Systematic Study of Forward and Reverse Shock Afterglow Emission from Two-Component Jets

Abstract

Two-component jets are frequently invoked to explain complex features in gamma-ray burst (GRB) afterglows, such as late-time rebrightening and chromatic breaks. While many studies fit these models to individual events, a systematic exploration mapping the broader parameter space, particularly the reverse shock contribution, is currently lacking. To address this, we present a comprehensive systematic analysis of two-component jet signatures using numerical modeling with the VegasAfterglow code. Our modeling shows that observable rebrightenings in the forward shock require the wing to carry substantially more energy, while for the reverse shock the energies can be comparable. Because the two components can occupy different spectral regimes, spectral breaks may arise when the wing emission overtakes the core. When the wing's initial velocity is high, relativistic beaming can render its emission invisible to the on-axis observer. As the flow decelerates, the resulting debeaming produces a steeper rise in the observed emission, reaching temporal slopes as steep as about $4.5$ and peaking shortly after the core jet break. In this case, the wing masks the core's break, leaving only a single late-time break. Slower wings that are not initially beamed away do not obscure the core, allowing the observer to see two distinct jet breaks. At late times, the decaying post-jet-break slopes are unaffected and limited to temporal slopes of about $-p$. Additionally, the forward shock dominates the emission across most of the parameter space, while the reverse shock contributes noticeably only under conditions of high magnetization and long engine durations.