Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis

Abstract: On 2017 August 17, gravitational waves were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical and infrared light curves of SSS17a extending from 10.9 hours to 18 days post-merger. We constrain the radioactively-powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in r-process nucleosynthesis in the Universe.

• The paper analyzes the light curves of GW170817/SSS17a, revealing evidence for multiple ejecta components and significant r-process element production during the merger.
• The study estimates the GW170817 merger produced at least 0.05 solar masses of r-process products, strongly supporting neutron star mergers as primary sites for heavy element synthesis.
• Analysis of the light curves reveals multiple ejecta components with differing opacity and lanthanide abundance, supporting models of dynamical ejecta and accretion disk winds.

Light Curves of the Neutron Star Merger GW170817/SSS17a: Implications for R-Process Nucleosynthesis

The merger of neutron stars, GW170817, accompanied by a gamma-ray burst (GRB170817A) and its optical counterpart, SSS17a, presents a paradigm-shifting observational opportunity for astrophysics, particularly in the context of r-process nucleosynthesis. This paper offers a comprehensive analysis of the light curves in ultraviolet, optical, and infrared spectra, covering a timeframe from 10.9 hours to 18 days post-merger. It highlights the ejecta dynamics and the implications for the formation of heavy elements in the universe—a key to understanding cosmic chemical abundance.

Key Findings and Discussion

SSS17a's light curves show rapid initial brightening, subsequent decay, and swift color evolution, all of which suggest multiple ejecta components with varying lanthanide abundances. One of the standout results of this study is the significant production of heavy elements. Specifically, a minimum of approximately 0.05 solar masses worth of r-process nucleosynthesis products—a major indicator of the role of neutron star mergers in synthesizing the universe's heavy elements.

The paper's key arguments are structured around several critical analytical frameworks:

1. Gravitational Waves and Electromagnetic Counterpart: The integration of gravitational wave observations with electromagnetic spectra provides robust localization and novel insight into the neutron star merger dynamics. This dual-modal observation extends the interpretive framework offered by GW signatures alone.
2. Macronova/Kilonova Phenomena: SSS17a's observation affirms the hypothesized kilonova as an isotropic electromagnetic signature produced by the radioactive decay of neutron-rich ejecta. Prior observational evidence for kilonovae remains tentative, positioning GW170817/SSS17a as a critical empirical validation for these theoretical models.
3. R-Process Nucleosynthesis: The formation of approximately half the elements heavier than iron remains a significant scientific enigma. This study provides strong evidence for compact binary mergers, rather than core-collapse supernovae, as the predominant r-process production sites. The amount of r-process material and subsequent energy radiation profile reveals valuable insights into the general properties and elemental yields of such astrophysical events.
4. Opacity and Ejecta Dynamics: Evidence from the light curves indicates the presence of multiple ejecta components with differing compositions. Early optical depths suggest a lanthanide-free, low-opacity component followed by late-time lanthanide-rich material. This bifurcation is consistent with theoretical models of dynamical ejecta followed by accretion disk winds.

Future Implications

This paper's findings elucidate the significance of neutron star mergers in universal element distribution. The implications stretch from galactic chemical makeup to the rates and detectability of future neutron star mergers by gravitational wave and electromagnetic observatories. As advanced LIGO and Virgo detectors enhance sensitivity, the occurrence of such detections will likely increase, offering further empirical opportunities to refine r-process modeling and nucleosynthesis predictions.

Overall, this study advances the discourse on cosmic r-process element formation and neutron star merger dynamics. It stands as a touchstone for future multi-messenger astrophysics, harnessing the collaborative potential of gravitational wave and electromagnetic observations to solve cosmic chemical evolution puzzles.