The Optical Afterglow of GW170817 at One Year Post-merger

We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between 2018 February and August, up to one year after the neutron star merger GW170817. The afterglow shows a rapid decline beyond 170. days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for failed-jet scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with a Lorentz factor of Gamma greater than or similar to 100, forms a narrow core (similar to 5 degrees) and is surrounded by a wider angular component that extends to similar to 15 degrees, which is itself relativistic (Gamma greater than or similar to 5). For a two-component model of this structure, the late-time optical decline, where F proportional to t(-a), is alpha = 2.20 +/- 0.18, and for a Gaussian structure the decline is alpha = 2.45 +/- 0.23. We find the Gaussian model to be consistent with both the early similar to 10 days and late greater than or similar to 290 days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster with a luminosity less than or similar to 4000 L-circle dot (M-F606W greater than or similar to -4.3).