TY - JOUR
T1 - The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin
AU - Fong, W.
AU - Blanchard, P.~K.
AU - Alexander, K.~D.
AU - Strader, J.
AU - Margutti, R.
AU - Hajela, A.
AU - Villar, V.~A.
AU - Wu, Y.
AU - Ye, C.~S.
AU - Berger, E.
AU - Chornock, R.
AU - Coppejans, D.
AU - Cowperthwaite, P.~S.
AU - Eftekhari, T.
AU - Giannios, D.
AU - Guidorzi, C.
AU - Kathirgamaraju, A.
AU - Laskar, T.
AU - Macfadyen, A.
AU - Metzger, B.~D.
AU - Nicholl, M.
AU - Paterson, K.
AU - Terreran, G.
AU - Sand, D.~J.
AU - Sironi, L.
AU - Williams, P.~K.~G.
AU - Xie, X.
AU - Zrake, J.
PY - 2019/9/17
Y1 - 2019/9/17
N2 - 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 $\langle \beta \rangle =-0.583\pm 0.013$, 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 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.
AB - 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 $\langle \beta \rangle =-0.583\pm 0.013$, 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 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.
KW - Neutron stars
KW - Gravitational waves
KW - Gamma-ray bursts
KW - Hubble Space Telescope
KW - Astrophysics - High Energy Astrophysical Phenomena
U2 - 10.3847/2041-8213/ab3d9e
DO - 10.3847/2041-8213/ab3d9e
M3 - Article
VL - 883
JO - The Astrophysical Journal Letters
JF - The Astrophysical Journal Letters
IS - 1
ER -