Electromagnetic Evidence that SSS17a is the Result of a Binary Neutron Star Merger

Abstract: 11 hours after the detection of gravitational wave source GW170817 by the Laser Interferometer Gravitational-Wave Observatory and Virgo Interferometers, an associated optical transient SSS17a was discovered in the galaxy NGC 4993. While the gravitational wave data indicate GW170817 is consistent with the merger of two compact objects, the electromagnetic observations provide independent constraints of the nature of that system. Here we synthesize all optical and near-infrared photometry and spectroscopy of SSS17a collected by the One-Meter Two-Hemisphere collaboration. We find that SSS17a is unlike other known transients. The source is best described by theoretical models of a kilonova consisting of radioactive elements produced by rapid neutron capture (the r-process). We find that SSS17a was the result of a binary neutron star merger, reinforcing the gravitational wave result.

Overview of the Study on Electromagnetic Evidence for a Binary Neutron Star Merger

This paper provides substantial evidence that SSS17a resulted from a binary neutron star (BNS) merger, supported by both gravitational wave and electromagnetic observations. The detection of GW170817 by LIGO and Virgo marked a significant event, identifying the merger of compact objects. The identification of an optical counterpart SSS17a, 11 hours post detection, enabled a comprehensive analysis of the electromagnetic spectrum. This study synthesizes optical and near-infrared (NIR) data from the One-Meter Two-Hemisphere (1M2H) Collaboration to propose the transient as a kilonova characterized by radioactive decay processes.

Host Galaxy Analysis

NGC 4993, an S0-type galaxy, serves as the host for SSS17a. Observations through the Hubble Space Telescope provided insights into the morphology, surface brightness, and stellar population, which are typical for lenticular galaxies. The calculated stellar mass aligns with typical hosts of short gamma-ray bursts (SGRBs). SSS17a's location within NGC 4993 suggests a progenitor system closely situated to the galaxy's dynamical center, discounting the likelihood of recent progenitor ejection from a globular cluster, given the absence of recent star formation.

Spectral Analysis and Light Curves

The paper meticulously details the spectral and light curve analysis demonstrating the unique properties of SSS17a. The rapid rise and decline in brightness, coupled with the shift in color from blue to red, distinguish SSS17a from previously-known transients. This behavior aligns more closely with theoretical kilonova models rather than typical supernovae, highlighting the role of heavy, neutron-rich element formation through the r-process.

Kilonova Characteristics

The study emphasizes a two-component kilonova model comprised of a lanthanide-rich red component and a less opaque blue component. The inferred parameters include an ejecta mass of approximately 0.035 M_\odot for the red kilonova and distinct optical properties suggesting varied lanthanide fractions. The complex spectral evolution corroborates the presence of both component types, suggesting dynamic ejecta with stratified elemental compositions. Such a model robustly corroborates the rapid transition and color evolution observed in SSS17a, providing insight into its progenitor system.

Implications for r-Process Synthesis

The analysis sets a precedent for understanding the astrophysical sites of the r-process, alluding to binary neutron star mergers as significant contributors to the galaxy's r-process nucleosynthesis. The observed heavy element abundance matches predictions within the mass and energy constraints of inhomogeneous r-process dispersions seen in Galactic halo stars.

Conclusion and Future Implications

The paper concludes that SSS17a is consistent with a binary neutron star progenitor system. The event offers substantial evidence of BNS mergers as significant sites for r-process element formation, aligning with observational constraints from metal-poor Galactic stars. Future observations could further decipher mass ratios and ejecta properties, enhancing the predictive accuracy of binary neutron star merger models. This research strengthens the understanding of galactic chemical evolution and the dynamic universe of compact binary mergers.