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

SOARES-SANTOS, M.; REST, A.; MASSARO, F.; DUNLAP, B.; CHEN, H.-Y.; BAHRAMIAN, A.; WILLIAMS, P. K. G.; STRADER, J.; FONG, W.; CHORNOCK, R.; RICCI, F.; BRICEÑO, C.; MOSKOWITZ, N.; KASEN, D.; HOLZ, D. E.; CLEMENS, J. C.; BROUT, D.; BROWN, W.; SAKO, M.; MARGUTTI, R.; PELISOLI, I.; COWPERTHWAITE, P. S.; MARCHESINI, E.; ALEXANDER, K. D.; DENNIHY, E.; METZGER, B. D.; BROWN, D. A.; NICHOLL, M.; ANNIS, J.; VILLAR, V. A.; EFTEKHARI, T.; BLANCHARD, P. K.; ELIAS, J.; BERGER, E.

The Astrophysical Journal

IOPscience

Lugar: BRISTOL; Año: 2017 vol. 848

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 ($Tapprox 5500$ 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 $tlesssim 4.5$ days. The colors, 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 $Alesssim 140$. This indicates a sightline within ${heta }_{mathrm{obs}}lesssim 45^circ $ of the orbital axis. Comparison to models suggests ~0.03 M ⊙ of blue ejecta, with a velocity of $sim 0.3c$. The required lanthanide fraction is $sim {10}^{-4}$, but this drops to $lt {10}^{-5}$ 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.