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

Abstract

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.