Ejecta Width and Magnetization Reflected in Gamma-Ray Burst Early Afterglows: Implication for Reverse Shock Component and Shallow Decay Phase

TL;DR

This paper uses relativistic magneto-hydrodynamic simulations to explore how ejecta magnetization and width influence gamma-ray burst afterglows, revealing their roles in reverse shock behavior and early emission features.

Contribution

It provides new semi-analytical models linking ejecta properties to afterglow light curves, especially the shallow decay phase and early TeV emission.

Findings

Magnetization and ejecta width significantly affect reverse shock light curves.

The transition phase can explain the shallow decay observed in GRB afterglows.

Inverse Compton emission during magnetic acceleration may cause steep early TeV emission.

Abstract

To study the ejecta property dependence of the gamma-ray burst (GRB) afterglow, we carry out spherically symmetrical one-dimensional special relativistic magneto-hydrodynamic simulations of magnetized outflows with an adaptive mesh refinement method. The Lorentz factor evolutions of forward and reverse shocks induced by the interaction between magnetized ejecta and an ambient medium are investigated for a wide range of magnetization and width of the ejecta. The forward shock evolution is described by the magnetic acceleration, coasting, transition, and self-similar deceleration phases. According to our simulation results, we numerically calculate the corresponding radiation. Based on our numerical results, to model afterglow light curves in general cases, we construct semi-analytical formulae for the Lorentz factor evolutions. The magnetization and ejecta width dependence are clearly seen in the reverse shock light curves. The transition phase with a reasonable ejecta width can reproduce the shallow decay phase in the observed GRB afterglow. The inverse Compton emission in the magnetic acceleration phase can be responsible for the very steep rise of the early TeV emission in GRB221009A.