Relativistic MHD simulations of merging and collapsing stars and effects on GRB transient

TL;DR

This work uses state-of-the-art general relativistic magnetohydrodynamic simulations to study how mergers and collapsars produce relativistic jets and how surrounding environments—disk winds, dynamical ejecta, and self-gravity—affect jet launching, collimation, and prompt emission. The authors quantify jet energetics, structure, and opening angles, showing strong dependencies on black hole spin and disk mass, and reveal substantial $r$-process nucleosynthesis in disk winds with a red kilonova signature. They demonstrate that jet-disk wind interactions can significantly reshape collimation in NS-NS and BH-NS mergers, while self-gravity can suppress jet propagation and potentially yield observational signatures like quiescent or plateau features in the prompt phase. Overall, the results inform multimessenger GRB modeling by linking engine physics to observable high-energy and kilonova signatures using comprehensive GRMHD frameworks.

Abstract

Compact binary mergers and the collapse of massive stars can produce intense transients observable across high-energy wavelengths. Events such as gamma-ray bursts and kilonova emissions are often accompanied by gravitational wave detections, making them crucial sources for multimessenger astrophysics. To explore these phenomena theoretically, state-of-the-art approaches of General Relativistic magnetohydrodynamic simulations are used. We present recent findings from our simulations, and discuss observational consequences of the stellar/post-merger environment on the gamma ray burst prompt emission properties.

Figures (5)

• Figure 1: Jet structure of an evolved 3D model assuming presence of Dynamical Ejecta. The parameter represented by color scale is jet magnetization $\sigma$, and the bounding box is $500 r_{g}$
• Figure 2: Histograms of electron fraction distribution versus velocity in the outflows.
• Figure 3: Abundance pattern from r-process nucleosynthesis, averaged over tracers following disk wind outflows. The model 3D-kn represents an engine composed of a 8.2 $M_{\odot}$ black hole, mildly rotating with spin $a=0.8$ and surrounded by a disk with initial mass of $0.3 M_{\odot}$. Solid and dashed lines show results from nuclear reaction network runs where the onset of r-process after the Nuclear Statistical Equilibrium was either at $T=10 GK$ or at $T=5 GK$.
• Figure 4: Schematic cartoon of a post-merger (NSNS or BHNS) system. The ejecta are composed of the dynamical ejecta (DE) launched before the merger or soon after the HMNS was formed, and the disk wind, formed after accreting black hole starts powering the GRB central engine. The DE undergo initial expansion into the circum-burst environment where the jet and disk wind are propagating. In this system, the DE, disk wind, and jet are interacting with each other.
• Figure 5: Jet opening angle (left) and terminal Lorentz factor (right) for the simulations of magnetized collapsars. Self-gravitating model is represented by blue lines, and the non self-gravitating case is shown with red lines.