Late-time accretion in neutron star mergers: Implications for short gamma-ray bursts and kilonovae

Abstract

ABSTRACT We study the long-term ($t\gg 10\rm \, s\!$ ) evolution of the accretion disc after a neutron star(NS)–NS or NS–black hole merger, taking into account the radioactive heating by r-process nuclei formed in the first few seconds. We find that the cumulative heating eventually exceeds the disc’s binding energy at $t\sim 10^2\mathrm{\, s}\, (\alpha /0.1)^{-1.8}(M/2.6\, \mathrm{M}_{\odot })^{1.8}$ after the merger, where α is the viscosity parameter and M is the mass of the remnant object. This causes the disc to evaporate rapidly and the jet power to shut off. We propose that this corresponds to the steep flux decline at the end of the extended emission (EE) or X-ray plateau seen in many short gamma-ray bursts (GRBs). The shallow flux evolution before the steep decline is consistent with a plausible scenario where the jet power scales linearly with the disc mass. We suggest that the jets from NS mergers have two components – a short-duration narrow one producing the prompt gamma-rays and a long-lasting wide component producing the EE. This leads to a prediction that ‘orphan EE’ (without short GRB) may be a promising electromagnetic counterpart for NS mergers observable by future wide-field X-ray surveys. The long-lived disc produces a slow ejecta component that can efficiently thermalize the β-electrons’ energy up to $t\sim 100\rm \, d$ and contributes $\sim \!10~{{\ \rm per\ cent}}$ of the kilonova’s bolometric luminosity at these late epochs. We predict that future ground-based and JWST near-IR spectroscopy of nearby ($\lesssim 100\rm \, Mpc\!$ ) NS mergers will detect narrow (Δv ∼ 0.01c) line features a few weeks after the merger.