The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin

Abstract 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 ≈584 days post-merger, which provide a robust optical template. The light curve spans ≈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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">〈</mml:mo> <mml:mi>β</mml:mi> <mml:mo stretchy="false">〉</mml:mo> <mml:mo>=</mml:mo> <mml:mo>−</mml:mo> <mml:mn>0.583</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.013</mml:mn> </mml:math> , 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 gamma-ray bursts 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 (GC) of L ≲ 6.7 × 10 3 L ⊙ , or M ≲ 1.3 × 10 4 M ⊙ , at least 4 standard deviations below the peak of the GC mass function of the host galaxy, NGC 4993. This limit provides a direct and strong constraint that GW170817 did not form and merge in a GC. As highlighted here, HST (and soon the James Webb Space Telescope ) enables critical observations of the optical emission from neutron star merger jets and outflows.