The electromagnetic counterpart of the binary neutron star merger LIGO/Virgo GW170817. III. Optical and UV spectra of a blue kilonova from fast polar ejecta

We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational-wave (GW)source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post merger, using the Southern Astrophysical Research and Magellan telescopes; the UV spectrum was obtained with the Hubble Space Telescope at 5.5 days. Our data reveal a rapidly fading blue component (T » 5500 K at 1.5 days)that quickly reddens; spectra later than 4.5 days peak beyond the optical regime. The spectra are mostlyfeatureless, although we identify a possible weak emission line at ∼7900 Å at t  4.5 days. The colors, rapidevolution, and featureless spectrum are consistent with a “blue” kilonova from polar ejecta comprised mainly oflight r-process nuclei with atomic mass number A  140. This indicates a sightline within qobs  45 of the orbitalaxis. Comparison to models suggests ∼0.03 Me of blue ejecta, with a velocity of ~0.3c. The required lanthanidefraction is ~ - 10 4, but this drops to < - 10 5 in the outermost ejecta. The large velocities point to a dynamical origin,rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (asopposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors asmall neutron star radius of 12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis