千与千寻：宇宙射线向银河系星系际介质的平流损失解释了大麦哲伦星系的低伽马射线光度
Spirited Away: Advective Loss of Cosmic Rays into the Milky Way's Circumgalactic Medium Explains the Large Magellanic Cloud's Low Gamma-ray Luminosity
AI总结：
研究大麦哲伦星系伽马射线光度被高估问题，通过将宇宙射线传输代码应用于其与银河系星系际介质相互作用模拟，发现宇宙射线平流损失使伽马射线光度降低，模型在低能与观测光谱匹配，高能仍有高估，揭示传输或注入谱问题。
AI中文摘要：
星系尺度宇宙射线产生和传输模型成功再现了许多星系的射电和伽马射线光谱，但大麦哲伦星系（LMC）是个例外，现有模型一直高估其伽马射线通量。本文通过将CRIPTIC宇宙射线传输代码应用于LMC与银河系星系际介质相互作用的磁流体动力学模拟来研究这一差异。对于多种宇宙射线传输模型，我们恢复的模拟伽马射线光度接近观测值，而孤立LMC的对照模拟显示出与之前研究相同的预测过高问题。包含星系际介质相互作用的模拟产生较低的伽马射线光度，因为宇宙射线主要通过平流逃离星系，显著减少了碰撞过程的发射，并将主导的伽马射线发射机制从介子衰变变为逆康普顿发射。详细光谱形状的比较表明，我们的相互作用LMC模型在光子能量低于约10 GeV时与观测到的伽马射线光谱匹配，但在较高能量下仍略微高估通量，这表明要么传输比我们探索的模型更强烈地依赖能量，要么宇宙射线注入光谱比我们采用的$p^{-2.2}$更陡。
英文摘要：
Models of galactic-scale cosmic ray production and transport have successfully reproduced the radio and $γ$-ray spectra of many galaxies; however, one notable exception is the Large Magellanic Cloud (LMC), where models that successfully fit other galaxies consistently overestimate its $γ$-ray flux. Here we investigate this discrepancy by applying the CRIPTIC cosmic ray transport code to recent magnetohydrodynamic simulations of the LMC's interactions with the Milky Way circumgalactic medium that accurately reproduce observations of the LMC's magnetic field structure. We recover a simulated $γ$-ray luminosity that is close to the observed luminosity for a wide range of cosmic ray transport models, while our control simulations of an isolated LMC not interacting with the Milky Way show the same over-prediction problem as previous investigations. Simulations including the CGM interaction yield lower $γ$-ray luminosities because in them cosmic rays escape the galaxy primarily via advection, significantly decreasing the emission from collisional processes and changing the dominant $γ$-ray emission mechanism from pion decay to inverse Compton emission. Comparisons of the detailed spectral shape show that our interacting LMC models match the observed $γ$-ray spectrum at photon energies below $\sim 10$ GeV, but still slightly overestimate the flux at higher energies, suggesting either more strongly energy-dependent transport than the models we have explored, or a cosmic ray injection spectrum steeper than the $p^{-2.2}$ that we adopt.
