From Neutrino- to Photon-Cooled in Three Years: Can Fallback Accretion Explain the X-ray Excess in GW170817?

Understand

An excess in the X-ray emission from the neutron star merger GW170817 above the predicted afterglow was recently detected t~3.4 years post-merger.

• One possible origin for the excess is accretion onto the newly unshrouded black hole (BH) remnant.
• While fall-back of the bound dynamical ejecta is insufficient to generate the excess luminosity, L_X ~ 5e38 erg/s, fall-back from the disk wind ejecta-due to their larger mass and lower velocity-remains a viable possibility.
• We present hydrodynamic alpha-viscosity simulations of the post-merger disk evolution which extend to an unprecedently long timescale t ~ 35 s post-merger, as necessary to capture the end of photodissociation and the asymptotic evolution into the radiatively inefficient regime.