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arXiv 2408.07640astro-ph.SRastro-ph.HE
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通过利用uGMRT观测研究沃尔夫-拉叶星深入了解粒子加速的发生机制

Insight into the occurrence of particle acceleration through the investigation of Wolf-Rayet stars using uGMRT observations

A. B. Blanco, M. De Becker, A. Saha, A. Tej, P. Benaglia

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AI总结：

本研究利用uGMRT观测5颗沃尔夫-拉叶星，仅探测到WR 110的热射电辐射，其余4颗未检出同步辐射，认为强自由-自由吸收是射电波段识别粒子加速源的主要障碍。

摘要：

大质量恒星会产生强劲的恒星风，由速度达数千公里每秒的物质持续外流构成。这些恒星风携带着大量动能，沃尔夫-拉叶（Wolf-Rayet，WR）星的情况尤为突出。当这些恒星风与附近物质相互作用时，很可能产生激波。在诸多过程中，预计会发生粒子加速。这一点在大质量双星系统中已得到充分证实：这类系统中恒星风相互碰撞，得益于激波中加速的相对论粒子群产生的同步射电辐射被探测到，人们得以识别这类系统。我们的研究目标是探究大质量恒星在超新星前演化阶段中粒子加速的发生情况。为此，我们利用位于印度的升级巨型米波射电望远镜（upgraded Giant Metrewave Radio Telescope，uGMRT）在射电波段观测了5颗沃尔夫-拉叶星组成的子样本。所有目标源的观测均在4波段（550-950 MHz）和5波段（1050-1450 MHz）开展。我们仅在4和5波段探测到了WR 110的射电辐射。其热谱在低至uGMRT的波段上呈现出一致的+0.74的谱指数。其余4颗目标源未被探测到，我们推导出了它们的3σ上限。我们得到的4波段上限是这些目标源在1 GHz以下的首批上限结果。在这些射电波段中，没有任何目标源被认定为同步射电辐射源。如果这些系统中确实产生了部分同步辐射，那么uGMRT未探测到的情况最有可能归因于强自由-自由吸收（free-free absorption，FFA）。这一点对于WR 98a尤其值得关注——根据此前更高射电频率的测量结果，该星被归类为粒子加速源。我们讨论了自由-自由吸收的显著效应如何成为在射电波段识别粒子加速源的严重障碍。

英文摘要：

Massive stars produce strong stellar winds that consist of continuous outflows of material at speeds of thousands of km/s. These winds convey large amounts of kinetic power, especially in the case of Wolf-Rayet (WR) stars. When these winds interact with nearby material, they will likely produce shocks. Among other processes, particle acceleration is expected to occur. This is particularly well established in the case of massive binary systems, where the stellar winds collide, allowing these systems to be identified thanks to the detection of synchrotron radio emission, produced by a population of relativistic particles accelerated in the shocks. Our goal is to investigate the occurrence of particle acceleration among massive stars in their pre-supernova evolution phases. To this end, we observed a subset of five WR stars in the radio domain using the upgraded Giant Metrewave Radio Telescope (uGMRT), located in India. The observations were carried out in bands 4 (550-950 MHz) and 5 (1050-1450 MHz) for all the targets. We detected radio emission for only WR 110 in bands 4 and 5. Its thermal spectrum displays a consistent index of +0.74 down to uGMRT bands. The four other targets were not detected and we derived 3$σ$ upper limits. Our upper limits in Band 4 are the first provided for these targets below 1 GHz. None of the targets was identified as a synchrotron radio emitter in these radio bands. If some synchrotron emission is produced in these systems, the non-detection with uGMRT can be most likely attributed to strong free-free absorption (FFA). This is especially relevant for WR 98a, which is catalogued as a particle accelerator based on previous measurements at higher radio frequencies. We discuss how the prominence of FFA constitutes a severe obstacle to identifying particle accelerators in the radio domain.

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