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

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...

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Detalles Bibliográficos
Autores principales: Nicholl, M., Berger, E., Kasen, D., Metzger, B. D., Elias, J., Briceño, C., Alexander, K. D., Blanchard, P. K., Chornock, R., Cowperthwaite, P. S., Eftekhari, T., Fong, W., Margutti, R., Villar, V. A., Williams, P. K. G., Brown, W., Annis, J., Bahramian, A., Brout, D., Brown, D. A., Chen, H. Y., Clemens, J. C., Dennihy, E., Dunlap, B., Holz, D. E., Marchesini, Ezequiel Joaquín, Massaro, F., Moskowitz, N., Pelisoli, I., Rest, A., Ricci, F., Sako, M., Soares-Santos, M., Strader, J.
Formato: Articulo
Lenguaje:Inglés
Publicado: 2017
Materias:
Física
binaries: close
gravitational waves
nuclear reactions, nucleosynthesis, abundances
stars: neutron
Acceso en línea:http://sedici.unlp.edu.ar/handle/10915/87367
Aporte de:
SEDICI (UNLP) de Universidad Nacional de La Plata
Descripción
Sumario: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 (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 ≲ 45° of the orbital axis. Comparison to models suggests ∼0.03 M o of blue ejecta, with a velocity of . 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.

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