Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger
Creators: name: Pian, E.
name: D'Avanzo, P.
name: Benetti, S.
name: Branchesi, M.
name: Brocato, E.
name: Campana, S.
name: Cappellaro, E.
name: Covino, S.
name: D'Elia, V.
name: Fynbo, J. P. U.
name: Getman, F.
name: Ghirlanda, G.
name: Ghisellini, G.
name: Grado, A.
name: Greco, G.
name: Hjorth, J.
name: Kouveliotou, C.
name: Levan, A.
name: Limatola, L.
name: Malesani, D.
name: Mazzali, P. A.
name: Melandri, A.
name: Moller, P.
name: Nicastro, L.
name: Palazzi, E.
name: Piranomonte, S.
name: Rossi, A.
name: Salafia, O. S.
name: Selsing, J.
name: Stratta, G.
name: Tanaka, M.
name: Tanvir, N. R.
name: Tomasella, L.
name: Watson, D.
name: Yang, S.
name: Amati, L.
name: Antonelli, L. A.
name: Ascenzi, S.
name: Bernardini, M. G.
name: Boer, M.
name: Bufano, F.
name: Bulgarelli, A.
name: Capaccioli, M.
name: Casella, P. G.
name: Castro-Tirado, A. J.
name: Chassande-Mottin, E.
name: Ciolfi, R.
name: Copperwheat, C. M.
name: Dadina, M.
name: De Cesare, G.
name: Di Paola, A.
name: Fan, Y. Z.
name: Gendre, B.
name: Giuffrida, G.
name: Giunta, A.
name: Hunt, L. K.
name: Israel, G.
name: Jin, Z. -P.
name: Kasliwal, M.
name: Klose, S.
name: Lisi, M.
name: Longo, F.
name: Maiorano, E.
name: Mapelli, M.
name: Masetti, .
name: Nava, L.
name: Patricelli, B.
name: Perley, D.
name: Pescalli, A.
name: Piran, T.
name: Possenti, A.
name: Pulone, L.
name: Razzano, M.
name: Salvaterra, R.
name: Schipani, P.
name: Spera, M.
name: Stamerra, A.
name: Stella, L.
name: Tagliaferri, G.
name: Testa, V.
name: Troja, E.
name: Turatto, M.
name: Vergani, S. D.
name: Vergani, D.
Publisher: name: arXiv
Year: 2017
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of gamma-rays, a gravitational wave signal, and a transient optical/near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named "macronovae" or "kilonovae", are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short gamma-ray burst at z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational wave source GW 170817 and gamma-ray burst GRB 170817A associated with a galaxy at a distance of 40 Mpc from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03-0.05 solar masses of material, including high-opacity lanthanides.
description: version accepted for publication in Nature. Some minor changes are expected with respect to the journal version