论文标题
模拟费米气泡作为AGN喷气机驱动的前锋冲击
Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets
论文作者
论文摘要
费米气泡是位于银河中心（GC）上方和下方的伽马射线中的两个巨大气泡。尽管对气泡进行了许多研究，但它们的起源和排放机制仍然难以捉摸。在这里，我们使用一套流体动力模拟来研究气泡中宇宙射线（CRS）的场景，主要是在由SGR A*的一对相对喷射器驱动的前向冲击中加速。我们发现，发生了一个活跃的银河核（AGN）喷气事件发生了$ 5-6 $ MYR以前可以自然地再现气泡的双眼形态，并且气泡中的后震动气温加热到$ \ sim0.4 $ kev，与最近的X射线观测一致。正向冲击压缩热光环气体，在低纬度下，压缩气体显示出X形结构，自然地解释了Rosat 1.5 KeV映射的双色X射线结构，形态和X射线表面亮度均具有。在前2个MYR中，CR加速度在气泡的头部区域最有效。相对的喷气机释放$ \ sim 10^{55} $ erg的总能量，其爱丁顿的比率为$ \ sim 10^{ - 3} $，它在黑洞的热积聚流程模式的范围内很好。我们的模拟进一步表明，由球形风在GC驱动的正向冲击通常会产生比观察到的较宽要宽的气泡，并且无法在低纬度下重现双色调X射线结构。这表明在冲击场景中，Starburst或AGN风不太可能是气泡的起源。
The Fermi bubbles are two giant bubbles in gamma rays lying above and below the Galactic center (GC). Despite numerous studies on the bubbles, their origin and emission mechanism remain elusive. Here we use a suite of hydrodynamic simulations to study the scenario where the cosmic rays (CRs) in the bubbles are mainly accelerated at the forward shocks driven by a pair of opposing jets from Sgr A*. We find that an active galactic nucleus (AGN) jet event happened $5-6$ Myr ago can naturally reproduce the bilobular morphology of the bubbles, and the postshock gas temperature in the bubbles is heated to $\sim0.4$ keV, consistent with recent X-ray observations. The forward shocks compress the hot halo gas, and at low latitudes, the compressed gas shows an X-shaped structure, naturally explaining the biconical X-ray structure in the ROSAT 1.5 keV map in both morphology and X-ray surface brightness. CR acceleration is most efficient in the head regions of the bubbles during the first 2 Myrs. The opposing jets release a total energy of $\sim 10^{55}$ erg with an Eddington ratio of $\sim 10^{-3}$, which falls well in the range of the hot accretion flow mode for black holes. Our simulations further show that the forward shocks driven by spherical winds at the GC typically produce bubbles with much wider bases than observed, and could not reproduce the biconical X-ray structure at low latitudes. This suggests that starburst or AGN winds are unlikely the origin of the bubbles in the shock scenario.
