AI 中文总结
本研究采用两种顶帽喷流模型拟合Swift/XRT余辉光变曲线，发现模型1拟合效果更优，多数GRB观测角接近喷流轴，离轴比不随宇宙纪元演化。
AI 中文摘要
伽马射线暴（GRB）是宇宙中最剧烈的现象之一，其余辉光变曲线包含喷流几何结构与观测角的信息。为约束GRB的观测角，我们分析了Swift X射线望远镜（XRT）余辉光变曲线中的喷流拐折特征，采用两种顶帽喷流模型：不含高纬度辐射的简化几何模型（模型1）和包含高纬度辐射的综合模型（模型2）。两种模型分别应用于20个星际介质（ISM）中的GRB样本与20个风介质中的GRB样本，这些样本的选择依据是喷流拐折由边缘效应导致且数据覆盖充足，采用马尔可夫链蒙特卡罗方法进行拟合。我们在两种密度分布下研究了观测角与离轴比q=θobs/θjet，评估了高纬度辐射的影响。基于约化卡方与贝叶斯信息准则的比较，模型1对所有GRB的拟合效果更优。大多数GRB具有较小的离轴比（模型1的平均q=0.1851），表明观测角通常接近喷流轴；对数空间的观测角分布近似为高斯分布。柯尔莫哥洛夫-斯米尔诺夫检验显示，ISM与风介质之间、存在X射线平台与不存在X射线平台的GRB之间，离轴比无显著差异。尽管观测角随红移显著减小，但离轴比未出现显著演化，这与离轴取向不依赖宇宙纪元的结论一致。
英文摘要
Gamma-ray bursts (GRBs) are among the most energetic phenomena in the universe, and their afterglow light curves encode information about jet geometry and viewing angle. To constrain GRB viewing angles, we analyzed jet break features in Swift X-Ray Telescope afterglow light curves using two top-hat jet models: a simplified geometric model without high-latitude emission (model 1) and a comprehensive model including it (model 2). Both models were applied to a sample of 20 GRBs in an interstellar medium (ISM) and 20 in a wind medium, selected so that jet breaks are attributed to the edge effect with sufficient data coverage, and fitted with Markov Chain Monte Carlo methods. We examined viewing angles and off-axis ratios q = $θ_{\rm obs}/θ_{\rm jet}$ under both density profiles, evaluating the impact of high-latitude emission. Based on reduced chi-squared and Bayesian information criterion comparisons, model 1 fits all GRBs better. Most GRBs have small off-axis ratios (mean q = 0.1851 for model 1), indicating viewing angles generally close to the jet axis; the log-space viewing-angle distribution is approximately Gaussian. A Kolmogorov-Smirnov test shows no significant difference in off-axis ratios between ISM and wind media, nor between bursts with and without an X-ray plateau. While viewing angles decrease significantly with redshift, the off-axis ratio shows no significant evolution, consistent with off-axis alignment being independent of cosmic epoch.
Comments22 pages, 14 figures, 3 tables. Accepted for publication in The Astrophysical Journal
