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Fading of the X-Ray Afterglow of Neutron Star Merger GW170817/GRB 170817A at 260 Days

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journal contribution
posted on 30.08.2019, 08:27 by Melania Nynka, John J. Ruan, Daryl Haggard, Phil A. Evans
The multi-wavelength electromagnetic afterglow from the binary neutron star merger GW170817/GRB 170817A has displayed long-term power-law brightening, and has presented challenges to post-merger models of the nonthermal emission. The most recent radio observations up to 200 days post-merger suggest that the afterglow has finally peaked and may now be fading, but fading has not been confirmed in the X-rays. We present new, deep Chandra observations of GW170817/GRB 170817A at 260 days post-merger that reveal an X-ray flux of = F(0.3 - 8 keV) = 1.1 x 10^-14 erg s^−1 cm^−2 , and confirm that the X-ray light curve is now also fading. Through rigorous comparisons with previous Chandra observations of GW170817/GRB 170817A, X-ray fading is detected between 160 and 260 days post-merger at a 4.4σ significance on the basis of the X-ray data alone. We further constrain the X-ray photon index to steepen by <0.5 at 3.1σ significance during this period, which disfavors the passing of the synchrotron cooling frequency through the X-ray band as the cause of the observed fading. These observations remain consistent with optically thin synchrotron afterglow emission. If this afterglow emission arises from a quasi-spherical mildly relativistic outflow, the X-ray fading suggests that the outflow is now decelerating. Alternatively, if this afterglow arises from a successful off-axis structured jet, the X-ray fading suggests that emission from the jet core has already entered the line of sight.

Funding

The authors thank Belinda Wilkes and the Chandra scheduling, data processing, and archive teams for making these observations possible. This work was supported by Chandra Award Number GO7-18033X, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration (NASA) under contract NAS8-03060. J.J.R., M.N., and D.H. acknowledge support from a Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery grant and a Fonds de recherche du Québec–Nature et Technologies (FRQNT) Nouveaux Chercheurs grant. J.J.R. and M.N. acknowledge funding from the McGill Trottier Chair in Astrophysics and Cosmology. D.H. acknowledges support from the Canadian Institute for Advanced Research (CIFAR). P.A.E. acknowledges UKSA support. Facility: CXO - Chandra X-ray Observatory satellite. Software: Matplotlib (Hunter 2007).

History

Citation

The Astrophysical Journal Letters, 2018, 862(2)

Author affiliation

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

Version

VoR (Version of Record)

Published in

The Astrophysical Journal Letters

Publisher

American Astronomical Society, IOP Publishing

issn

2041-8205

eissn

2041-8213

Acceptance date

12/07/2018

Copyright date

2018

Available date

30/08/2019

Publisher version

https://iopscience.iop.org/article/10.3847/2041-8213/aad32d

Language

en