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arXiv 2403.16403astro-ph.HE
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GRB瞬发辐射中的偏振角翻转

The polarization-angle flip in GRB prompt emission

Kangfa Cheng, Xiaohong Zhao, Jirong Mao, Zhifu Chen

摘要:

目的。我们的目标是提供一个解释的PA旋转伽玛暴和找到的物理条件,导致旋转90度的环形磁场(MF)模型。此外,我们提出了一些可观察到的极化性质的MF模型,可以在未来进行测试。结果我们发现,在环形MF的PA旋转主要是有关的三个关键因素:视角,射流开口角,和射流洛伦兹因子。此外,PA可以经历两次90度翻转。翻转的条件是$q \gtrsim 0.5$($q\simeq 1$除外)和$y_j =(Γθ_j)^2 \gtrsim 4$。然而,由于通量的限制,PA中的两个翻转可能无法同时观察到。考虑到这些条件,并假设喷流轴和视线(LOS)之间的随机取向,我们得到的理论上限(没有任何限制)的观察到的伽马射线暴在X射线或$γ$ -射线波段显示的翻转PA作为$R_{ch} \lesssim 80\%$。根据最大翻转后极化通量水平,我们进一步将观察到的速率约束为$R_{ch} \sim 16\%$,其中观察到的单次和双次翻转的速率分别占$\sim 8\%$。应该注意的是,在各个波段中,观察到的速率是不同的。在光学波段观察到的第二次PA翻转速率应该高于X射线或$γ$射线波段,因为光学波段的通量下降比X射线或$γ$射线波段慢得多。此外,当LOS接近喷流边缘($q\to 1$)时,由于翻转后的偏振通量水平相对较高,因此最容易观察到90度PA翻转。

英文摘要:

Aims. We aim to provide an explanation for the PA rotation in GRBs and find the physical conditions that lead to the rotation by 90 degrees in the toroidal magnetic-field (MF) model. Moreover, we present some observable polarization properties in the MF model that can be tested in the future. Results. We find that the PA rotation in the toroidal MF is primarily related to three critical factors: the viewing angle, the jet opening angle, and the jet Lorentz factor. Additionally, the PA can experience flips of 90 degrees twice. The conditions for the flips are $q \gtrsim 0.5$ (except for $q\simeq 1$) and $y_j =(Γθ_j)^2 \gtrsim 4$. However, the two flips in the PA might not be concurrently observable due to the constraint of flux. Taking these conditions into account and assuming a random orientation between the jet axis and the line of sight (LOS), we obtain a theoretical upper limit (without any constraints) for the observed rate of GRBs in the X-ray or $γ$-ray band displaying the flips in PA as $R_{ch} \lesssim 80\%$. We further constrain the observed rate as $R_{ch} \sim 16\%$ according to the maximal post-flip polarized flux level, where the observed rate of single and double flips each account for $\sim 8\%$. It should be noted that the observed rates are different in various wavebands. The observed rate of the second PA flip in the optical bands should be higher than that in the X-ray or $γ$-ray band since the flux in the optical band declines much slower than that in the X-ray or $γ$-ray band. Moreover, when the LOS is close to the jet edge ($q\to 1$), it is the easiest case in which to observe the 90-degree PA flip due to the relatively high post-flip polarized flux level.

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