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

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  • C. Briceno
  • K. D. Alexander
  • P. K. Blanchard
  • R. Chornock
  • P. S. Cowperthwaite
  • T. Eftekhari
  • W. Fong
  • R. Margutti
  • V. A. Villar
  • P. K. G. Williams
  • W. Brown
  • J. Annis
  • A. Bahramian
  • D. Brout
  • D. A. Brown
  • H. -Y. Chen
  • J. C. Clemens
  • E. Dennihy
  • B. Dunlap
  • D. E. Holz
  • E. Marchesini
  • F. Massaro
  • N. Moskowitz
  • I. Pelisoli
  • A. Rest
  • F. Ricci
  • M. Sako
  • M. Soares-Santos
  • J. Strader

Colleges, School and Institutes


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 ≳ 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 ≲ 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 A ≲ 140. This indicates a sightline within θobs ≲ 45o of the orbital axis. Comparison to models suggests ∼0.03 M 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 ≲12 km. This mass also supports the idea that neutron star mergers are a major contributor to r-process nucleosynthesis.


Original languageEnglish
Article numberL18
Number of pages8
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 16 Oct 2017


  • astro-ph.HE, astro-ph.CO, astro-ph.SR, binaries: close, gravitational waves, nuclear reactions, stars: neutron, abundances, nucleosynthesis