Gal Birenbaum, Ramandeep Gill, Omer Bromberg, Paz Beniamini and Jonathan Granot
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引用次数: 0
Abstract
Gamma-ray bursts (GRBs) are powered by ultrarelativistic jets. The launching sites of these jets are surrounded by dense media, which the jets must cross before they can accelerate and release high-energy emission. Interaction with the medium leads to the formation of a mildly relativistic sheath around the jet, resulting in an angular structure to the jet’s asymptotic Lorentz factor and energy per solid angle, which modifies the afterglow emission. We build a semi-analytical tool to analyze the afterglow light curve and polarization signatures of jets observed from a wide range of viewing angles, and focus on ones with slowly declining energy profiles known as shallow jets. We find overall lower polarization compared to the classical top hat jet model. We provide an analytical expression for the peak polarization degree as a function of the energy profile power-law index, magnetic field configuration, and viewing angle, and show that it occurs near the light-curve break time for all viewers. When applying our tool to GRB 221009A, suspected to originate from a shallow jet, we find that the suggested jet structures for this event agree with the upper limits placed on the afterglow polarization in the optical and X-ray bands. We also find that at early times the polarization levels may be significantly higher, allowing for a potential distinction between different jet structure models and possibly constraining the magnetization in both forward and reverse shocks at that stage.
伽马射线暴(GRB)由超相对论射流驱动。这些喷流的发射地点周围都是致密介质,喷流必须穿过这些介质才能加速并释放出高能量。与介质的相互作用导致在喷流周围形成轻度相对论鞘,从而使喷流的渐近洛伦兹系数和每固角能量的角度结构发生变化,从而改变了余辉发射。我们建立了一个半分析工具,用于分析从各种视角观测到的喷流的余辉光曲线和偏振特征,并重点关注能量曲线缓慢下降的喷流,即浅喷流。与经典的 "顶帽 "喷流模型相比,我们发现其偏振程度总体较低。我们提供了峰值偏振度与能量剖面幂律指数、磁场配置和观测角度的函数关系的分析表达式,并表明它出现在所有观测者的光曲线断裂时间附近。当把我们的工具应用于怀疑源自浅喷流的 GRB 221009A 时,我们发现该事件的喷流结构与光学和 X 射线波段中余辉偏振的上限一致。我们还发现,在早期,极化水平可能要高得多,从而有可能区分出不同的喷流结构模型,并有可能在这一阶段对正向和反向冲击中的磁化进行约束。