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

By E. Pian, P. D’Avanzo, S. Benetti, M. Branchesi, E. Brocato, S. Campana, E. Cappellaro, S. Covino, V. D’Elia, Johan Peter Uldall Fynbo and 74 more. Nature, 2017

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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.

Subfields: Astronomy and Astrophysics

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Citations in the corpus

The Combined Ultraviolet, Optical, and Near-infrared Light Curves of the Kilonova Associated with the Binary Neutron Star Merger GW170817: Unified Data Set, Analytic Models, and Physical Implications (2017, background): “This multi-component behavior is also seen in optical and NIR spectroscopic observations of the transient (Chornock et al. 2017; Nicholl et al. 2017; Pian et al. 2017; Shappee et al. 2017; Smartt et al. 2017).”
The Emergence of a Lanthanide-rich Kilonova Following the Merger of Two Neutron Stars (2017, background): “Overall, the spectra match well those seen in the extensive ground-based spectroscopic sequence of (Pian et al. 2017), although the absence of atmospheric absorption, compared to ground-based spectra, is particularly beneficial in revealing clearly the 1.4μm feature.”
Chemical evolution with rotating massive star yields - I. The solar neighbourhood and the s-process elements (2018, methodology): “The NSM scenario is given support by the recent joint detection of electromagnetic and gravitational signal from the γ-ray burst GW170817/GRB170817A (Pian et al. 2017 and references therein), but there is still no consensus on the role of that class of objects in the production and evolution of r-el”
The Binary Neutron Star Event LIGO/Virgo GW170817 160 Days after Merger: Synchrotron Emission across the Electromagnetic Spectrum (2018, background): “…KN; Metzger 2017; Chornock et al. 2017; Coulter et al. 2017; Cowperthwaite et al. 2017; Drout et al. 2017; Kasliwal et al. 2017; Nicholl et al. 2017; Pian et al. 2017; Smartt et al. 2017; Soares-Santos et al. 2017; Tanvir et al. 2017; Valenti et al. 2017; Villar et al. 2017) and nonthermal…”
A Magnetar Origin for the Kilonova Ejecta in GW170817 (2018, background): “…peaked at UV/optical frequencies (e.g., Evans et al. 2017; McCully et al. 2017; Nicholl et al. 2017), before rapidly evolving over the course of a few days to become dominated by emission with a spectral peak in the near-infrared (NIR; Chornock et al. 2017; Pian et al. 2017; Tanvir et al. 2017).”
The evolution of the X-ray afterglow emission of GW 170817 / GRB 170817A in XMM-Newton observations (2018, background): “…Li & Paczyński 1998), providing the first compelling observational evidence for the existence of such elusive transient sources (Cowperthwaite et al. 2017; Drout et al. 2017; Evans et al. 2017; Kasliwal et al. 2017; Nicholl et al. 2017; Pian et al. 2017; Smartt et al. 2017; Villar et al. 2017).”