The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. IV. Detection of Near-infrared Signatures of r-process Nucleosynthesis with Gemini-South

Abstract: We present a near-infrared spectral sequence of the electromagnetic counterpart to the binary neutron star merger GW170817 detected by Advanced LIGO/Virgo. Our dataset comprises seven epochs of J+H spectra taken with FLAMINGOS-2 on Gemini-South between 1.5 and 10.5 days after the merger. In the initial epoch, the spectrum is dominated by a smooth blue continuum due to a high-velocity, lanthanide-poor blue kilonova component. Starting the following night, all subsequent spectra instead show features that are similar to those predicted in model spectra of material with a high concentration of lanthanides, including spectral peaks near 1.07 and 1.55 microns. Our fiducial model with 0.04 M_sun of ejecta, an ejection velocity of v=0.1c, and a lanthanide concentration of X_lan=1e-2 provides a good match to the spectra taken in the first five days, although it over-predicts the late-time fluxes. We also explore models with multiple fitting components, in each case finding that a significant abundance of lanthanide elements is necessary to match the broad spectral peaks that we observe starting at 2.5 d after the merger. These data provide direct evidence that binary neutron star mergers are significant production sites of even the heaviest r-process elements.

• The paper reveals that near-infrared spectral observations of GW170817 provide empirical evidence for r-process nucleosynthesis.
• It employs seven epochs of J+H band spectroscopy from Gemini-South to compare with theoretical kilonova models and pinpoint lanthanide-rich ejecta.
• The findings validate a two-component kilonova structure, reinforcing neutron star mergers as significant sources of heavy element production.

Near-Infrared Spectroscopy of the Electromagnetic Counterpart to GW170817

The paper "The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. IV. Detection of Near-Infrared Signatures of r-process Nucleosynthesis With Gemini-South" by Chornock et al. presents a comprehensive analysis of the near-infrared (NIR) spectral observations of the kilonova associated with the binary neutron star merger GW170817. This study is crucial because it provides empirical evidence for the production of r-process elements in such cosmological events.

Spectral Observations and Analysis

The research utilized a sequence of seven epochs of $J+H$ band spectroscopy taken over a span of approximately ten days following the gravitational wave detection of GW170817. The observations were conducted using the FLAMINGOS-2 instrument on the Gemini-South telescope. The initial spectra exhibited a smooth blue continuum attributed to a high-velocity, lanthanide-poor blue kilonova component. Over time, the presence of spectral features, notably peaks around 1.07 and 1.55 microns, consistent with lanthanide-rich material, became predominant.

The spectral data were compared against theoretical kilonova emission models to deduce the properties of the ejecta. A single-component model with 0.04 solar masses of ejecta, velocities of 0.1c, and a lanthanide concentration of $10^{-2}$ provided a good theoretical match to the early spectra but over-predicted late-time fluxes. This model validates the prediction that binary neutron star mergers are significant sites for the synthesis of heavy r-process elements, which has long been hypothesized but not empirically confirmed until these observations.

Theoretical Implications

The observations corroborate theoretical models suggesting a two-component kilonova structure comprising a blue component, indicative of lanthanide-poor ejecta, and a red component, representing lanthanide-rich material. The developed models assert that adequate lanthanide fractions, ranging between $10^{-2}$ and $10^{-3}$, are necessary to replicate the observed spectral features. The red kilonova component, with substantial lanthanide presence, manifests as the dominant feature shortly after the merger and suggests significant contributions to the galactic r-process nucleosynthesis.

Future Directions

The study underscores the importance of comprehensive spectral models that incorporate realistic velocity and abundance distributions within the ejecta. These will refine our understanding of the kilonova components and their contribution to the r-process nucleosynthesis. With advancements in observational capabilities, notably the upcoming James Webb Space Telescope, more detailed spectral and temporal analyses of similar events could provide deeper insights into the dynamics of heavy element formation in the universe. Additionally, further gravitational wave observations will help ascertain the frequency of such neutron star mergers, thereby enabling a better estimation of their contribution to the cosmic r-process element budget.

In conclusion, Chornock et al.'s findings underscore the role of binary neutron star mergers in synthesizing heavy elements and offer a significant step forward in our understanding of nucleosynthesis processes in the universe. This study lays a foundation for further exploration and model refinement in the field of astrophysical transients, highlighting the interplay between theoretical astrophysics and empirical observations.