Polarization of gamma-ray burst afterglows in the synchrotron self-Compton process from a highly relativistic jet
Polarization of gamma-ray burst afterglows in the synchrotron self-Compton process from a highly relativistic jet
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DOI：
10.1088/1674-1137/41/4/045101
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发表时间：
2016-12
期刊：
影响因子：
--
通讯作者：
Hai-Nan Lin;Xin Li;Z. Chang
中科院分区：
文献类型：
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作者：
Hai-Nan Lin;Xin Li;Z. Chang
在一些明亮的伽玛暴（GRB）的瞬发相和余辉中都观察到了线偏振。在瞬发阶段的极化跨越一个很宽的范围，并可能高达50%。然而，在余辉阶段，它通常低于10%。根据标准火球模型，伽玛暴是由中心发动机喷出的高度相对论喷流中的同步辐射和康普顿散射过程产生的。人们普遍认为，当不同速度的壳体相互碰撞时，内激波中会发生瞬发，并且中心发动机喷流平流产生的磁场可以在大尺度上有序化。另一方面，当喷流与星际介质碰撞时，通常假设余辉发生在外部激波中，并且激波通过例如韦伯不稳定性产生的磁场可能是随机的。本文计算了激波平面上磁场随机分布的高度相对论性喷流的同步自康普顿过程的偏振特性。我们还考虑了垂直于激波平面的均匀磁分量叠加在随机磁分量上的一般情况。我们发现，如果种子电子是各向同性的喷流框架中的极化是很难大于10%。这可以解释观测到的余辉阶段的伽玛暴的极化上限。此外，如果随机磁分量和均匀磁分量随时间以不同的速度衰减，则极化角在时间演化期间可能改变90°。
Linear polarization has been observed in both the prompt phase and afterglow of some bright gamma-ray bursts (GRBs). Polarization in the prompt phase spans a wide range, and may be as high as ≳ 50%. In the afterglow phase, however, it is usually below 10%. According to the standard fireball model, GRBs are produced by synchrotron radiation and Compton scattering process in a highly relativistic jet ejected from the central engine. It is widely accepted that prompt emissions occur in the internal shock when shells with different velocities collide with each other, and the magnetic field advected by the jet from the central engine can be ordered on a large scale. On the other hand, afterglows are often assumed to occur in the external shock when the jet collides with interstellar medium, and the magnetic field produced by the shock through, for example, Weibel instability, is possibly random. In this paper, we calculate the polarization properties of the synchrotron self-Compton process from a highly relativistic jet, in which the magnetic field is randomly distributed in the shock plane. We also consider the generalized situation where a uniform magnetic component perpendicular to the shock plane is superposed on the random magnetic component. We show that it is difficult for the polarization to be larger than 10% if the seed electrons are isotropic in the jet frame. This may account for the observed upper limit of polarization in the afterglow phase of GRBs. In addition, if the random and uniform magnetic components decay with time at different speeds, then the polarization angle may change 90° during the temporal evolution.