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

Understand

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.