The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin
Berger, E.; Chornock, R.; Laskar, T.; Fong, W.; Nicholl, M.; Eftekhari, T.; Metzger, B. D.; Sand, D. J.; Guidorzi, C.; Terreran, G.; Wu, Y.; Strader, J.; Margutti, R.; Alexander, K. D.; Blanchard, P. K.; Hajela, A.; Paterson, K.; Williams, P. K. G.; Villar, V. A.; Ye, C. S.; Coppejans, D.; Cowperthwaite, P. S.; Giannios, D.; Kathirgamaraju, A.; Macfadyen, A.; Sironi, L.; Xie, X.; Zrake, J.
United States, Italy, United Kingdom
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 < β > =-0.583+/- 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 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 × 103 L ⊙, or M ≲ 1.3 × 104 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.