The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin. (arXiv:1908.08046v1 [astro-ph.HE])

The Optical Afterglow of GW170817: An Off-axis Structured Jet and Deep Constraints on a Globular Cluster Origin. (arXiv:1908.08046v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fong_W/0/1/0/all/0/1">Wen-fai Fong</a> (Northwestern/CIERA), <a href="http://arxiv.org/find/astro-ph/1/au:+Blanchard_P/0/1/0/all/0/1">P. K. Blanchard</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Alexander_K/0/1/0/all/0/1">K. D. Alexander</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Strader_J/0/1/0/all/0/1">J. Strader</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Margutti_R/0/1/0/all/0/1">R. Margutti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hajela_A/0/1/0/all/0/1">A. Hajela</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Villar_V/0/1/0/all/0/1">V. A. Villar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wu_Y/0/1/0/all/0/1">Y. Wu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Ye_C/0/1/0/all/0/1">C. S. Ye</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Berger_E/0/1/0/all/0/1">E. Berger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Chornock_R/0/1/0/all/0/1">R. Chornock</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coppejans_D/0/1/0/all/0/1">D. Coppejans</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Cowperthwaite_P/0/1/0/all/0/1">P. S. Cowperthwaite</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Eftekhari_T/0/1/0/all/0/1">T. Eftekhari</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Giannios_D/0/1/0/all/0/1">D. Giannios</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guidorzi_C/0/1/0/all/0/1">C. Guidorzi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kathirgamaraju_A/0/1/0/all/0/1">A. Kathirgamaraju</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Laskar_T/0/1/0/all/0/1">T. Laskar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+MacFadyen_A/0/1/0/all/0/1">A. MacFadyen</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Metzger_B/0/1/0/all/0/1">B. D. Metzger</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Nicholl_M/0/1/0/all/0/1">M. Nicholl</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Paterson_K/0/1/0/all/0/1">K. Paterson</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Terreran_G/0/1/0/all/0/1">G. Terreran</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sand_D/0/1/0/all/0/1">D. J. Sand</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sironi_L/0/1/0/all/0/1">L. Sironi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Williams_P/0/1/0/all/0/1">P. K. G. Williams</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Xie_X/0/1/0/all/0/1">X. Xie</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zrake_J/0/1/0/all/0/1">J. Zrake</a>

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.

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