[论文解读] A deep spectromorphological study of the $\gamma$-ray emission surrounding the young massive stellar cluster Westerlund 1

F. Aharonian, H. Ashkar+157 more|arXiv (Cornell University)|Jan 1, 2022

Stellar, planetary, and galactic studies被引用 3

一句话总结

本研究基于164小时的H.E.S.S.数据，对Westerlund 1周围扩展的TeV γ射线发射进行了深度光谱形态学分析，揭示了一种大尺度壳状结构，其能谱在数十TeV能量范围内保持均匀。该发射最可能由强子宇宙射线相互作用解释，其中Westerlund 1本身被确定为主要源，可能通过星团风终端激波处的粒子加速实现，尽管具体机制尚未明确。

ABSTRACT

Context. Young massive stellar clusters are extreme environments and potentially provide the means for efficient particle acceleration. Indeed, they are increasingly considered as being responsible for a significant fraction of cosmic rays (CRs) that are accelerated within the Milky Way. Westerlund 1, the most massive known young stellar cluster in our Galaxy, is a prime candidate for studying this hypothesis. While the very-high-energy $\gamma$-ray source HESS J1646−458 has been detected in the vicinity of Westerlund 1 in the past, its association could not be firmly identified.Aims. We aim to identify the physical processes responsible for the γ-ray emission around Westerlund 1 and thus to understand the role of massive stellar clusters in the acceleration of Galactic CRs better.Methods. Using 164 h of data recorded with the High Energy Stereoscopic System (H.E.S.S.), we carried out a deep spectromorphological study of the $\gamma$-ray emission of HESS J1646−458. We furthermore employed H I and CO observations of the region to infer the presence of gas that could serve as target material for interactions of accelerated CRs.Results. We detected large-scale (∼2° diameter) $\gamma$-ray emission with a complex morphology, exhibiting a shell-like structure and showing no significant variation with $\gamma$-ray energy. The combined energy spectrum of the emission extends to several tens of TeV, and it is uniform across the entire source region. We did not find a clear correlation of the $\gamma$-ray emission with gas clouds as identified through H I and CO observations.Conclusions. We conclude that, of the known objects within the region, only Westerlund 1 can explain the majority of the $\gamma$-ray emission. Several CR acceleration sites and mechanisms are conceivable and discussed in detail. While it seems clear that Westerlund 1 acts as a powerful particle accelerator, no firm conclusions on the contribution of massive stellar clusters to the flux of Galactic CRs in general can be drawn at this point.

研究动机与目标

• 确定Westerlund 1附近扩展TeV γ射线发射的物理起源，该星团为年轻大质量恒星簇。
• 确定发射是否源于强子过程或轻子过程，特别是宇宙射线与周围气体的相互作用。
• 评估大质量恒星簇在加速银河系宇宙射线中的作用。
• 评估已知致密源和气体结构对观测到的γ射线形态的贡献。
• 约束可能的加速机制，包括恒星风或超新星的激波特异性。

提出的方法

• 对覆盖164小时观测的H.E.S.S.数据执行深度光谱形态学分析。
• 在多个能量能段绘制γ射线发射形态，以检测能量依赖的结构特征。
• 结合γ射线数据与H I和CO谱线观测，追踪周围气体分布。
• 构建发射区域的联合能量谱，以评估谱形和截止特征。
• 通过比较γ射线发射与气体云的空间相关性，检验强子相互作用模型。
• 评估竞争性情景：轻子（逆康普顿）与强子（π介子衰变）起源，以及各种激波特异性加速位置。

实验结果

研究问题

• RQ1 围绕Westerlund 1的大尺度壳状TeV γ射线发射的物理起源是什么？
• RQ2 γ射线发射是否与H I和CO观测所追踪的星际气体相关，如强子情景所预期？
• RQ3 已知的致密源（如脉冲星）能否解释观测到的γ射线发射的大部分？
• RQ4 哪种粒子加速机制——风-风、超新星-风或星团风终端激波——最符合观测到的形态和谱形？
• RQ5 Westerlund 1在多大程度上可被视为银河系宇宙射线的代表性加速器？

主要发现

• γ射线发射在约2°的角直径范围内延伸，呈现出明显的壳状形态。
• 能量谱在源区域保持均匀，并延伸至数十TeV，未见显著的能量依赖性谱软化。
• γ射线发射与H I或CO气体云之间未发现显著空间相关性，对简单强子相互作用模型构成挑战。
• 在该区域所有已知天体中，仅Westerlund 1能够解释γ射线发射的主体部分。
• 缺乏能量依赖性形态特征，不利于持续注入的轻子逆康普顿情景。
• 星团风终端激波被确定为最可能的加速位置，可解释壳状结构和持续发射。