The γ-ray emission from stars is induced by the interaction of cosmic rays with stellar atmospheres and photon fields. This emission is expected to come in two components: a stellar disc emission, where γ-rays are mainly produced in atmospheric showers generated by hadronic cosmic rays, and an extended halo emission, where the high density of soft photons in the surroundings of stars create a suitable environment for γ-ray production via inverse Compton (IC) scattering by cosmic ray electrons. Besides the Sun, no other disc or halo from single stars has ever been detected in γ-rays. However, by assuming a cosmic ray spectrum similar to that observed on Earth, the predicted γ-ray emission of superluminous stars, e.g. Betelgeuse and Rigel, could be high enough to be detected by the Fermi Large Area Telescope (LAT) after its first decade of operations. In this work, we use 12 yr of Fermi-LAT observations along with IC models to study nine superluminous nearby stars, both individually and via stacking analysis. Our results show no significant γ-ray emission, but allow us to restrict the stellar γ-ray fluxes to be on average <3.3 × 10−11 ph cm−2 s−1 at a 3σ confidence level, which translates to an average local density of electrons in the surroundings of our targets to be less than twice of that observed for the Solar system.

中文翻译：

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用费米大面积望远镜研究超亮恒星

来自恒星的 γ 射线发射是由宇宙射线与恒星大气和光子场的相互作用引起的。这种发射预计将分为两部分：恒星盘发射，其中 γ 射线主要在由强子宇宙射线产生的大气簇射中产生，以及扩展的光晕发射，其中恒星周围的高密度软光子产生通过宇宙射线电子的逆康普顿（IC）散射产生γ射线的合适环境。除了太阳之外，在 γ 射线中还没有发现任何来自单星的圆盘或光晕。然而，假设宇宙射线光谱类似于在地球上观察到的光谱，预测的超亮恒星的 γ 射线发射，例如参宿四和参宿七，费米大区域望远镜（LAT）在其第一个十年运行后可能会探测到。在这项工作中，我们使用 12 年的 Fermi-LAT 观测和 IC 模型来研究附近的九颗超亮恒星，无论是单独还是通过叠加分析。我们的结果显示没有显着的 γ 射线发射，但允许我们在 3σ 置信水平上将恒星 γ 射线通量限制在平均 <3.3 × 10−11 ph cm−2 s−1，这意味着平均我们的目标周围的局部电子密度不到太阳系观察到的电子密度的两倍。