Early Spectra of the Gravitational Wave Source GW170817: Evolution of a Neutron Star Merger

TL;DR

This paper presents early spectroscopic observations of the neutron star merger GW170817, revealing rapid ejecta expansion, cooling, and spectral evolution consistent with r-process nucleosynthesis and lanthanide-free material.

Contribution

First detailed time-series spectroscopy of GW170817's optical counterpart, showing spectral evolution and ejecta properties that support neutron star merger models.

Findings

Ejecta rapidly expanded and cooled within the first hour.

Spectra showed distinct blue and red components over time.

Late-time spectra matched r-process-enriched ejecta models.

Abstract

On 2017 August 17, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after merger. Over the first hour of observations the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measure the photosphere cooling from $11,000^{+3400}_{-900}$ K to $9300^{+300}_{-300}$ K, and determine a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a begin displaying broad features after 1.46 days, and evolve qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high velocity, lanthanide-free material.