Recently, ground-based Imaging Atmospheric Cherenkov Telescopes have reported the detection of very-high-energy (VHE) gamma-rays from some gamma-ray bursts (GRBs). One of them, GRB 190829A, was triggered by the Swift satellite, and about 2 × 104 s after the burst onset the VHE gamma-ray emission was detected by H.E.S.S. with ∼5σ significance. This event had unusual features of having much smaller isotropic equivalent gamma-ray energy than typical long GRBs and achromatic peaks in X-ray and optical afterglow at about 1.4 × 103 s. Here, we propose an off-axis jet scenario that explains these observational results. In this model, the relativistic beaming effect is responsible for the apparently small isotropic gamma-ray energy and spectral peak energy. Using a jetted afterglow model, we find that the narrow jet, which has the initial Lorentz factor of 350 and the initial jet opening half-angle of 0.015 rad, viewed off-axis can describe the observed achromatic behaviour in the X-ray and optical afterglow. Another wide, baryon-loaded jet is necessary for the later-epoch X-ray and radio emissions. According to our model, the VHE gamma rays observed by H.E.S.S. at 2 × 104 s may come from the narrow jet through the synchrotron self-Compton process.

中文翻译：

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低光度伽马暴GRB 190829A早期余辉发射的离轴喷射情景

最近，地面成像大气切伦科夫望远镜报告了从一些伽马射线暴 (GRB) 中检测到超高能 (VHE) 伽马射线。其中之一，GRB 190829A，由 Swift 卫星触发，在爆发开始后约 2 × 104 秒，HESS 检测到 VHE 伽马射线发射，显着性约为 5σ。该事件具有不同寻常的特征，即各向同性等效伽马射线能量比典型的长伽马射线小得多，X 射线中的消色差峰和大约 1.4 × 103 s 的光学余辉。在这里，我们提出了一个解释这些观测结果的离轴喷射场景。在这个模型中，相对论射束效应是造成明显小的各向同性伽马射线能量和光谱峰值能量的原因。使用喷射余辉模型，我们发现窄喷射，其初始洛伦兹因子为 350，初始射流开口半角为 0.015 弧度，离轴观察可以描述在 X 射线和光学余辉中观察到的消色差行为。对于后期的 X 射线和无线电发射，另一个宽的、装载重子的喷流是必要的。根据我们的模型，HESS 在 2 × 104 s 观测到的 VHE 伽马射线可能来自窄射流，通过同步加速器自康普顿过程。