2017/10/16 by M. Nicholl, E. Berger, D. Kasen +31 · 2 citations
Physics and Astronomy · #astro-ph.HE #astro-ph.CO #astro-ph.SR
paper · pdf · doi:10.3847/2041-8213/aa9029
ApJL, in press (GW170817, LVC)
arxiv created 2017/10/16 · arxiv updated 2017/10/17
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 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 \gtrsim 4.5 days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at ∼ 7900 Å 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. This indicates a sight-line within θ\rm obs\lesssim 45∘ of the orbital axis. Comparison to models suggests ∼0.03 M_\odot of blue ejecta, with a velocity of ∼ 0.3c. The required lanthanide fraction 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 (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.