Effects of magnetically driven shocks on nucleosynthesis and kilonovae from neutron star mergers

TL;DR

This study investigates how magnetically driven shocks from neutron star merger remnants influence ejecta composition, nucleosynthesis, and kilonova emission, revealing potential sources of observed kilonova diversity.

Contribution

The paper introduces a parametric study using relativistic MHD simulations to connect shock heating with nucleosynthesis and kilonova light curves, highlighting the impact of magnetic shocks.

Findings

Strong shocks can induce nuclear statistical equilibrium in ejecta.

Shock heating alters the electron fraction and entropy, affecting r-process yields.

Observable effects include changes in kilonova color evolution and late-time light curves.

Abstract

Neutron-star mergers can launch mildly relativistic to moderately relativistic outflows whose interaction with the ejecta can reshape kilonova emission. We parametrically study magnetically powered outbursts from long-lived merger remnants, such as flare-like eruptions and collapse-driven shocks, and quantify their impact on ejecta dynamics, composition, and observables. Using two-dimensional special-relativistic magnetohydrodynamic simulations, we follow magnetized blast waves injected into expanding merger ejecta for early- and late-launch scenarios across a range of shock strengths. We then post-process Lagrangian tracer histories with the nuclear reaction network WinNet and the radiative-transfer code SuperNu with realistic opacities, to connect shock heating directly to nucleosynthesis and kilonova light curves. We find that sufficiently strong shocks can reheat portions of the ejecta to nuclear statistical equilibrium, increase the electron fraction in the shocked material, and deposit entropy, leading to systematic changes in $r$-process yields. These thermodynamic and compositional changes can leave observable imprints on kilonova emission -- especially in color evolution and late-time light-curve behavior -- indicating that magnetically driven remnant variability can potentially contribute to kilonova diversity.