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

• Published in: Astrophysical journal. Letters Vol. 883; no. 1; p. L1
• Main Authors: Fong, W., Blanchard, P. K., Alexander, K. D., Strader, J., Margutti, R., Hajela, A., Villar, V. A., Wu, Y., Ye, C. S., Berger, E., Chornock, R., Coppejans, D., Cowperthwaite, P. S., Eftekhari, T., Giannios, D., Guidorzi, C., Kathirgamaraju, A., Laskar, T., Macfadyen, A., Metzger, B. D., Nicholl, M., Paterson, K., Terreran, G., Sand, D. J., Sironi, L., Williams, P. K. G., Xie, X., Zrake, J.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 20.09.2019; IOP Publishing
• Subjects: Binary stars; Emission; Galaxies; Gamma ray bursts; Gamma rays; Globular clusters; Gravitational waves; Hubble Space Telescope; James Webb Space Telescope; Light curve; Luminosity; Neutron stars; Radio; Space telescopes
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/ab3d9e

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.