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An excess in the X-ray emission from the neutron star merger GW170817 abovethe predicted afterglow was recently detected t~3.4 years post-merger. Onepossible origin for the excess is accretion onto the newly unshrouded blackhole (BH) remnant. While fall-back of the bound dynamical ejecta isinsufficient to generate the excess luminosity, L_X ~ 5e38 erg/s, fall-backfrom the disk wind ejecta-due to their larger mass and lower velocity-remains aviable possibility. We present hydrodynamic alpha-viscosity simulations of thepost-merger disk evolution which extend to an unprecedently long timescale t ~35 s post-merger, as necessary to capture the end of photodissociation and theasymptotic evolution into the radiatively inefficient regime. Due toinefficient neutrino cooling, the BH accretion rate decays rapidly at latetimes (Mdot_BH ~ t^(-\beta_BH), where \beta_BH ~ 2.4-2.8), seeminglyincompatible with generating the late-time excess. However, the rate at whichmatter falls back to the inner disk from the equatorial regions (as inferred bythe rate matter is unbound in outflows by viscous heating at higher latitudes)decays more gradually, Mdot_fb ~ t^(-\beta_fb) with \beta_fb ~ 1.43 in ouralpha ~ 0.03 simulations. By the present epoch, the fall-back rate has becomesub-Eddington and the disk can again accrete efficiently, i.e. Mdot_BH ~Mdot_fb, this time as a result of photon cooling instead of neutrino cooling.The predicted X-ray accretion luminosity at the present epoch is L_X ~ 0.1Mdot_BH c^2 ~ (2-70)e38 erg/s for beta_FB ~ 1.43-1.66, thus supporting (withcaveats) an accretion-powered origin for the X-ray excess in GW170817. Thesuppressed BH accretion rate prior to the radiatively efficient (sub-Eddington)transition, weeks to months after the merger, is key to avoid overproducing thekilonova luminosity via reprocessing.

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