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

Understand

The multi-wavelength electromagnetic afterglow from the binary neutron star merger GW170817/GRB170817A has displayed long-term power-law brightening, and presented challenges to post-merger models of the non-thermal 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/GRB170817A at 260 days post-merger that reveal an X-ray flux of F{0.3-8keV} = 1.1 x 10^-14 erg/s/cm^2, and confirm that the X-ray light curve is now also fading.
• Through rigorous comparisons to previous Chandra observations of GW170817/GRB170817A, X-ray fading is detected between 160 and 260 days post-merger at a 4.4 sigma significance, based on the X-ray data alone.