演化激波景观中的宇宙线输运与加速
Cosmic ray transport and acceleration in an evolving shock landscape
AI总结：
本文利用CRPropa中的随机微分方程并结合Athena++流体模拟，研究银河系晕中由局域爆发产生的演化激波对宇宙线的输运与再加速，发现大量宇宙线可返回银河系并贡献膝区以上能谱，且激波碰撞会使高能谱暂时变硬。
摘要：
膝区与踝区之间的宇宙线起源仍存在争议。银河风及其终止激波被认为可能促成这一银河系与河外起源之间的过渡，但另一种可能是来自银河系内局域源的大尺度激波结构。本文研究了银河系晕中随时间演化的激波景观下的宇宙线输运。这些激波可能源于局域爆发，例如恒星形成区和超级气泡。在此类激波处被再加速的宇宙线可以达到膝区以上的能量。由于这些激波比终止激波更靠近银河系，且宇宙线在下游逃逸，它们更容易传播回银河系。由于此类爆发频繁发生，激波将发生相互作用。这种相互作用可能调整宇宙线能谱，尤其是对于能够同时在两个激波处被加速的粒子。激波处宇宙线的输运与加速通过公开宇宙线传播框架CRPropa中的随机微分方程（SDE）进行建模。我们为随时间变化的风剖面开发了扩展，并首次将该代码与流体力学模拟相连接，这些模拟使用公开的Athena++代码运行。我们发现，取决于扩散张量的具体实现，相当大比例的宇宙线能够返回银河系。这些宇宙线可能对膝区及以上（$E \gtrsim 10\\,\mathrm{PeV}$）观测到的能谱作出贡献。与简化模型相比，简单幂律并不能很好地描述能谱。相反，对于单一激波，我们发现在低能区存在平坦能谱（$E^{-2}$），该能谱逐渐变陡，直至达到指数下降。当激波碰撞时，能谱在高能区暂时变得比$E^{-2}$更硬。
英文摘要：
The sources of cosmic rays between the knee and the ankle are still debated. The Galactic wind and its termination shock have been proposed to contribute to this transition between Galactic and extragalactic origin, but another possibility is large-scale shock structures from local sources in the Milky Way. In this paper, we investigate CR transport in a time-dependent landscape of shocks in the Galactic halo. These shocks could result from local outbursts, e.g. starforming regions and superbubbles. CRs re-accelerated at such shocks can reach energies above the knee. Since the shocks are closer to the Galaxy than a termination shock and CRs escape downstream, they can propagate back more easily. With such outbursts happening frequently, shocks will interact. This interaction could adjust the CR spectrum, particularly for the particles that are able to be accelerated at two shocks simultaneously. The transport and acceleration of CRs at the shock is modeled by Stochastic Differential Equations (SDEs) within the public CR propagation framework CRPropa. We developed extensions for time-dependent wind profiles and for the first time connected the code to hydrodynamic simulations, which were run with the public Athena++ code. We find that, depending on the concrete realization of the diffusion tensor, a significant fraction of CRs can make it back to the Galaxy. These could contribute to the observed spectrum around and above the CR knee ($E \gtrsim 10\,\mathrm{PeV}$). In contrast to simplified models, a simple power-law does not describe the energy spectra well. Instead, for single shocks, we find a flat spectrum ($E^{-2}$) at low energies, which steepens gradually until it reaches an exponential decline. When shocks collide, the energy spectra transiently become harder than $E^{-2}$ at high energies.
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