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Title: Fading of the X-Ray Afterglow of Neutron Star Merger GW170817/GRB 170817A at 260 Days
Authors: Nynka, Melania
Ruan, John J.
Haggard, Daryl
Evans, Phil A.
First Published: 31-Jul-2018
Publisher: American Astronomical Society, IOP Publishing
Citation: The Astrophysical Journal Letters, 2018, 862(2)
Abstract: 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.
DOI Link: 10.3847/2041-8213/aad32d
ISSN: 2041-8205
eISSN: 2041-8213
Version: Publisher Version
Status: Peer-reviewed
Type: Journal Article
Rights: Copyright © 2018, American Astronomical Society, IOP Publishing. Deposited with reference to the publisher’s open access archiving policy. (
Appears in Collections:Published Articles, Dept. of Physics and Astronomy

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