Chemical Evolution and Kilonova Implications of Post-Merger Accretion Disk Winds

TL;DR

This study addresses how post-merger accretion-disk winds from compact binary mergers produce kilonova emission via $r$-process nucleosynthesis. It combines time-dependent GR MHD simulations with a three-parameter EOS and a neutrino leakage scheme, tracer particles, and SkyNet nuclear networks to map ejecta composition and heavy-element yields. The results show substantial neutron-rich winds in SANE disks, producing diverse abundance patterns with Lanthanides and Actinides in some scenarios, and reveal how disk mass, spin, and magnetization shape kilonova outcomes and jet viability. The work links central-engine physics to observable kilonova signals and Galactic chemical evolution, with implications for events like GRB 211211A and for long-duration GRBs from eccentric mergers.

Abstract

Several gamma ray bursts have recently been associated with a kilonova emission. We study the mechanisms which could account for this effect, by means of radioactive decay of elements synthesized in accretion disk wind. We model the r-process nucleosynthesis in the accretion disk wind system, asscociated with the prompt GRB phase. We compute the time-dependent GR MHD evolution of a GRB central engine where the newly formed black hole is accreting the mass from post-merger remnant. We explore the wind properties, for a range of the initial parameters of the system, and study representative cases for compact binary merger progenitors. We compute a suite of 2D and 3D time-dependent General Relativistic numerical simulations with a tabulated 3-parameter equation of state that allows for evolution of chemical composition evolution of the accretion flow. The neutrino emission is accounted for by incorporating the leakage scheme, where neutrino optical depth is calculated along the radial rays. We parameterize the optically thick and thin tori with different values of the pressure maximum and entropy in the disk, while the strength of large-scale poloidal magnetic fields is parameterized according to the chosen gas-to-magnetic pressure ratio. To probe the winds, we follow the particle trajectories. Upon this, we derive the nucleosynthetic yields of heavy elements in the outflows, and we map the regions of Lanthanide rich and poor ejecta. We find that the outflow carries high mass of neutron rich material expanding with mildly relativistic velocities. Our accretion disks operating under the SANE mode can power the GRB jets via neutrino annihilation, if the disk to BH mass ratio is larger than about 0.01 and the black hole is spinning. Slowly spinning black holes surrounded by massive post-merger disks can power these jets, and also be the sites of efficient nucleosynthesis of Lanthanides.

Figures (16)

• Figure 1: Snapshots from the 2D simulation of the GRB central engine from model 2D-Thin. The top panel shows the density and magnetic field streamlines distribution in the innermost part of the accretion disk. The bottom panel shows its chemical composition parameter - the electron fraction. Snapshots are taken at time $t=18000 ~t_{g}$
• Figure 2: Snapshots from the same 2D simulation of the short GRB central engine, as shown in Fig.\ref{['fig:GRrun']}. Here, the top panel shows the neutrino cooling rate in the innermost part of the accretion disk. The bottom panel shows the color map of neutrino optical depth.
• Figure 3: Neutrino lightcurves for the 2D models, optically thin and thick disk Plots show 2D-Thin-s and 2D-Thin models, with thin dot-dashed and dashed lines, respectively, and 2D-Thick-s and 2D-Thick models, with thicker dotted and solid lines, respectively. Three colors, blue, green, and magenta, represent luminosities in electron neutrinos, electron anti-neutrinos, and heavy lepton neutrinos, respectively.
• Figure 4: Mass accretion rates for Thin and Thick models. The 2D-Thin-s, 2D-Thin and 3D-Thin models are shown in red, with dotted, dashed and solid lines, respectively. The 2D-Thick-s, 2D-Thick and 3D-Thin models are shown in blue, with dotted, dashed and solid lines, respectively.
• Figure 5: Distributions of density $\rho$ (code units) with superimposed velocity field, and magnetisation, $\sigma=B^{2}/\rho$ and magnetic field vector fields, at time t=18000 $t_{g}$ for the 3D 'kilonova' model 3D-kn.