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 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.

• The paper provides direct evidence of r-process nucleosynthesis through detailed spectral evolution of the kilonova event from GW170817.
• It employs multi-wavelength observations from UV to near-IR to analyze ejecta dynamics, revealing fast-moving material (~0.2c) and substantial lanthanide opacity.
• The findings refine theoretical models of neutron star mergers and pave the way for enhanced multi-messenger astronomy strategies in future kilonova detections.

Spectroscopic Identification of R-Process Nucleosynthesis in a Double Neutron Star Merger

The paper under review presents a comprehensive spectroscopic analysis of a kilonova event, associated with the neutron star merger corresponding to the gravitational wave source GW170817 and the short gamma-ray burst GRB170817A. The study provides a meticulous account of the properties and behaviors of the kilonova, a transient luminous event expected in the aftermath of neutron star mergers. The authors employed extensive spectroscopic observations spanning from ultraviolet to near-infrared wavelengths, conducted with ground-based observatories.

Key Observations and Numerical Results

The GW170817 event marked the first confirmed detection of both gravitational waves and electromagnetic radiation from a double neutron star merger, occurring at a distance of 40 Mpc in the galaxy NGC 4993. Spectroscopic data revealed rapidly expanding ejecta with a velocity approximating 0.2c, reaching a radius of ~50 AU within 1.5 days. Notably, the emission evolved from a predominantly blue spectral continuum into one embodying broad absorption lines, consistent with theoretical predictions of r-process nucleosynthesis. The analyses estimate an ejected mass ranging from 0.03 to 0.05 solar masses, inclusive of high-opacity lanthanides, and indicate expansion dynamics involving both dynamical ejecta and slower wind regions.

Implications and Theoretical Significance

The identification of r-process nucleosynthesis, as inferred from spectroscopic features, offers direct evidence for the production of heavy elements such as gold and platinum in such cosmic collisions, corroborating the hypothesized role of neutron star mergers as significant sites for these elements' synthesis. This finding not only enhances the understanding of nucleosynthesis processes but also integrates the multimessenger astronomy approach, combining gravitational and electromagnetic observations for more holistic universe explorations.

The spectral observations provide crucial insights into the complexity of kilonova emissions, requiring adaptations in existing models to account for observed luminosities and spectral features. The conclusions drawn emphasize the necessity to refine models, incorporating a wide range of neutron-rich environments and multi-velocity ejection dynamics, aligning theoretical constructs with empirical spectral data.

Future Prospects

The data from GW170817 and its associated kilonova open new research avenues for refined modeling of kilonova emissions, consideration of viewing-angle dependencies, and exploration of the variability across neutron star merger events. The study sets a foundational template for future detections, guiding spectroscopic analysis frameworks and observational strategies. Further research will likely deepen the understanding of the elemental abundance contributions from such astrophysical events and enhance the precision of multimessenger locate-and-identify technologies.

Overall, the paper significantly advances the field's knowledge of neutron star mergers and the r-process, contributing both practical methodologies for observing these phenomena and theoretical insights into the nucleosynthesis in extreme astrophysical environments.