The Spatially Uniform Spectrum of the Fermi Bubbles: The Leptonic Active Galactic Nucleus Jet Scenario
The Spatially Uniform Spectrum of the Fermi Bubbles: The Leptonic Active Galactic Nucleus Jet Scenario
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DOI：
10.3847/1538-4357/aa9434
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发表时间：
2017-06
期刊：
影响因子：
--
通讯作者：
H. Yang;M. Ruszkowski
中科院分区：
文献类型：
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作者：
H. Yang;M. Ruszkowski
费米气泡是费米伽马射线太空望远镜最重要的发现之一；然而，它们的起源仍然难以捉摸。气泡的独特特征之一是它们的伽马射线光谱，包括在~ 110 GeV的高能截止点和光谱的整体形状，在空间上几乎是均匀的。高能光谱截止表明，由于同步加速器和宇宙射线（CR）电子的逆康普顿冷却，轻子起源；然而，即使对于轻子模型，谱在空间上均匀的原因也不明显。在这项工作中，我们研究了轻子活动星系核（AGN）射流场景中的气泡形成，使用FLASH中的新CRSPEC模块，使我们能够在模拟过程中跟踪CR光谱的演变。我们表明，高能切断是由喷射喷射时银河系中心（GC）附近的快速电子冷却引起的。之后，动态时间尺度成为所有相关时间尺度中最短的，因此光谱基本上是平流的，只有轻微的冷却损失。这可以解释为什么气泡光谱在空间上几乎是均匀的：今天看到的来自气泡不同部分的cr在气泡膨胀的早期阶段都在GC附近共享同一个起源。我们发现，预测的CR空间和光谱分布可以同时与观测到的伽玛射线光谱的归一化、光谱形状和高能截止点及其空间均匀性相匹配，这表明过去的AGN喷流活动可能是费米气泡形成的机制。
The Fermi bubbles are among the most important findings of the Fermi Gamma-ray Space Telescope; however, their origin is still elusive. One of the unique features of the bubbles is that their gamma-ray spectrum, including a high-energy cutoff at ∼110 GeV and the overall shape of the spectrum, is nearly spatially uniform. The high-energy spectral cutoff is suggestive of a leptonic origin due to synchrotron and inverse-Compton cooling of cosmic-ray (CR) electrons; however, even for a leptonic model, it is not obvious why the spectrum should be spatially uniform. In this work, we investigate the bubble formation in the leptonic active galactic nucleus (AGN) jet scenario using a new CRSPEC module in FLASH that allows us to track the evolution of a CR spectrum during the simulations. We show that the high-energy cutoff is caused by fast electron cooling near the Galactic center (GC) when the jets were launched. Afterwards, the dynamical timescale becomes the shortest among all relevant timescales, and therefore the spectrum is essentially advected with only mild cooling losses. This could explain why the bubble spectrum is nearly spatially uniform: the CRs from different parts of the bubbles as seen today all share the same origin near the GC at an early stage of the bubble expansion. We find that the predicted CR spatial and spectral distribution can simultaneously match the normalization, spectral shape, and high-energy cutoff of the observed gamma-ray spectrum and their spatial uniformity, suggesting that past AGN jet activity is a likely mechanism for the formation of the Fermi bubbles.