2019MNRAS.485.4404D

Mergers of compact binaries containing two neutron stars (NS-NS), or a neutron star and a stellar mass black hole (NS-BH), are likely progenitors of short-duration gamma-ray bursts (SGRBs). A fraction ≳20 per cent of SGRBs is followed by temporally extended (>=minute-long), variable X-ray emission, attributed to ongoing activity of the central engine. One source of late-time engine activity is fall-back accretion of bound tidal ejecta; however, observed extended emission light curves do not track the naively anticipated, uninterrupted t^–5/3 power-law decay, instead showing a lull or gap in emission typically lasting tens of seconds after the burst. Here, we re-examine the impact of heating due to rapid neutron capture (r-process) nucleosynthesis on the rate of the fall-back accretion, using ejecta properties extracted from numerical relativity simulations of NS-BH mergers. Heating by the r-process has its greatest impact on marginally bound matter, hence its relevance to late-time fall-back. Depending on the electron fraction of the ejecta and the mass of the remnant black hole, r-process heating can imprint a range of fall-back behaviour, ranging from temporal gaps of up to tens of seconds to complete late-time cut-off in the accretion rate. This behaviour is robust to realistic variations in the nuclear heating experienced by different parts of the ejecta. Central black holes with masses ≲3 M_☉ typically experience absolute cut-offs in the fall-back rate, while more massive ≳6-8 M_☉ black holes instead show temporal gaps. We thus propose that SGRBs showing extended X-ray emission arise from NS-BH, rather than NS-NS, mergers. Our model implies an NS-BH merger detection rate by LIGO that, in steady state, is comparable to or greater than that of NS-NS mergers.