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 SOAR and Magellan telescopes; the UV spectrum was obtained with the $\mathit{\text{Hubble}}\mathit{\text{Space}}\mathit{\text{Telescope}}$ at 5.5 days. Our data reveal a rapidly-fading blue component ($T\approx 5500$ K at 1.5 days) that quickly reddens; spectra later than $\gtrsim 4.5$ days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at $\sim 7900$\mathring{A}at$t\lesssim 4.5$ days. The colours, rapid evolution and featureless spectrum are consistent with a “blue” kilonova from polar ejecta comprised mainly of light $r$-process nuclei with atomic mass number $A\lesssim 140$\mathrm{.}Thisindicatesasight-linewithin$\theta_{\rm obs}\lesssim 45^{\circ}$oftheorbitalaxis\mathrm{.}Comparisontomodelssuggests$\sim0.03$M$_\odot$ of blue ejecta, with a velocity of $\sim 0.3c$. The required lanthanide fraction is $\sim {10}^{-4}$, 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 (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of $\lesssim 12$ km. This mass also supports the idea that neutron star mergers are a major contributor to $r$-process nucleosynthesis.