Fallback Accretion Halted by R-process Heating in Neutron Star Mergers and Gamma-Ray Bursts

The gravitational wave event GW170817 with a macronova/kilonova shows that a merger of two neutron stars ejects matter with radioactivity including \(r\)-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB), such as extended and plateau emissions. We investigate the fallback accretion with the \(r\)-process heating by performing one-dimensional hydrodynamic simulations and developing a semi-analytical model. We show that the usual fallback rate \(dM/dt \propto t^{-5/3}\) is halted by the heating because pressure gradients accelerate ejecta beyond an escape velocity. The suppression is steeper than Chevalier's power-law model through Bondi accretion within a turn-around radius. The characteristic halting timescale is \(\sim 10^5\)--\(10^9\) sec for the GW170817-like \(r\)-process heating, which is long enough to continue the long-lasting emission of sGRBs. The halting timescale is sensitive to the uncertainty of the \(r\)-process. Future observation of fallback halting could constrain the \(r\)-process heating on the year scale.