# The r-process nucleosynthesis in the outflows from short GRB accretion disks

## TL;DR

This study uses relativistic magneto-hydrodynamic simulations to analyze the formation, composition, and nucleosynthesis of outflows from accretion disks around black holes in short gamma-ray bursts, explaining kilonova observations.

## Contribution

It provides new insights into the properties of outflows from post-merger accretion disks, including their composition, velocity, and contribution to kilonova lightcurves, using realistic nuclear equations of state.

## Key findings

- Fast wind outflows with velocities 0.11-0.23c are launched.
- Outflows have electron fractions Y_e of 0.1-0.4, explaining kilonova components.
- Mass loss from the disk ranges from 2% to 17% of initial disk mass.

## Abstract

Short gamma-ray bursts require a rotating black hole, surrounded by a magnetized relativistic accretion disk, such as the one formed by coalescing binary neutron stars or neutron star - black hole systems. The accretion onto a Kerr black hole is the mechanism of launching a baryon-free relativistic jet. An additional uncollimated outflow, consisting of sub-relativistic neutron-rich material which becomes unbound by thermal, magnetic and viscous forces, is responsible for blue and red kilonova. We explore the formation, composition and geometry of the secondary outflow by means of simulating accretion disks with relativistic magneto-hydrodynamics and employing realistic nuclear equation of state. We calculate the nucleosynthetic r-process yields by sampling the outflow with a dense set of tracer particles. Nuclear heating from the residual r-process radioactivities in the freshly synthesized nuclei is expected to power a red kilonova, contributing independently from the dynamical ejecta component, launched at the time of merger, and neutron-poor broad polar outflow, launched from the surface of the hypermassive neutron star by neutrino wind. Our simulations show that both magnetisation of the disk and high black hole spin are able to launch fast wind outflows ($v/c\sim 0.11-0.23$) with a broad range of electron fraction $Y_{\rm e}\sim 0.1-0.4$, and help explain the multiple components observed in the kilonova lightcurves. The total mass loss from the post-merger disk via unbound outflows is between 2\% and 17\% of the initial disk mass.