[论文解读] Time-dependent galactic winds I. Structure and evolution of galactic outflows accompanied by cosmic ray acceleration

E. A. Dorfi, D. Breitschwerdt|arXiv (Cornell University)|Apr 4, 2013

Astrophysics and Cosmic Phenomena参考文献 59被引用 5

一句话总结

该论文提出了一种时变的星系风动力学模型，自洽地耦合了宇宙射线（CR）通过扩散和对流的输运过程与自激磁流体（MHD）波的动量传递。结果表明，连续的超新星爆发在星系晕中激发出激波，使宇宙射线在约3 kpc的银道面附近高效地加速至10^17–10^18 eV，为能量范围约10^15–10^18 eV之间‘膝’与‘踝’之间的银河系宇宙射线能谱提供了自然解释。

ABSTRACT

Cosmic rays are transported out of the galaxy by diffusion and advection due to streaming along magnetic field lines and resonant scattering off self-excited MHD waves. Thus momentum is transferred to the plasma via the frozen-in waves as a mediator assisting the thermal pressure in driving a galactic wind. The bulk of the Galactic CRs are accelerated by shock waves generated in SNRs, a significant fraction of which occur in OB associations on a timescale of several $10^7$ years. We examine the effect of changing boundary conditions at the base of the galactic wind due to sequential SN explosions on the outflow. Thus pressure waves will steepen into shock waves leading to in situ post-acceleration of GCRs. We performed simulations of galactic winds in flux tube geometry appropriate for disk galaxies, describing the CR diffusive-advective transport in a hydrodynamical fashion along with the energy exchange with self-generated MHD waves. Our time-dependent CR hydrodynamic simulations confirm the existence of time asymptotic outflow solutions (for constant boundary conditions). It is also found that high-energy particles escaping from the Galaxy and having a power-law distribution in energy ($\propto E^{-2.7}$) similar to the Milky Way with an upper energy cut-off at $\sim 10^{15}$ eV are subjected to efficient and rapid post-SNR acceleration in the lower galactic halo up to energies of $10^{17} - 10^{18}$ eV by multiple shock waves propagating through the halo. The particles can gain energy within less than $3\,$kpc from the galactic plane corresponding to flow times less than $5\cdot 10^6\,$years. The mechanism described here offers a natural solution to explain the power-law distribution of CRs between the "knee" and the "ankle". The mechanism described here offers a natural and elegant solution to explain the power-law distribution of CRs between the "knee" and the "ankle".

研究动机与目标

• 研究恒星形成区中时变的超新星活动如何影响星系风动力学及宇宙射线加速。
• 在星系风框架下，对宇宙射线输运、MHD波生成与流体动力学之间的自洽耦合进行建模。
• 确定在星系晕中形成的激波是否能解释‘膝’（~10^15 eV）至‘踝’（~10^18 eV）之间观测到的幂律宇宙射线能谱。
• 评估在边界条件变化下，时变星系风解的稳定性和渐近行为。

提出的方法

• 采用通量管几何的流体动力学模拟，通过反映超新星驱动压力变化的时变内边界条件来建模星系风。
• 通过福克-普朗克方程的矩方法描述宇宙射线输运，包含沿磁场线的扩散和对流输运。
• 包含宇宙射线与自激MHD波之间的能量交换，实现对离子等离子体的动量传递并驱动风。
• 考虑非线性反馈，使宇宙射线压力能够改变激波特性和避免测试粒子近似。
• 模拟追踪了风结构、激波特性和粒子加速随时间的演化，包括激波传播和能谱变陡。
• 通过改变边界条件模拟连续的超新星爆发，诱导晕中产生瞬态激波和接触间断面。

实验结果

研究问题

• RQ1 由连续超新星爆发引起的时变边界条件是否能导致形成与稳态模型一致的稳定、渐近的喷流解？
• RQ2 在星系晕中传播的激波如何影响宇宙射线加速至10^15 eV以上的能量？
• RQ3 宇宙射线能谱在‘膝’至‘踝’区域的能谱变陡在多大程度上源于晕中激波特性的加速机制？
• RQ4 宇宙射线多快能被加速至10^17–10^18 eV？该过程在距离银河系盘面多近的区域发生？
• RQ5 激波下游粒子的对流在塑造高能宇宙射线在地球观测到的各向同性和通量方面起什么作用？

主要发现

• 时变星系风模拟收敛至渐近解，与Breitschwerdt等人（1991）的稳态风解高度一致。
• 由于超新星活动引起的时变压力，星系晕中形成的激波可在5×10^6年以内、在距离银河平面约3 kpc的范围内，将宇宙射线加速至10^17–10^18 eV。
• 该加速机制高效且迅速，发生在下部晕区，多个激波在周围介质中传播，与银河系宇宙射线的观测能量分布一致。
• 激波处的宇宙射线谱比盘面更陡，谱指数低于-4，这是由于晕区马赫数较低，解释了‘膝’之后的能谱变陡现象。
• 宇宙射线被对流输运至激波下游并朝向银河系盘面运动，使其可能被观测到，并对地球处的相对各向同性通量有贡献。
• 星系晕被识别为超新星爆发后宇宙射线加速的有利场所，尤其在像银河系这样处于平静状态的恒星形成区，激波马赫数较低，能谱变陡现象更显著。