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

TL;DR

This paper demonstrates that r-process heating in neutron star merger ejecta can halt fallback accretion, potentially explaining long-lasting gamma-ray burst emissions and providing a new way to probe nucleosynthesis processes.

Contribution

The study introduces a semi-analytical model and hydrodynamic simulations showing fallback accretion is halted by r-process heating, affecting gamma-ray burst afterglow modeling.

Findings

Fallback rate is halted by r-process heating.

Halting timescale ranges from 10^5 to 10^9 seconds.

Future observations can constrain r-process heating effects.

Abstract

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.