The effects of r-process heating on fall-back accretion in compact object mergers

Abstract

We explore the effects of r-process nucleosynthesis on fall-back accretion in neutron star(NS)-NS and black hole-NS mergers, and the resulting implications for short-duration gamma-ray bursts (GRBs). Though dynamically important, the energy released during the r-process is not yet taken into account in merger simulations. We use a nuclear reaction network to calculate the heating (due to beta-decays and nuclear fission) experienced by material on the marginally-bound orbits nominally responsible for late-time fall-back. Since matter with longer orbital periods t_orb experiences lower densities, for longer periods of time, the total r-process heating rises rapidly with t_orb, such that material with t_orb > 1 seconds can become completely unbound. Thus, r-process heating fundamentally changes the canonical prediction of an uninterrupted power-law decline in the fall-back rate dM/dt at late times. When the timescale for r-process to complete is > 1 second, the heating produces a complete cut-off in fall-back accretion after ~ 1 second; if robust, this would imply that fall-back accretion cannot explain the late-time X-ray flaring observed following some short GRBs. However, for a narrow, but physically plausible, range of parameters, fall-back accretion can resume after ~ 10 s, despite having been strongly suppressed for ~ 1-10 s after the merger. This suggests the intriguing possibility that the gap observed between the prompt and extended emission in short GRBs is a manifestation of r-process heating.