paper

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

arXiv:1710.05456 · doi:10.3847/2041-8213/aa9029

Abstract

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 \textit{Hubble Space Telescope} at 5.5 days. Our data reveal a rapidly-fading blue component ( K at 1.5 days) that quickly reddens; spectra later than days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at Å at days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light -process nuclei with atomic mass number . This indicates a sight-line within of the orbital axis. Comparison to models suggests M of blue ejecta, with a velocity of . The required lanthanide fraction is , but this drops to 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 km. This mass also supports the idea that neutron star mergers are a major contributor to -process nucleosynthesis.

ApJL, in press (GW170817, LVC)

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