Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger

Abstract

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-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 γ-ray burst at redshift 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 GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs 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 to 0.05 solar masses of material, including high-opacity lanthanides.

The Environment of the Binary Neutron Star Merger GW170817

“…Numerous groups undertook searches of the resulting GW-error region, revealing a counterpart in NGC 4993 (Coulter et al 2017a(Coulter et al , 2017b, independently confirmed by several groups (Allam et al 2017;Arcavi et al 2017;Lipunov et al 2017;Yang et al 2017). The counterpart, known as SSS17a/AT2017gfo, was seen to brighten in the IR and then dramatically redden in the following nights (Evans et al 2017;Pian et al 2017;Smartt et al 2017;, revealing broad features consistent with the expectations for a transient driven by heavy element (r-process) nucleosynthesis, often dubbed a kilonova (Li & Paczyński 1998;Metzger & Berger 2012;Barnes & Kasen 2013). These properties cement the association of the optical counterpart with both the GRB and the gravitationalwave trigger.…”

“…This has changed with the discovery of GW170817, an unambiguous neutron star merger directly measured in gravitational waves (LIGO Scientific Collaboration & Virgo Collaboration 2017b), associated with an SGRB (LIGO Scientific Collaboration & Virgo Collaboration, Fermi-GBM & Integral 2017, in preparation) as well as a radioactively powered kilonova (e.g., LIGO Scientific Collaboration & Virgo Collaboration 2017, in preparation; Pian et al 2017;. For the first time this provides a route for studying the properties of a confirmed neutron star binary merger in detail.…”

Evaluating radiation transport errors in merger simulations using a Monte Carlo algorithm

“…We also observe the usual hierarchy of temperatures, with ν x being hotter thanν e neutrinos, which are themselves hotter than ν e . Global quantities show relative differences of (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)% between the M1 and MC results, which could be due either to the approximations made in the M1 scheme or to the slightly different microphysics implemented in each algorithm. We also note that the net flux of lepton number in the two schemes (i.e., number of ν e minus number ofν e leaving the grid) would likely be in closer agreement if the MC scheme was coupled to the fluid, as the fluid would evolve towards a new, slightly modified equilibrium composition.…”

“…This event also shows the current limits of our ability to reliably extract information about merging compact objects using EM observations. For example, GW170817 was followed by a bright kilonova [3][4][5][6][7][8][9][10][11][12][13][14][15], an optical/infrared transient powered by radioactive decays in the neutron-rich ejecta produced by the merger [16][17][18][19][20]. EM observations of that kilonova have been used to infer plausible properties of the ejecta, and the outcome of r-process nucleosynthesis in the outflows.…”

Continued Brightening of the Afterglow of GW170817/GRB 170817A as Being Due to a Delayed Energy Injection

“…About 1.7s later, a faint short gamma-ray burst (GRB) GRB 170817A was detected by Fermi Gamma-Ray Telescope and International Gamma-Ray Astrophysics Laboratory (INTEGRAL), which is considered to be associated with GW170817 (Abbott et al 2017b;Goldstein et al 2017;Savchenko et al 2017). About ∼11hr after the event, an optical counterpart was identified (a kilonova) and named AT2017gfo (Arcavi et al 2017;Coulter et al 2017;Drout et al 2017;Kasen et al 2017;Kasliwal et al 2017;Pian et al 2017;Smartt et al 2017;Soares-Santos et al 2017;Valenti et al 2017), which verifies the hypothesis that r-process-induced kilonovae are associated with binary NS mergers (Li & Paczyński 1998;Rosswog 2005;Metzger et al 2010;Metzger 2017), and confirms that at least some short GRBs originate from binary NS mergers (e.g., Eichler et al 1989;Mészáros & Rees 1992;Narayan et al 1992;Oechslin & Janka 2006;Piran 2011, andsee Berger 2014 for a recent review).…”