Early afterglows from radially structured outflows and the application to X-ray shallow decays

TL;DR

This paper extends the standard GRB afterglow model to include radially nonuniform ejecta, demonstrating how energy injection from slower materials influences afterglow decay and explains early optical and X-ray plateau features.

Contribution

It introduces a modified reverse-forward shock model accounting for radial ejecta structure, classifying cases into thick and thin shells, and explains plateau phenomena in early afterglows.

Findings

In thick shell cases, reverse shock emission dominates high-energy afterglows.

Energy injection from low Lorentz factor ejecta causes slower afterglow decay.

Model explains early optical and X-ray plateau features in GRB afterglows.

Abstract

In the fireball model, it is more physically realistic that gamma-ray burst (GRB) ejecta have a range of bulk Lorentz factors (assuming $M\propto \Gamma^{-s}$). The low Lorentz factor part of the ejecta will catch up with the high Lorentz factor part when the latter is decelerated by the surrounding medium to a comparable Lorentz factor. Such a process will develop a long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile, the forward shocked materials are gradually supplied with energy from the ejecta that are catching-up, and thus the temporal decay of the forward shock emission will be slower than that without an energy supply. However, the reverse shock may be strong. Here, we extend the standard reverse-forward shock model to the case of radially nonuniform ejecta. We show that this process can be classified into two cases: the thick shell case and the thin shell case. In the thin shell case, the reverse shock is weak and the temporal scaling law of the afterglow is the same as that in Sari & Mesz (2000). However, in the thick shell case, the reverse shock is strong and thus its emission dominates the afterglow in the high energy band. Our results also show slower decaying behavior of the afterglow due to the energy supply by low Lorentz factor materials, which may help the understanding of the plateau observed in the early optical and X-ray afterglows.