We present a revised and complete optical afterglow light curve of the binary neutron star merger GW170817 , enabled by deep Hubble Space Telescope ( HST ) F606W observations at \approx 584 days post-merger , which provide a robust optical template .
The light curve spans \approx 110 - 362 days , and is fully consistent with emission from a relativistic structured jet viewed off-axis , as previously indicated by radio and X-ray data .
Combined with contemporaneous radio and X-ray observations , we find no spectral evolution , with a weighted average spectral index of \langle \beta \rangle = -0.583 \pm 0.013 , demonstrating that no synchrotron break frequencies evolve between the radio and X-ray bands over these timescales .
We find that an extrapolation of the post-peak temporal slope of GW170817 to the luminosities of cosmological short GRBs matches their observed jet break times , suggesting that their explosion properties are similar , and that the primary difference in GW170817 is viewing angle .
Additionally , we place a deep limit on the luminosity and mass of an underlying globular cluster of L \lesssim 6.7 \times 10 ^ { 3 } L _ { \odot } , or M \lesssim 1.3 \times 10 ^ { 4 } M _ { \odot } , at least 4 standard deviations below the peak of the globular cluster mass function of the host galaxy , NGC4993 .
This limit provides a direct and strong constraint that GW170817 did not form and merge in a globular cluster .
As highlighted here , HST ( and soon JWST ) enables critical observations of the optical emission from neutron star merger jets and outflows .