We present observations of the optical afterglow of GRB 170817A , made by the Hubble Space Telescope , between February and August 2018 , 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 Lorentz factor \Gamma \gtrsim 100 , forms a narrow core ( \sim 5 ^ { \circ } ) and is surrounded by a wider angular component that extends to \sim 15 ^ { \circ } , which is itself relativistic ( \Gamma \gtrsim 5 ) .
For a two-component model of this structure , the late-time optical decline , where F \propto t ^ { - \alpha } , is \alpha = 2.20 \pm 0.18 , and for a Gaussian structure the decline is \alpha = 2.45 \pm 0.23 .
We find the Gaussian model to be consistent with both the early \sim 10 days and late \gtrsim 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 \lesssim 4000 L _ { \odot } ~ { } ( M _ { F 606 W } \gtrsim - 4.3 ) .