Late-time post-merger modeling of a compact binary: effects of relativity, r-process heating, and treatment of transport

Abstract

Detectable electromagnetic counterparts to gravitational waves from compact binary mergers can be produced by outflows from the black hole-accretion disk remnant during the first ten seconds after the merger. Two-dimensional axisymmetric simulations with effective viscosity remain an efficient and informative way to model this late-time post-merger evolution. In addition to the inherent approximations of axisymmetry and modeling turbulent angular momentum transport by a viscosity, previous simulations often make other simplifications related to the treatment of the equation of state and turbulent transport effects.

In this paper, we test the effect of these modeling choices. By evolving with the same viscosity the exact post-merger initial configuration previously evolved in Newtonian viscous hydrodynamics, we find that the Newtonian treatment provides a good estimate of the disk ejecta mass but underestimates the outflow velocity. We find that the inclusion of heavy nuclei causes a notable increase in ejecta mass. An approximate inclusion of r-process effects has a comparatively smaller effect, except for its designed effect on the composition. Diffusion of composition and entropy, modeling turbulent transport effects, has the overall effect of reducing ejecta mass and giving it a speed with lower average and more tightly-peaked distribution. Also, we find significant acceleration of outflow even at distances beyond 10,000 km, so that thermal wind velocities only asymptote beyond this radius and at somewhat higher values than previously reported.

Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole–Neutron Star Mergers

“…We find no evidence for late fallback accretion, as all models exhibit a smooth power-law decline in the mass accretion rate (Figure 1(a)). However, in the absence of neutrino cooling, heavy nuclei, and radioactive heating (e.g., Desai et al 2019;Haddadi et al 2023), we cannot preclude a definitive conclusion regarding late-time fallback of the tail. In a recent study, Metzger & Fernández (2021) found that the marginally bound tail progressively falls back into the disk, but it quickly becomes unbound due to its inability to efficiently cool down.…”

“…However, this necessarily requires the jet launching to be longer than a typical sGRB duration of 1 s. We stress that the exclusion of the alpha recombination effect in our simulations is unlikely to impact this conflict. The reason is that these factors only start to influence the mass accretion rate after the neutrino luminosity decreases at t ∼ 0.5 s, shifting the accretion rate from  ~-M t 2 to t 3 (Haddadi et al 2023). As a result, their impact on the jet luminosity is negligible within the typical duration of sGRBs, t  1 s.…”

Emergent Nucleosynthesis from a 1.2 s Long Simulation of a Black Hole Accretion Disk

“…MHD disk models typically drive an early, fast outflow powered by heat and magnetic forces (Siegel & Metzger 2017;Christie et al 2019) and a late, slow outflow powered by turbulent viscosity (Shakura & Sunyaev 1973). The latter outflow is enhanced by nuclear recombination incorporated into the nuclear statistical equilibrium finite temperature equation of state (Fernández et al 2019;Fahlman & Fernández 2022;Just et al 2022;Haddadi et al 2023). Our disk is no exception, and the more massive late-time outflow is from the slower viscous mechanism.…”

A Unified Picture of Short and Long Gamma-Ray Bursts from Compact Binary Mergers

“…If the disrupted NS has a purely toroidal field configuration, Φ is not constant but slowly increases due to the dynamo process.11 The postmerger disk mass is negligible compared to the BH mass, so no appreciable spin-down is expected in binary mergers.12 More massive BHs generally only lead to more compact disks (e.g.,Fernández et al 2020), making this result robust.13 Energy injection from alpha-particle recombination can also act to steepen the mass accretion power law after neutrino cooling is no longer important at t  1 s(Metzger et al 2008a;Haddadi et al 2023). …”