AI 中文总结
研究伽马射线暴余辉平台，通过高分辨率数值计算，用JET和Firefly代码验证分层抛射物结构可产生余辉平台，其斜率能约束抛射物分层，还给出平台持续时间与抛射物特征洛伦兹因子的标度关系。
AI 中文摘要
自斯威夫特发现伽马射线暴（GRB）余辉中的平台以来，主要通过后期能量注入对其进行建模。但许多研究表明，平台可由反向激波穿越前的早期阶段建模。早期平台斜率对最快移动抛射物的分层有约束。数值研究通常未将喷流建模为分层流出，因准确演化少量高相对论性物质需极高分辨率。本研究进行高分辨率数值计算，验证分层抛射物结构能在风（$k = 2$）和星际介质（$k = 0$）环境中产生余辉平台并明确计算断裂时间。用JET代码演化相对论流体动力学，用Firefly代码后处理计算余辉。结果表明分层抛射物结构足以解释GRB余辉，平台斜率可用于约束抛射物分层，还给出平台持续时间与抛射物特征洛伦兹因子的精确标度关系。典型平台的长时间需要抛射物有适度的特征洛伦兹因子（$\gamma_0 \sim 10 - 50$），与其他余辉平台模型一致。
英文摘要
Since the discovery of plateaus in GRB afterglows by Swift, they have been modeled predominantly by late-time energy injection. However, many studies have suggested that the plateau may be modeled by an early phase before reverse shock crossing (either coasting with constant Lorentz factor or decelerating very slowly as the reverse shock crosses the ejecta). The slope of the early plateau provides some constraints the stratification of the fastest-moving ejecta, which could be the ejecta responsible for the prompt emission. However, numerical studies typically do not model the jet as a stratified outflow; the reason being the extremely high resolution required in order to accurately evolve this tiny amount of highly relativistic material. In this study, we perform high-resolution numerical calculations ($Δr/r \lesssim 10^{-5}$) verifying that a stratified ejecta structure can indeed produce an afterglow plateau in both wind ($k=2$) and ISM ($k=0$) environments, and computing break times explicitly. We evolve the relativistic hydrodynamics using the JET code, and post-process this to compute the afterglow using the Firefly code. Our results show that a stratified ejecta structure (which should generically be present in GRB jets) is sufficient to explain GRB afterglows, and the plateau slopes can be used to constrain the ejecta stratification. We additionally provide precise measured scalings for the plateau duration as a function of the characteristic Lorentz factor of the ejecta. The long duration of typical plateaus requires a very modest characteristic Lorentz factor for the ejecta ($γ_0 \sim 10-50$), in agreement with other afterglow plateau models.
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