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Title: 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

Here, 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 ($$T\approx5500$$ 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 $$\sim 7900$$ 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 $$\theta_{\rm obs}\lesssim 45^{\circ}$$ of the orbital axis. Comparison to models suggests $$\sim0.03$$ M$$_\odot$$ of blue ejecta, with a velocity of $$\sim 0.3c$$. The required lanthanide fraction is $$\sim 10^{-4}$$, 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.
Authors:
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  1. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States)
  2. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
  3. Columbia Univ., New York, NY (United States)
  4. Southern Astrophysical Research Telescope, La Serena (Chile)
  5. National Optical Astronomy Observatory, La Serena (Chile)
  6. Ohio Univ., Athens, OH (United States)
  7. Northwestern Univ., Evanston, IL (United States)
  8. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
  9. Michigan State Univ., East Lansing, MI (United States)
  10. Univ. of Pennsylvania, Philadelphia, PA (United States)
  11. Syracuse Univ., Syracuse, NY (United States)
  12. Univ. of Chicago, Chicago, IL (United States)
  13. Univ. of North Carolina, Chapel Hill, NC (United States)
  14. Univ. Navional de La Plata, La Plata (Argentina); Instituto de Astrofisica de La Plata, La Plata (Argentina); Univ. degli Studi di Torino, Torino (Italy); Istituto Nazionale di Fisica Nucleare, Torino (Italy); INAF-Istituto di Astrofisico Spaziale e Fisica Cosmica di Bologna, Bologna (Italy)
  15. Univ. degli Studi di Torino, Torino (Italy); Istituto Nazionale di Fisica Nucleare, Torino (Italy); INAF-Osservatorio Astrofisico di Torino, Pino Torinese (Italy)
  16. Lowell Observatory, Flagstaff, AZ (United States)
  17. Univ. Federal do Rio Grande do Sul, Rio Grande do Sul (Brazil); Univ. of Warwick, Coventry (United Kingdom)
  18. Space Telescope Science Institute, Baltimore, MD (United States); Johns Hopkins Univ., Baltimore, MD (United States)
  19. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA (United States); Univ. Roma Tre, Roma (Italy)
  20. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States); Brandeis Univ., Waltham, MA (United States)
Publication Date:
Report Number(s):
arXiv:1710.05456; FERMILAB-PUB-17-475-AE-CD
Journal ID: ISSN 2041-8213; 1630796
Grant/Contract Number:
AC02-07CH11359
Type:
Accepted Manuscript
Journal Name:
The Astrophysical Journal. Letters
Additional Journal Information:
Journal Volume: 848; Journal Issue: 2; Journal ID: ISSN 2041-8213
Publisher:
Institute of Physics (IOP)
Research Org:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Country of Publication:
United States
Language:
English
Subject:
79 ASTRONOMY AND ASTROPHYSICS; binaries: close; gravitational waves; nuclear reactions, nucleosynthesis, abundances; stars: neutron
OSTI Identifier:
1409351