On the Continued Brightening of the Afterglow of GW170817/GRB 170817A: Delayed Energy Injection due to Fall-back Accretion

Recently, it was found that the multi-wavelength afterglow of GRB 170817A is steadily increasing even $\sim$ 160 days after the associate gravitational event. This brightening is completely unexpected in the frame work of the normal off-axis top-hat jet model. Here, we suggest that the brightening can be caused by the fall-back accretion process. During the coalescence of the double neutron star system, a significant amount of the dynamical ejecta may fall back toward the central remnant, naturally causing a delayed energy release. The energy is injected into the outflow that originally gave birth to the gamma-ray burst and re-energizes the external shock, causing a continued brightening in the afterglow emission. We show that the observed multi-wavelength brightening can be well explained in our frame work. According to our calculations, the total fall-back material should be $\sim 0.01$ solar mass, which is in good agreement with the mass ejection revealed by numerical simulations on double neutron star mergers. It is also consistent with the previous kilo-nova observations of GW170817.