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

M. Nicholl, E. Berger, D. Kasen, B. D. Metzger, J. Elias, C. Briceo, 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. BrownH. Y. Chen, J. C. Clemens, E. Dennihy, B. Dunlap, D. E. Holz, E. Marchesini, F. Massaro, N. Moskovitz, I. Pelisoli, A. Rest, F. Ricci, M. Sako, M. Soares-Santos, J. Strader

Research output: Contribution to journalArticlepeer-review

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 Hubble Space Telescope at 5.5 days. Our data reveal a rapidly-fading blue component (T ≈ 5500K 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 ∼ 7900Å at t ≲ 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 ≲ 140. This indicates a sight-line within θobs ≲ 45° of the orbital axis. Comparison to models suggests ∼ 0.03M⊙ of blue ejecta, with a velocity of ∼ 0.3c. The required lanthanide fraction is ∼10-4, but this drops to < 10-5in 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 (US)
JournalUnknown Journal
StatePublished - Oct 16 2017

Keywords

  • Gravitational waves
  • Nuclear reactions, nucleosynthesis, abundances
  • Stars: Neutron

ASJC Scopus subject areas

  • General

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