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The optical afterglow of GW170817 at one year post-merger

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posted on 12.09.2019, 15:31 by GP Lamb, JD Lyman, AJ Levan, NR Tanvir, T Kangas, AS Fruchter, B Gompertz, J Hjorth, I Mandel, SR Oates, D Steeghs, K Wiersema
We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between 2018 February and August, up to one year after the neutron star merger GW170817. The afterglow shows a rapid decline beyond 170 days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for "failed-jet" scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultra-relativistic jet, with a Lorentz factor of Γ ~> 100, forms a narrow core (~5°) and is surrounded by a wider angular component that extends to ~15°, which is itself relativistic (Γ ~> 5). For a two-component model of this structure, the late-time optical decline, where F ∝ t −α , is α = 2.20 ± 0.18, and for a Gaussian structure the decline is α = 2.45 ± 0.23. We find the Gaussian model to be consistent with both the early ~10 days and late ~> 290 days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster with a luminosity ~< 4000 L ⊙ (M F606W ~> −4.3).

Funding

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute (STScI). STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. These observations are associated with programs GO 15482 (J.D.L.) and GO 14771 (N.R.T.). G.P.L. is supported by STFC grant ST/N000757/1 and thanks Om Salafia for useful discussions. J.D.L. acknowledges support from STFC grant ST/P000495/1. A.J.L. acknowledges that this project received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant agreement 725246). J.H. acknowledges support by a VILLUM FONDEN Investigator grant (project number 16599). S.R.O. acknowledges support of the Leverhulme Trust Early Career Fellowship. We thank the anonymous referee for the comments. Facility: HST(WFC3). - Software: astropy (Astropy Collaboration et al. 2013), DrizzlePac STSCI Development Team (2012), emcee (Foreman-Mackey et al. 2013), SExtractor (Bertin & Arnouts 1996).

History

Citation

Astrophysical Journal Letters, 2019, 870 L15

Author affiliation

/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy

Version

VoR (Version of Record)

Published in

Astrophysical Journal Letters

Publisher

American Astronomical Society, IOP Publishing

eissn

2041-8213

Acceptance date

18/12/2018

Copyright date

2019

Available date

12/09/2019

Publisher version

https://iopscience.iop.org/article/10.3847/2041-8213/aaf96b/meta

Language

en

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