Exploring Jet Structure and Dynamics in Short Gamma Ray Bursts: A Case Study on GRB 090510

TL;DR

This work uses high-resolution GRMHD simulations to model the central-engine jet of GRB 090510, aiming to reproduce its energetics, jet opening angle, and variability. By comparing 2D and a limited 3D configuration, and by incorporating dynamical ejecta, the authors identify models MD-0.07-2D, DE1-0.07-2D, and DE2-0.07-2D that reproduce $E_{\rm GRB}$ and $\theta_j$ consistent with observations when assuming a radiative efficiency of $\eta\sim$10%. The simulations predict jet properties including $\theta_j$ in the range $\sim9$–$11^{\circ}$, $E_{\rm jet}\sim(1.1$–$1.6)\times10^{52}$ erg, and Lorentz-factor distributions with $\Gamma_{\infty}$ up to several hundred, plus variability timescales $\sim$4–5 ms that match wavelet-based MTS analyses. The results support disk winds as the dominant jet-collimating mechanism, with dynamical ejecta playing a secondary role within the modeled domain, and they demonstrate the potential to extend such modeling to a broader set of short GRBs. The study provides a framework for linking GRMHD jet dynamics to observed high-energy GRB properties and highlights the importance of 3D effects and extended ejecta domains for future work.

Abstract

Gamma-ray bursts observed in high-energies allow the investigation of the emission processes of these still puzzling events. In this study, we perform general relativistic magnetohydrodynamic (GRMHD) simulations to investigate GRB 090510, a peculiar short GRB detected by Fermi-LAT. Our primary goal is to model the energetics, jet structure, variability, and opening angle of the burst to understand its underlying physical conditions. We tested the 2D and 3D models and estimated the time scale of variability. The predicted energetics and the jet opening angle reconcile with the observed ones with 1$σ$ when considering that the jet opening angles also evolve with redshift. Furthermore, we extend our analysis by incorporating dynamical ejecta into selected models to study its impact on jet collimation at smaller distances. In addition, we investigated a suite of models exhibiting a broad range of observable GRB properties, thereby extending our understanding beyond this specific event.

Figures (8)

• Figure 1: Time evolution of mass accretion rate ($\Dot{M}$) in the upper panel and luminosity ($L_{BZ}$, lower panel) for the best models.
• Figure 2: Density distribution and jet structure at $t = 0.3$ seconds for three different models. Top row: Density plots of the accretion disk illustrating variations in the disk's evolution across the three models. Bottom row: $\mu$ and $\sigma$ parameters for each model, reflecting the jet energy distribution and magnetic field strength variations, respectively.
• Figure 3: Snapshots of disk density profile with magnetic field streamlines of model MD-0.07-2D at 0s (left panel), 0.25s (middle panel) and 0.60s (right panel) respectively.
• Figure 4: Time evolution of the jet opening angle ($\theta_{j}$) at 1000$R_{\rm g}$ for selected models. The opening angle and their standard deviation are computed at 0.05s intervals to track its temporal variation. For clarity, only a subset of the models is shown. The grey shaded region represents the range between the highest and lowest opening angle models, each extended by its respective standard deviation.
• Figure 5: Jet structure of 3D model in our sample, HD-0.10-3D. The parameter represented is jet magnetisation $\sigma$, and the bounding box is 500 $R_g$.