AI 中文总结
利用LAST高节奏观测，发现GRB 251013C光学余辉存在多尺度变异性，主再增亮归因于刷新外部激波，谱硬化源于更硬电子群，快速波动揭示喷流内部结构。
AI 中文摘要
背景。伽马射线暴（GRB）的光学余辉通常被描述为平滑的幂律衰减，但耀发和再增亮等偏差为潜在辐射过程提供了重要约束。目的。我们利用高节奏观测研究GRB 251013C光学余辉的时间变异性，并评估所观测变异性的起源。方法。我们分析了由大型巡天望远镜（LAST）获得的连续光学观测数据，并辅以Swift-XRT公开可用的X射线数据。光学光变曲线采用幂律衰减叠加变异性成分进行建模，同时利用同期光学和X射线测量来约束宽带谱。结果。光学光变曲线在多个时间尺度上显示出显著的变异性，包括宽再增亮事件（Δt ~ t）以及叠加的更快波动（Δt/t < 1）。主要再增亮呈现结构化上升，在峰值前有明显的陡化。快速变异性包含不对称的快升-慢降特征，其持续时间随峰值时间增加。光学通量位于X射线谱的外推线上，且谱在X射线波段内测量或从光学到X射线测量时硬化程度相同。结论。我们倾向于用刷新外部激波解释主要再增亮。宽带谱和峰后衰减与风状介质中的慢冷却同步辐射一致。谱硬化需要新占主导的、更硬的电子群，而更快的光学变异性表明刷新喷流物质或激波区域内存在结构。
英文摘要
Context. Optical afterglows of gamma-ray bursts (GRBs) are often described by smooth power-law decays, but deviations such as flares and rebrightenings provide important constraints on the underlying emission processes. Aims. We investigate the temporal variability of the optical afterglow of GRB 251013C using high-cadence observations, and assess the origin of the observed variability. Methods. We analyze continuous optical observations obtained with the Large Array Survey Telescope (LAST), complemented by publicly available X-ray data from Swift-XRT. The optical light curve is modeled using a power-law decay with superposed variability components, and contemporaneous optical and X-ray measurements are used to constrain the broadband spectrum. Results. The optical light curve shows pronounced variability on multiple timescales, including broad rebrightening episodes with $Δt \sim t$ and superposed faster fluctuations with $Δt/t < 1$. The main rebrightening exhibits a structured rise with a clear steepening prior to the peak. The fast variability comprises asymmetric fast-rise, slow-decay features whose durations increase with peak time. The optical flux lies on the extrapolation of the X-ray spectrum, and the spectrum hardens by the same amount whether measured within the X-ray band or from optical to X-ray. Conclusions. We favor a refreshed external-shock interpretation for the main rebrightening. The broadband spectrum and post-peak decay are consistent with slow-cooling synchrotron emission in a wind-like medium. The spectral hardening requires a newly dominant, harder electron population, while the faster optical variability indicates structure within the refreshed ejecta or shocked region.
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