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The multi-wavelength electromagnetic afterglow from the binary neutron starmerger GW170817/GRB170817A has displayed long-term power-law brightening, andpresented challenges to post-merger models of the non-thermal emission. Themost recent radio observations up to 200 days post-merger suggest that theafterglow has finally peaked and may now be fading, but fading has not beenconfirmed in the X-rays. We present new, deep Chandra observations ofGW170817/GRB170817A at 260 days post-merger that reveal an X-ray flux ofF{0.3-8keV} = 1.1 x 10^-14 erg/s/cm^2, and confirm that the X-ray light curveis now also fading. Through rigorous comparisons to previous Chandraobservations of GW170817/GRB170817A, X-ray fading is detected between 160 and260 days post-merger at a 4.4 sigma significance, based on the X-ray dataalone. We further constrain the X-ray photon index to steepen by <0.5 at 3.1sigma significance during this period, which disfavors the passing of thesynchrotron cooling frequency through the X-ray band as the cause of theobserved fading. These observations remain consistent with optically thinsynchrotron afterglow emission. If this afterglow emission arises from aquasi-spherical mildly relativistic outflow, the X-ray fading suggests that theoutflow is now decelerating. Alternatively, if this afterglow arises from asuccessful off-axis structured jet, the X-ray fading suggests that emissionfrom the jet core has already entered the line of sight.

Fading of the X-ray Afterglow of Neutron Star Merger GW170817/GRB170817A at 260 days