the gravitational wave event GW 170817

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.
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%).