伽马射线暴早期光学余辉中喷流进动导致的退偏振
Depolarization by jet precession in early optical afterglows of gamma-ray bursts
AI总结：
本文通过计算反向激波主导阶段有、无喷流进动时的偏振度，发现喷流进动可导致退偏振，并可作为解释伽马射线暴早期光学余辉低偏振度的替代机制。
摘要：
偏振观测为探究伽马射线暴（GRBs）中喷流磁场的性质提供了一种独特途径。目前，一些伽马射线暴已被探测到在其早期光学余辉中呈现偏振。然而，这些伽马射线暴的实测偏振度（PDs）远低于理论模型预测值。在本工作中，我们结合伽马射线暴早期光学余辉在反向激波（RS）主导阶段（约10^2-10^3秒）的实测偏振度，研究喷流进动引起的退偏振。我们在两种磁场位形（即有序磁场和环形磁场）下，分别计算了有喷流进动和无喷流进动时反向激波辐射的偏振度，同时探讨了不同参数对偏振度的影响。我们发现，偏振度受有序磁场位形的影响较小，且与进动周期呈正相关。此外，偏振度对观测角十分敏感，实测的低偏振度倾向于支持较小的观测角。因此，作为结构化喷流的可能起源之一，喷流进动可被视为解释伽马射线暴早期光学余辉中观测到的低偏振度的一种替代机制。
英文摘要：
Polarization observations provide a unique way to probe the nature of jet magnetic fields in gamma-ray bursts (GRBs). Currently, some GRBs have been detected to be polarized in their early optical afterglows. However, the measured polarization degrees (PDs) of these GRBs are much lower than those predicted by theoretical models. In this work, we investigate the depolarization induced by jet precession in combination with the measured PDs of the GRB early optical afterglows in the reverse shock (RS) dominated phase ($\sim 10^2-10^3 \,{\rm s}$). We calculate the PDs of RS emission with and without jet precession in both magnetic field configurations, i.e., aligned and toroidal magnetic fields, and meanwhile explore the effect of different parameters on the PDs. We find that the PDs are slightly affected by the configurations of the ordered magnetic fields and are positively related to the precession period. Moreover, the PDs are sensitive to the observed angle and the measured low PDs favor a small one. Thus, as one of the plausible origins of the structured jets, jet precession could be considered as an alternative mechanism for the low PDs observed in GRB early optical afterglows.
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