The Electromagnetic Counterpart of the Binary Neutron Star Merger LIGO/VIRGO GW170817. III. Optical and UV Spectra of a Blue Kilonova From Fast Polar Ejecta

Abstract: We present optical and ultraviolet spectra of the first electromagnetic counterpart to a gravitational wave (GW) source, the binary neutron star merger GW170817. Spectra were obtained nightly between 1.5 and 9.5 days post-merger, using the SOAR and Magellan telescopes; the UV spectrum was obtained with the \textit{Hubble Space Telescope} at 5.5 days. Our data reveal a rapidly-fading blue component ($T\approx5500$ K at 1.5 days) that quickly reddens; spectra later than $\gtrsim 4.5$ days peak beyond the optical regime. The spectra are mostly featureless, although we identify a possible weak emission line at $\sim 7900$ \AA\ at $t\lesssim 4.5$ days. The colours, rapid evolution and featureless spectrum are consistent with a "blue" kilonova from polar ejecta comprised mainly of light $r$-process nuclei with atomic mass number $A\lesssim 140$. This indicates a sight-line within $\theta_{\rm obs}\lesssim 45^{\circ}$ of the orbital axis. Comparison to models suggests $\sim0.03$ M$_\odot$ of blue ejecta, with a velocity of $\sim 0.3c$. The required lanthanide fraction is $\sim 10^{-4}$, but this drops to $<10^{-5}$ in the outermost ejecta. The large velocities point to a dynamical origin, rather than a disk wind, for this blue component, suggesting that both binary constituents are neutron stars (as opposed to a binary consisting of a neutron star and a black hole). For dynamical ejecta, the high mass favors a small neutron star radius of $\lesssim 12$ km. This mass also supports the idea that neutron star mergers are a major contributor to $r$-process nucleosynthesis.

• The paper reveals that optical and UV spectra of GW170817 demonstrate a blue kilonova emerging from fast-moving polar ejecta.
• The analysis quantifies a polar ejecta mass of about 0.03 solar masses and an expansion velocity around 0.3c with low lanthanide content.
• The findings support r-process nucleosynthesis in neutron star mergers and suggest a softer equation of state with smaller neutron star radii.

Optical and UV Spectra of the Neutron Star Merger GW170817

Introduction

The detection of the binary neutron star merger GW170817 via both gravitational waves (GWs) and electromagnetic (EM) signals represents a significant scientific milestone. This paper specifically addresses the optical and ultraviolet (UV) spectral observations that followed GW170817. The relevance of these observations lies in the insight they provide into the nature of kilonovae, EM emissions produced by such mergers. These kilonovae are believed to result from the radioactive decay of neutron-rich ejecta following neutron star coalescence.

Observations and Spectral Characteristics

The research team obtained optical and UV spectra of GW170817's electromagnetic counterpart over a timeframe of approximately nine days post-merger, utilizing telescopes including the Southern Astrophysical Research (SOAR) telescope and the Hubble Space Telescope. The initial spectra depicted a rapidly changing blue component with a temperature of around 5500 K at 1.5 days post-merger that showed a significant redshift over time. The lack of strong spectral features and the rapid transition in color are consistent with predictions of a "blue" kilonova resulting from fast-moving polar ejecta dominated by light r-process nuclei.

Analysis of Ejecta Properties

The analyzed spectra suggest that the polar ejecta mass is approximately 0.03 solar masses, with an expansion velocity of about 0.3c. The line features in the spectra indicate a low lanthanide fraction of roughly $10^{-4}$, consistent with rapid expansion velocities that facilitate line blending and result in a featureless spectrum. The theoretical models that best fit the data are indicative of neutron stars with a small radius, possibly less than 12 kilometers, suggesting a softer equation of state. These results are instrumental in deepening the understanding of how neutron star mergers contribute to r-process nucleosynthesis, supporting the hypothesis that such mergers are significant asteroid factories for heavy elements in the universe.

Implications

The findings of this study have significant implications for both theoretical and observational astrophysics. The data supports the hypothesis that binary neutron star mergers are major contributors to the universe's supply of elements heavier than iron. Practically, it demonstrates the viability of combined GW and EM observation campaigns for understanding cosmic phenomena. Future developments could refine the models of neutron star mergers and the mechanisms of r-process nucleosynthesis, leveraging the method and findings of this study.

In conclusion, this analysis of GW170817 adds to the body of knowledge concerning kilonova characteristics and neutron star mergers, with implications for astrophysics that extend from stellar evolution models to theories of heavy element genesis. Continued observation of similar events will enhance this understanding, fostering further integration of observational techniques across electromagnetic and gravitational spectra.