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On 2017 August 17, Swope Supernova Survey 2017a (SSS17a) was discovered asthe optical counterpart of the binary neutron star gravitational wave eventGW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until8.5 days after merger. Over the first hour of observations the ejecta rapidlyexpanded and cooled. Applying blackbody fits to the spectra, we measure thephotosphere 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. Thespectra of SSS17a begin displaying broad features after 1.46 days, and evolvequalitatively over each subsequent day, with distinct blue (early-time) and red(late-time) components. The late-time component is consistent with theoreticalmodels of r-process-enriched neutron star ejecta, whereas the blue componentrequires high velocity, lanthanide-free material.

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